Title of Invention	"AN INTRINSICALLY SAFE ZENER REGULATED POWER SUPPLY UNIT USEFUL IN HAZARDOUS LOCATIONS"
Abstract	An intrinsically safe zener regulated power supply unit useful in hazardous locations, characterised by : a pair of bipolar transistors (12a, 12b) connected in parallel to a resistor (11), the said bipolar transistor pair (12a, 12b) being provided with a series current limiting resistor (SCLR) (10), a filter capacitor (2) with a series resistor (1), control circuits (13a, 13b) for controlling the conduction of the said bipolar transistor (12a, 12b), an over current sensing circuits (OCSC) (19a, 19b) and a fuse, a plurality of zener regulators (7,8,9) being provided across the output.

Title of Invention

"AN INTRINSICALLY SAFE ZENER REGULATED POWER SUPPLY UNIT USEFUL IN HAZARDOUS LOCATIONS"

Abstract

An intrinsically safe zener regulated power supply unit useful in hazardous locations, characterised by : a pair of bipolar transistors (12a, 12b) connected in parallel to a resistor (11), the said bipolar transistor pair (12a, 12b) being provided with a series current limiting resistor (SCLR) (10), a filter capacitor (2) with a series resistor (1), control circuits (13a, 13b) for controlling the conduction of the said bipolar transistor (12a, 12b), an over current sensing circuits (OCSC) (19a, 19b) and a fuse, a plurality of zener regulators (7,8,9) being provided across the output.

Full Text	The present invention relates to an intrinsically safe zener regulated power supply unit useful in hazardous locations. The present invention particularly relates to an intrinsically safe zener regulated power supply unit useful to provide significant current and stable output voltage to electronic instruments placed in hazardous locations, such as under ground mines. The basic function of an intrinsically safe power unit is to provide power at its output incapable of igniting or causing ignition of a mixture of flammable or combustible material in air in the mixtures most easily ignitable concentration. Intrinsic safety is achieved by limiting the energy flow from the output of the power supply unit. While designing an intrinsically safe circuit it is important to make sure that it functions properly under normal operating conditions but still is safe under fault conditions. There are governmental testing or certifying organizations such as Eastern Regional Testing Laboratory (E.R.T.L), Central Mining Research Institute (CM.R.I), & Director General of Mines Safety (D.G.M.S) which provide prescribed test conditions for determining equipment is intrinsically safe. It is mandatory to get the instrument tested from such organization before its use in the hazardous location. To protect the intrinsically safe apparatus in a hazardous area, an energy limiting device must be installed. There are three components in intrinsically safe circuit to limit current and voltage; a resistor, at least two zener diodes, and a fuse. The resistor limits the current to a specific value known as short circuit current. The zener diode limits the voltage to a value referred to as an open circuit voltage. The fuse will prevent the diode from burning and allowing excess voltage to reach hazardous area. The problem in using a simple resistor barrier to limit current flow is that the voltage regulation at the output terminal of the power source is poor as the voltage drop across the barrier resistor increases with the increase in load current. Moreover, the output voltage will vary with the change in input voltage level connected to the input terminal of the intrinsically safe power supply source. Extensive efforts have been made to the development of suitable intrinsically safe power unit to provide required voltage and current to instruments used in the hazardous locations. To highlight the continual efforts made by inventors to develop intrinsically safe power source, a few patent references are provided below for illustration. Reference may be made to U.S. Patent No. 5,982,594, titled: 'Intrinsically safe power source'. The invention relates to an intrinsically safe power supply unit, provided for conditioning power supplied by a power source. An input power converter connected to the power source receives current from the power source. A direct output crowbar and discrete impedance elements dissipate and limit the energy in the power supply unit that would otherwise be delivered to an external fault. In addition, the direct output crowbar extracts energy from any external storage elements. An adaptive shut down circuit distinguishes nominal load conditions including load changes from an external fault. A multistage LC power filter is used to minimize the stored energy that would be deliverable to an external fault or dissipated by the direct output crowbar and discrete impedance elements. The combination of the direct output crowbar, discrete impedance elements, adaptive shut down detection circuitry and multistage LC power filter significantly improves the level and quality of intrinsically safe power delivered by the power supply unit. Reference may be made to U.S. Patent No. 5,144,517. The invention relates to intrinsically safe barrier device. A protection barrier provides voltage and current necessarily conveyed to hazardous, such as explosive, areas. The system includes a current sensing circuit which triggers a silicon control rectifier based crowbar circuit in the event over current is detected. This triggering, in turn, trips a relay severing the current path entirely. A reverse polarity circuit provides for immediate clamping of improperly bias connections in a similar crowbar arrangement, and also provides for tripping of the circuit breaker. Thermistors are also provided in current series in the event the circuit breaker fails to trip, or is forcibly bypassed. The circuit breaker additional provides a visual indicator when it is in the tripped position. The intrinsic barrier system is suitable to serving as a first stage protection to zener barrier modules as utilized in the prior art. Reference may be made to U.S. Patent No.4,831,484, an electrical safety barrier for protection of electrical load elements placed in potentially hazardous locations. The barrier input and output each have two terminals. In at least one connection between the barrier input and the barrier output, there is an electronic in-line control element having a control input as well as, in series with the in-line control element, a current sensing element that emits an output voltage corresponding to the current. A control circuit having a control input controls the conductive state of the in-line control element and to this end receives the output voltage of the current sensing element. In accordance with the invention, to obtain a steeper limiting characteristic, the control voltage for the control circuit is the sum of at least part of the output voltage of the current sensing element and at least part of a voltage corresponding to the voltage drop in the in-line control element. The control element is formed by two serially connected transistors; the control circuit includes four active elements which are connected to individual summing circuits and provide, in pairs, output signals to control the serially connected transistors. Reference may be made to U.S. Patent No. 4,438,473 titled: 'Power supply for an intrinsically safe circuit'. An intrinsically safe power supply employing a binary current interrupter connected between the power source and the electrical load. Normally the load is situated in a potentially dangerous environment, like a coal mine, and the intrinsically safe power supply at a safe remote location, e.g., on the earth's surface. The interrupter has a pass and switching transistor, current sensor, gating circuitry, a flip-flop switch, and means to delay the turning on of the transistor - but not its turning off. If an over current or over voltage condition is sensed between the input and output of the intrinsically safe power supply, load current will cease to flow. In normal operation, only the current interrupter pass transistor will open. A reset signal from an oscillator internal of the interrupter may be used to reset the flip-flop after actuation or upon its initial setting, thereby causing the flow of power into the load. U.S. Patent No. 4,412,265, illustrates an intrinsic barrier in which attempt has been made to provide controlled output voltage by using a combination of transistor and zener diode. Since the transistor is kept turned on in all conditions a fuse having fast destruction time compared to that of the transistor is provided to protect the transistor from over current heating in fault conditions. Zener diode used in the circuit is only for limiting the output voltage level when the transistor fails. U.S Patent No.5,050,060 illustrates an intrinsically safe power supply unit that attempts to provide high output power and improved voltage regulation at the output terminal by using a post regulator after the barrier resistors. In this technique current limiting resistor passes the rated current in all conditions and dissipates proportionate wattage. Moreover, the post regulator passes all of the rated current to maintain output voltage regulation when load gets disconnected from the output terminal and there will be wastage of power. So this type of intrinsically safe power supply circuit can not be a right choice for battery powered applications. Zener diode used in the circuit is exclusively for control purposes and the actual voltage regulation is via the transistors. Hence, it is clear from the hitherto known prior art that no attempt has yet been made to use zener diode in intrinsically safe power supply unit as voltage regulator to provide significant current and stable output voltage due to the drawbacks associated with zener regulated power supply circuit. Strategy for designing a zener regulated power supply circuit using zener diode and series current limiting resistor is to design for a maximum power dissipated in the zener diode when the regulator is open circuited. The logic of this design strategy is to protect the zener diode. The value of series current limiting resistor limits the current so that the zener diode is not burned out if the load gets disconnected. So the serious drawback in situations where we require significant output voltage and current is that the series current limiting resistor always pass the total current and must dissipate proportionate wattage even when no output is required. In another situation when load is not connected, all of this current will pass through the zener and dissipate high power. So for high power applications, a suitable zener diode capable of dissipating high power has to be selected which is expensive. Moreover, the rating of zener diode in zener regulated intrinsically safe power supply circuit should be such that it will be capable of carrying 1.5 times the current circulated at normal condition in the particular conditions of mounting and in the ambient temperature specified (CI. No. 9.2.2 of IS: 5780-1980). So, intrinsically safe zener regulated power supply is normally restricted to low power applications only. The main object of the present invention is to provide an intrinsically safe zener regulated power supply unit useful in hazardous locations. Another object of the present invention is to provide an intrinsically safe zener regulated power supply unit useful to provide significant current and stable output voltage to electronic instruments placed in hazardous locations, such as under ground mines. Still another object of the present invention is to provide an intrinsically safe zener regulated power supply unit, which enables to provide stable output voltage at varying load currents. Yet another object of the present invention is to provide an intrinsically safe zener regulated power supply unit which enables reduction in power dissipation in the series current limiting resistor and zener regulator when load is disconnected from power supply unit. Still yet another object of the present invention is to provide an intrinsically safe zener regulated power supply unit, which enables to limit the energy flow to the hazardous area when over current is sensed due to external fault. A further object of the present invention is to provide an intrinsically safe zener regulated power supply unit which enables to maintain output voltage under regulation even when the input unregulated voltage changes due to the fluctuations in main power supply voltage. Still further object of the present invention is to provide an intrinsically safe zener regulated power supply unit which also enables to limit excess current flow to zener regulator by melting fuse connected in series with Zener regulator thereby limits output voltage. In the present invention there is provided an intrinsically safe zener regulated power supply unit useful to provide significant current and stable output voltage to electronic instrument placed in the hazardous location. Combination of bipolar transistor and resistor, connected in parallel with bipolar transistor, is connected in series with zener regulator to control the current flow in zener regulator. Control circuit controls the conduction of bipolar transistor to achieve controlled current flow in zener regulator at varied load conditions. When load is not connected to the power supply unit or the zener regulator is open circuited control circuit turns off bipolar transistor and the resistor connected in parallel with bipolar transistor keeps zener regulator under conduction. Since the value of the resistor connected in parallel with bipolar transistor is high, energy dissipation in zener regulator as well as in current limiting resistor is maintained at lower value. Whenever load is connected to the power supply unit, control circuit turns on bipolar transistor and controls its conduction to provide the required current to the load and maintains zener regulator current more or less unchanged. Hence the power supply unit according to the present invention overcomes the drawbacks associated with zener regulated power supply circuit. The present invention is also provided with means to monitor the excess current flowing in the circuit during fault condition and acts if current flowing exceeds specific limit to signal control circuit to turn off the bipolar transistor thereby limits the energy which would otherwise be delivered to hazardous area. To improve safety two bipolar transistors are connected in series to ensure the safe operation of power supply unit when one transistor fails. In the event of failure of both the bipolar transistors the excess current, delivered to the zener regulator, is limited by fuse. The rating of bipolar transistor is such that it can dissipate sufficient heat to maintain output voltage regulation if the unregulated voltage at the input changes due to fluctuations in main power supply voltage. The intrinsically safe zener regulated power supply unit of the present invention includes bipolar transistors, resistor connected in parallel with bipolar transistors and control circuit. Combination of bipolar transistors and resistor is connected in series with current limiting resistor and the control circuit provided controls the operation of bipolar transistor to control current flow to zener regulator and thereby to improve output voltage regulation. Bipolar transistor is turned off by control circuit, when output terminals get disconnected from load, and the zener regulator is kept in conduction by the resistor connected in parallel with bipolar transistors. Due to the higher value of the said resistor, minimum current will flow in the circuit. So the power consumption of series current limiting resistor and zener regulator are maintained at a lower value. Thus it becomes possible to avoid unwanted wastage of power which is common in the conventional zener regulated power supply circuit when no load is connected to the power supply unit. When load is connected to power supply unit control circuit turns on the bipolar transistor and controls its conduction to supply required current to the load and thus helps to check the changes in zener regulator current at varied load condition. As a result stable output voltage is obtained at varied load condition which is not practically possible in conventional zener regulated power supply circuit. It also provides means to monitor over current condition in the circuit. Whenever current beyond normal operating current, due to external fault, tries to flow in the circuit the said means trigger control circuit to reduce bias current of bipolar transistor and thus controls the conduction of bipolar transistor. Finally, bipolar transistor when current drawn due to external fault crosses a specific limit is brought into its blocking state and the fault current is supplied through resistor connected in parallel with bipolar transistors. Thus the power supply unit of the present invention restricts current flow to external fault. In normal operating condition output voltage is regulated by combination of control circuit, bipolar transistor and zener regulator but in the event of failure of bipolar transistor output voltage is limited by zener regulator and fuse. In figures 1 and 2 of the drawings accompanying this specification are detailed the various parts constituting the intrinsically safe zener regulated power supply unit of the present invention. In figure 1 of the drawings is shown a block diagram of the various parts constituting the intrinsically safe zener regulated power supply unit of the present invention is depicted. Figure 2 represents a circuit diagram of a preferred embodiment of the present invention. The intrinsically safe zener regulated power supply unit, useful to provide significant current and stable output voltage to electronic instrument placed in hazardous locations, of the present invention comprises bipolar transistors (12a, 12b), a resistor (11) connected in parallel with bipolar transistor (12a, 12b), current limiting resistor (10), filter capacitor (2) with a series resistor (1), control circuit (13a, 13b) to control the conduction of bipolar transistor (12a, 12b), fuse and zener regulator (7,8,9). A combination of zener diode (Z2) and resistors (R30, R31 & R32) of the control circuit (13b) is such as capable of setting the output voltage to control the current flow in zener regulator (7,8,9). Voltage drop across resistor (R32), when output voltage tends to increase due to decrease in load current, of the said combination also tends to increase to control the conduction of bipolar transistor (12b) through transistors (T6, T7 & T8) to control the zener regulator current to keep output voltage stable. A combination of zener diode Z2 and resistors (R30, R31 & R32) is such as capable of turning bipolar transistor (12b) into off condition when zener regulators (7, 8 & 9) are open circuited. Voltage drop across R32, when zener regulators (7, 8 & 9) are open circuited, is raised to provide forward bias to the base of transistor T6. Conduction of transistor T6 makes transistor T7 on and thereby provides reverse bias to the base of transistor T8 and helps to turn off bipolar transistor (12b). The over current sensing circuit (19b) consisting of optocoupler (26), resistors (R33, R34 & R35) and transistor T10 is such as capable of monitoring over current flowing through current limiting resistor R1. Voltage drop across current limiting resistor R1, when current flowing through it tends to cross the normal operating current due to external fault, becomes significant to activate the optocoupler (26) to make the transistor T10 on. Conduction of transistor T10 helps to provide back bias to the base of T9 and thereby to turn off the bipolar transistor (12b). The resistor R19 is such as capable of maintaining low current flow, when bipolar transistor (12b) is turned off by control circuit (13b) due to open circuiting of zener regulator (7, 8 & 9), in the power supply circuit. Resistor R19 thereby helps to reduce power consumption of series current limiting resistor and zener regulator when output terminal is open circuited. The said resistor R19 also helps to limit the short circuit current in case of external fault. The filter series resistor R2 and current limiting resistor R1 are such as capable of limiting the amount of energy to an external fault and zener regulator as well. The control circuit (13a) is such as capable of controlling the operation of bipolar transistor (12a) in the event of damage of bipolar transistor (12b) to maintain safe operation of power supply unit. The bipolar transistors (12a & 12b) are such as capable of dissipating sufficient heat without any damage to regulate output voltage when unregulated voltage at input terminals (3 & 4) varies due to fluctuations in supply voltage. The rating of zener regulator (7, 8 & 9) are such as capable of carrying, without open circuiting, the current which would flow if both the bipolar transistors (12a & 12b) fail. Voltage at the output terminal is limited, when current flow exceeds specific limit, by zener regulator by allowing fuse to melt and thus fuse protects zener regulator against wide variations in unregulated input voltage. Accordingly the present invention provides an intrinsically safe zener regulated power supply unit useful in hazardous locations, which comprises: a pair of bipolar transistors (12a, 12b) connected in parallel to a resistor (11), the said bipolar transistor pair (12a, 12b) being provided with a series current limiting resistor (SCLR) (10), a filter capacitor (2) with a series resistor (1), control circuits (13a, 13b) capable of controlling the conduction of bipolar transistor (12a, 12b), a plurality of zener regulators (7,8,9), over current sensing circuits (OCSC) (19a,19b)andafuse. In an embodiment of the present invention, the bipolar transistors (12a & 12b) are such as capable of dissipating sufficient heat without any damage to regulate output voltage when unregulated voltage at input terminals (3 & 4) varies due to fluctuations in supply voltage. In another embodiment of the present invention, the control circuit (13a) is such as capable of controlling the operation of bipolar transistor (12a) in the event of damage of bipolar transistor (12b) to maintain safe operation of power supply unit. In yet another embodiment of the present invention, the combination of zener diode (Z2) and resistors (R30, R31 & R32) of the control circuit (13b) is such as capable of setting the output voltage to control the current flow in zener regulator (7,8,9). In still another embodiment of the present invention, the said combination of zener diode Z2 and resistors (R30, R31 & R32) is such as capable of turning bipolar transistor (12b) into.off condition when zener regulators (7, 8 & 9) are open circuited. In still yet another embodiment of the present invention the rating of zener regulator (7, 8 & 9) are such as capable of carrying, without open circuiting, the current which would flow if both the bipolar transistors (12a & 12b) fail. In a further embodiment of the present invention, the over current sensing circuit (19b) is such as capable of monitoring over current flowing through current limiting resistor R1. In a yet further embodiment of the present invention, the over current sensing circuit (19b) consists of optocoupler (26), resistors (R33, R34 & R35) and transistor T10. In a still further embodiment of the present invention, the resistor R19 is such as capable of maintaining low current flow, when bipolar transistor (12b) is turned off by control circuit (13b) due to open circuiting of zener regulator (7, 8 & 9), in the power supply circuit. In another embodiment of the present invention, the filter series resistor R2 and current limiting resistor R1 are such as capable of limiting the amount of energy to an external fault and zener regulator as well. The embodiments, features and advantages of the present invention will be set forth in part in the description, and in part will come to be understood by those skilled in the art by the reference to the following description of the invention and referenced drawings. In figure 1 and 2 of the drawings accompanying this specification are detailed the various parts constituting the intrinsically safe zener regulated power supply unit of the present invention. The detailed description with reference to the drawing is given below. Figure 1, which is a simplified block diagram of the embodiment of the present invention, shows an intrinsically safe zener regulated power supply unit. The said power supply unit meets the requirements to limit the voltage and current on its intrinsically safe output side to a value, which is incapable of causing explosion. Bipolar transistors (12a & 12b), resistor (11) connected in parallel with bipolar transistor (12a & 12b) and control circuit (13a & 13b) play an important role in the operation of the power supply circuit of the present invention. Firstly, bipolar transistor (12b) act as a switch and remain in off condition when load gets disconnected from output terminal or output terminal is open circuited. In this situation resistor (11), connected in parallel with bipolar transistors (12a & 12b), acts to keep the power supply in operation by allowing minimum amount of current to flow in the circuit. Thus less energy is dissipated in the series current limiting resistor (10) and zener regulators (7, 8 & 9). Secondly, whenever load is connected to power supply unit, control circuit (13b) turn on bipolar transistor (12b) and controls its conduction to provide required amount of current at varied load conditions without significant variation in zener regulator (7, 8 & 9) current. If bipolar transistor (12b) is damaged, control circuit (13a) will act to control the operation of bipolar transistor (12a) for safe operation of power supply unit. Series current limiting resistor (10) primarily serves to limit maximum current escapes from power supply unit during external fault. But over current sensing circuit (19a & 19b) monitors voltage drop across current limiting resistor (10) and are activated, whenever current flowing through current limiting resistor (10) tends to cross the normal operating current due to external fault, to signal control circuit (13a & 13b) to turn off bipolar transistors (12a & 12b). Thus the energy delivered to the external fault is significantly reduced. Zener (7, 8 & 9), in the power supply circuit of the present invention, serves as regulator for regulating output voltage in normal operating condition and in the event of failure of bipolar transistors (12a & 12b) output voltage is limited by the said zener regulators (7, 8 & 9) and fuse. Thus the power supply unit of the present invention fulfils the requirements of intrinsic safety by limiting current and voltage on their intrinsically safe output side to avoid danger of explosion during fault condition. The said power supply unit along with step down transformer and rectifier circuit, not shown in the fig. (1), are enclosed in a flame proof housing so that it can be used in hazardous locations to provide intrinsically safe power to electronic instruments. Figure 2 represents circuit diagram of a presently preferred embodiment of the bipolar transistors (12a & 12b), resistor R19 connected in parallel with bipolar transistors (12a & 12b), control circuit (13a & 13b), over current sensing circuit (19a & 19b), series current limiting resistor JR1, filter capacitor C and zener regulators (7, 8 & 9) of the power supply unit of the present invention. Control circuit (13b) contains a combination of zener diode Z2 and resistors (R30, R31 & R32). The said combination is used, when no load is connected to the output terminals, to set the output voltage such that very small current passes through zener regulators (7, 8 & 9). In this condition voltage drop across resistor R32 is sufficient enough to turn on transistor T6 and thus to provide a forward bias to the base of transistor T7. Conduction of transistor T7 keeps the voltage at the junction (27) well below the voltage required to forward bias transistor T8 and thus helps to turn off bipolar transistor (12b). When load is connected to the power supply unit the voltage drop across resistor R32 is reduced and reverse bias the transistor T6, which in turn helps to generate reverse bias to the base of transistor T7 to turn it into off condition. Now voltage at the junction (27) is raised to a value to turn on transistor T8 and finally the bipolar transistor (12b) is turned on to supply current to load. Control circuit (13b) controls the conduction of bipolar transistor (12b) in such a way that it supplies current drawn only by the load and thus helps to maintain the zener regulator (7, 8 & 9) current more or less unchanged at varied load conditions. Thus voltage at output terminals (5 & 6) is not influenced by dynamic impedance effects and device temperature coefficient characteristics which are inherent with zener. So it becomes possible to achieve a very stable output voltage at varied load conditions. In the event of damage of bipolar transistor (12b) control circuit (13a) will act to control the operation of bipolar transistor (12a) for safe operation of power supply unit. Over current sensing circuit (19b) comprising optocoupler (26), resistor (R33, R34 & R35) and transistor T10 monitors the voltage drop across current limiting resistor R1. Voltage drop across current limiting resistor R1, when load attempts to draw excess current from power supply unit at fault conditions is raised to activate optocoupler (26) to provide forward bias to transistor T10. Which in turn helps to reduce bias current of bipolar transistor (12b) and the voltage at output terminals (5, 6) is reduced. Finally, bipolar transistor (12b) is completely turned off when current drawn by external fault crosses a specific value which reduces output voltage further to make it incapable of providing any bias current to bipolar transistor (12b). Over current sensing circuit (19a) consisting of optocoupler (25), resistors (R16, R17 & R18) and transistor T5 also acts simultaneously as above to reduce bias current to bipolar transistor (12a). Once the bipolar transistors (12a & 12b) are turned off resistor R19 connected between emitter and collector of bipolar transistors (12a & 12b) acts to supply the fault current. Since the value of resistor R19 is high fault current is significantly reduced and voltage across output terminals (5 & 6) becomes very low. So until the fault is cleared the voltage at the output terminal will not rise and the bipolar transistors (12a & 12b) will remain in off condition. In the event of damage of bipolar transistors (12a & 12b) short circuit current is limited by fuse. Control circuit (13b) along with resistor R19 and over current circuitry (19b) control the operation of bipolar transistor (12b) as above, when bipolar transistor (12a) fails, to maintain safe operation of power supply unit. In the event of failure of bipolar transistors (12a & 12b) zener regulator (7, 8 & 9) becomes more conductive and more current passes through fuse () which is finally burned to de-energize the power supply unit. Thus the output voltage is limited to zener regulator (7, 8 & 9) voltage. Capacitor C filters unwanted ripple and resistor R2 limits energy delivered by capacitor C to safe output terminals. In the present invention the intrinsically safe zener regulated power supply unit includes bipolar transistors and a resistor connected in parallel with bipolar transistors and control circuit. The combination of zener diode and resistors in the said control circuit helps to set the output voltage to control the current flow in zener regulator. Whenever the output voltage tends to increase due to changes in load current, voltage drop across resistors of the said combination also changes accordingly and helps to control the conduction of bipolar transistor to stabilize the output voltage at the preset value. Similarly, when zener regulator is open circuited tendency to increase the output voltage will immediately increase voltage drop across the said resistors and helps to turn off the bipolar transistor. Since the bipolar transistor is turned off resistor connected in parallel with bipolar transistors acts to limit current flow in the circuit and thus reduces the power consumption of series current limiting resistor and zener regulators. In the present invention intrinsically safe zener regulated power supply unit includes over current sensing circuit to monitor current flowing in the circuit. The over current sensing circuit comprises phototransistor optocoupler, resistors and transistor. Voltage drop across series current limiting resistor, when current flowing through it tends to exceed the normal operating current due to external fault, will be significant to forward bias the emitter of the optocoupler and enable the detector of the said optocoupler to turn on transistor. Which in turn enables to turn off the bipolar transistor to limit energy flow to external fault. In the present invention the intrinsically safe zener regulated power supply unit includes filter capacitor and filter series current limiting resistor to provide smooth output voltage. Filter series current limiting resistor helps to restrict the energy delivered from filter capacitor to output terminals to a safe value. In the present invention the intrinsically safe zener regulated power supply unit includes bipolar transistors which are capable of dissipating sufficient heat such as the output voltage remains within regulation during changes in unregulated input voltage due to fluctuations in main supply voltage. In the present invention the intrinsically safe zener regulated power supply unit includes zener regulator to regulate output voltage. Since the zener regulator, in normal operating condition, is kept in conduction mode with low zener current it helps to suppress the unwanted ripple generated due to the switching of bipolar transistor. Further, rating of zener regulator is such as capable of carrying, without open circuiting, the current which would flow in the case of damage of bipolar transistors and open circuiting of output terminals. In this circumstance if the unregulated voltage at the input terminals of the power supply unit varies due to the fluctuations in mains supply voltage, fuse limits excess energy flow to zener regulator and protects it against wide variations in input voltage. The novelty of the intrinsically safe zener regulated power supply unit, of the present invention, resides in the capability in controlling the zener regulator current at varied load conditions to ensure stable output voltage, thus making it capable of providing significant current and stable output voltage to electronic instruments placed in hazardous locations, such as underground mines. In the power supply unit of the present invention zener regulator current, when output terminals get disconnected from load or zener regulator is open circuited, is maintained at a minimum value to minimize the power consumption of series current limiting resistor and zener regulator. Further, it provides means to limit fault current, when over current is sensed due to external fault, to ensure that less energy is delivered to external fault. Zener regulator further acts to limit output voltage, when zener regulator is open circuited and unregulated voltage at the input terminals of the power supply unit varies widely due to fluctuations in mains supply voltage, by allowing fuse to melt and thus power supply unit is de-energized in fault condition. The novelty of intrinsically safe zener regulated power supply unit has been realized by the non-obvious inventive steps such as incorporation of bipolar transistors and resistor, connected in parallel with bipolar transistors, and control circuit in the present power supply circuit. Control circuit controls the operation of bipolar transistor which is connected in series with series current limiting resistor to maintain zener regulator current at lower value without significant variation as set by the combination of zener diode and resistors in the said control circuit during varied load condition. When the voltage across the output terminals tends to increase due to the decrease in load current, voltage drop across resistors of the said combination of control circuit is increased and it helps to control the conduction of bipolar transistor to stabilize the output voltage at the preset value. Thus the changes in zener regulator current is controlled even when load current varies due to changes in load resistance and it becomes possible keep the output voltage stable. Similarly when output terminals get disconnected from load the said control circuit turn off the bipolar transistor and then the resistor connected in parallel with bipolar transistor acts to maintain current in the circuit at lower value. So, the power consumption of series current limiting resistor and zener regulators are significantly controlled when output terminal is open circuited. In the hitherto known prior art, where zener is placed in parallel across a load to be regulated, the current through the zener is increased due to the decrease in load current and it reaches the maximum value when the zener regulator is open circuited. The changes in zener current influence the zener voltage due to dynamic impedance effects. Further, the zener voltage is also influenced by device temperature coefficient characteristic inherent with zener, particularly when applied power is significant relative to full rating or heat sinking is marginal. Moreover, heat is unnecessarily dissipated, which is nothing but wastage of power, in the series current limiting resistor and zener regulator when output terminals get disconnected from load. In present invention the over current sensing circuit monitors the current flowing through series current limiting resistor. In case of external fault, when excess current beyond the normal operating current is attempted to be drawn from the power supply unit, over current sensing circuit acts to reduce bias current and finally to turn off bipolar transistor. Thus the energy delivered to the external fault is significantly reduced by over current sensing circuit. In the event of failure of bipolar transistors over current is limited by fuse. Similarly, zener regulator is capable of dissipating sufficient heat, without open circuiting, if bipolar transistors fail and output terminal gets disconnected from load. Further increase in zener regulator current due to variations in unregulated voltage at the input terminals of power supply unit allows fuse to melt and thus limits the output voltage without any damage to zener regulator. The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention. The results of the experiments such as variation in voltage across intrinsically safe output terminals at varied load conditions and current limiting characteristic of the power supply unit according to present invention are detailed below. Example 1 Intrinsically safe power supply unit in accordance with the present specification was made to provide 12V DC output voltage. 13V DC was applied as input voltage to the power supply circuit and the value of the series current limiting resistor used was 3.9 ohms, 5W. Rating of zener used to regulate output voltage was 12V, 5W with tolerance of 5%. In conventional zener regulated power supply circuit zener current changes for regulation of output voltage against both variations in the input voltage from a unregulated power supply or variations in the load resistance. So in zener regulated power supply circuit using only series current limiting resistor and zener, output voltage should vary between 11.4V and 12.6V as per specifications of the zener mentioned above depending upon zener current and ambient temperature. Zener regulated power supply unit according to the present invention incorporates unique combination of bipolar transistors, resistor connected in parallel with bipolar transistors and control circuit. Control circuit controls changes in zener current against the variations in the load resistance by controlling the conduction of bipolar transistor. Figure 3, of the drawings accompanying this specification is a graph of the output voltage characteristic versus load current of the power supply unit configured as above. Combination of zener diode and resistors of control circuit of the said power supply unit was selected with values such that the output voltage was set at 12.00V when no load was connected to the output terminals. From the graph it is found that output voltage is maintained at 12.00V for load current up to 150mA. With further increase in load current voltage at output terminal starts decreasing and when load current reaches around 200mA over current sensing circuit starts functioning. Since the over current sensing circuit is activated bias current of bipolar transistor starts decreasing due to the decrease in potential at the junction 29 of control circuit (13b). As a result output voltage is decreased. Decrease in output voltage further lowers the potential at junction 29 and the conduction of bipolar transistor is further decreased. When load current reaches around 226mA, output voltage starts decreasing more rapidly due to the fast decrease in bias current of the bipolar transistor. When the bipolar transistor is brought into its blocking state output voltage is decreased to a very low value and load current is supplied through resistor R19. In this situation the magnitude of load current depends on the value of resistor R19 used. Thus by selecting a suitable value of resistor R19 it is possible to maintain the fault current at lower value. Example 2 In another situation with input voltage maintained at 13V, maximum current in the above power supply circuit when load is not connected to the output terminals and the bipolar transistor is short circuited will be around 256mA. This current is less than the rated maximum current (395mA) of zener regulator according to data sheet. Single zener used as regulator, is thus capable of carrying, without open circuiting, the current which would flow when bipolar transistor fails and output is open circuited. So two zeners, when connected at output terminals as per the requirements of intrinsic safety, will be able to carry sufficient current, without open circuiting, to melt the fuse provided in the circuit to limit current to protect zener regulator. Addition of more zeners at the output terminal will further improve safety and reliability of the power supply circuit. Zeners used as regulator thus help to limit output voltage too to restrict energy flow to external fault. In the event of failure of bipolar transistor fuse also acts to limit fault current. As those skilled in the art will realize that value of the components of the present zener regulated power supply unit may be changed to make it capable of providing any other required voltage and current. But for its use in hazardous locations the magnitude.of voltage and current has to be limited to a specific value which will pass the prescribed test conditions of intrinsic safety accepted by national or other testing authority. The present invention is an ideal one to design power supply unit for required voltage and current for battery powered application. Present invention may also be used to provide improved voltage regulation and significant current to electronic instrument used in non-hazardous area. The main advantages of the intrinsically safe zener regulated power supply unit of the present invention are: 1. Permits to provide significant current to electronic instrument placed in hazardous locations. 2. Makes it possible to keep the output voltage steady at varied load conditions. 3. Enables to reduce power consumption of series current limiting resistor and zener regulator when output terminals get disconnected from load. 4. Makes it a suitable one for battery powered applications. 5. In case of short circuit at the output terminals minimum current flows into the fault thereby ensures safety and protects the components of power supply unit from damage. We claim: 1. An intrinsically safe zener regulated power supply unit useful in hazardous locations, characterised by : a pair of bipolar transistors (12a, 12b) connected in parallel to a resistor (11), the said bipolar transistor pair (12a, 12b) being provided with a series current limiting resistor (SCLR) (10), a filter capacitor (2) with a series resistor (1) , control circuits (13a, 13b) for controlling the conduction of the said bipolar transistor (12a, 12b), an over current sensing circuits (OCSC) (19a, 19b) and a fuse, a plurality of zener regulators (7,8,9) being provided across the output. 2. An intrinsically safe zener regulated power supply unit, as claimed in claim 1, wherein the bipolar transistors (12a & 12b) are such so as to dissipate sufficient heat without any damage to regulate output voltage when unregulated voltage at input terminals (3 & 4) varies due to fluctuations in supply voltage. 3. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-2, wherein the control circuit (13a) is such so as to control the operation of bipolar transistor (12a) in the event of damage of bipolar transistor (12b) to maintain safe operation of power supply unit. 4. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-3, wherein the combination of zener diode (Z2) and resistors (R30, R31 & R32) of the control circuit (13b) is such so as to set the output voltage to control the current flow in zener regulator (7,8,9). 5. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-4, wherein the said combination of zener diode Z2 and resistors (R30, R31 & R32) is such so as to turn bipolar transistor (12b) into off condition when zener regulators (7, 8 & 9) are open circuited. 6. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-5, wherein the rating of zener regulator (7, 8 & 9) are such so as to carry current without open circuiting, the current which would flow if both the bipolar transistors (12a & 12b) fail. 7. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-6, wherein the over current sensing circuit (19b) is such so as to monitor over current flowing through current limiting resistor Rl. 8. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-7, wherein the over current sensing circuit (19b) consists of optocoupler (26), resistors (R33, R34 & R35) and transistor T10. 9. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-8, wherein the resistor R19 is such so as to maintain low current flow, when bipolar transistor (12b) is turned off by control circuit (13b) due to open circuiting of zener regulator (7, 8 & 9), in the power supply circuit. 10. An intrinsically safe zener regulated power supply unit, as claimed in claim 1- 9, wherein the filter series resistor R2 & current limiting resistor Rl are such so as to limit the energy amount to zener regulator & also an external fault.

Full Text

The present invention relates to an intrinsically safe zener regulated power supply unit useful in hazardous locations. The present invention particularly relates to an intrinsically safe zener regulated power supply unit useful to provide significant current and stable output voltage to electronic instruments placed in hazardous locations, such as under ground mines.
The basic function of an intrinsically safe power unit is to provide power at its output incapable of igniting or causing ignition of a mixture of flammable or combustible material in air in the mixtures most easily ignitable concentration. Intrinsic safety is achieved by limiting the energy flow from the output of the power supply unit. While designing an intrinsically safe circuit it is important to make sure that it functions properly under normal operating conditions but still is safe under fault conditions. There are governmental testing or certifying organizations such as Eastern Regional Testing Laboratory (E.R.T.L), Central Mining Research Institute (CM.R.I), & Director General of Mines Safety (D.G.M.S) which provide prescribed test conditions for determining equipment is intrinsically safe. It is mandatory to get the instrument tested from such organization before its use in the hazardous location. To protect the intrinsically safe apparatus in a hazardous area, an energy limiting device must be installed. There are three components in intrinsically safe circuit to limit current and voltage; a resistor, at least two zener diodes, and a fuse. The resistor limits the current to a specific value known as short circuit current. The zener diode limits the voltage to a value referred to as an open circuit voltage. The fuse will prevent the diode from burning and allowing excess voltage to reach hazardous area. The problem in using a simple resistor barrier to limit current flow is that the voltage regulation at the output terminal of the power source is poor as the voltage drop across the barrier resistor increases with the increase in load current. Moreover, the output voltage will vary with the change in input voltage level connected to the input terminal of the intrinsically safe power supply source.
Extensive efforts have been made to the development of suitable intrinsically safe power unit to provide required voltage and current to instruments used in the hazardous locations. To highlight the continual efforts made by inventors to develop intrinsically safe power source, a few patent references are provided below for illustration.
Reference may be made to U.S. Patent No. 5,982,594, titled: 'Intrinsically safe power source'. The invention relates to an intrinsically safe power supply unit, provided for conditioning power supplied by a power source. An input power converter connected to the power source receives current from the power source. A direct output crowbar and discrete impedance elements dissipate and limit the energy in the power supply unit that would otherwise be delivered to an external fault. In addition, the direct output crowbar extracts energy from any external storage elements. An adaptive shut down circuit distinguishes nominal load conditions including load changes from an external fault. A multistage LC power filter is used to minimize the stored energy that would be deliverable to an external fault or dissipated by the direct output crowbar and discrete impedance elements. The combination of the direct output crowbar, discrete impedance elements, adaptive shut down detection circuitry and multistage LC power filter significantly improves the level and quality of intrinsically safe power delivered by the power supply unit.
Reference may be made to U.S. Patent No. 5,144,517. The invention relates to intrinsically safe barrier device. A protection barrier provides voltage and current necessarily conveyed to hazardous, such as explosive, areas. The system includes a current sensing circuit which triggers a silicon control rectifier based crowbar circuit in the event over current is detected. This triggering, in turn, trips a relay severing the current path entirely. A reverse polarity circuit provides for immediate clamping of improperly bias connections in a similar crowbar arrangement, and also provides for tripping of the circuit breaker. Thermistors are
also provided in current series in the event the circuit breaker fails to trip, or is forcibly bypassed. The circuit breaker additional provides a visual indicator when it is in the tripped position. The intrinsic barrier system is suitable to serving as a first stage protection to zener barrier modules as utilized in the prior art.
Reference may be made to U.S. Patent No.4,831,484, an electrical safety barrier for protection of electrical load elements placed in potentially hazardous locations. The barrier input and output each have two terminals. In at least one connection between the barrier input and the barrier output, there is an electronic in-line control element having a control input as well as, in series with the in-line control element, a current sensing element that emits an output voltage corresponding to the current. A control circuit having a control input controls the conductive state of the in-line control element and to this end receives the output voltage of the current sensing element. In accordance with the invention, to obtain a steeper limiting characteristic, the control voltage for the control circuit is the sum of at least part of the output voltage of the current sensing element and at least part of a voltage corresponding to the voltage drop in the in-line control element. The control element is formed by two serially connected transistors; the control circuit includes four active elements which are connected to individual summing circuits and provide, in pairs, output signals to control the serially connected transistors.
Reference may be made to U.S. Patent No. 4,438,473 titled: 'Power supply for an intrinsically safe circuit'. An intrinsically safe power supply employing a binary current interrupter connected between the power source and the electrical load. Normally the load is situated in a potentially dangerous environment, like a coal mine, and the intrinsically safe power supply at a safe remote location, e.g., on the earth's surface. The interrupter has a pass and switching transistor, current sensor, gating circuitry, a flip-flop switch, and means to delay the turning on of the transistor - but not its turning off. If an over current or over voltage condition is sensed between the input and output of the intrinsically safe power supply,
load current will cease to flow. In normal operation, only the current interrupter pass transistor will open. A reset signal from an oscillator internal of the interrupter may be used to reset the flip-flop after actuation or upon its initial setting, thereby causing the flow of power into the load.
U.S. Patent No. 4,412,265, illustrates an intrinsic barrier in which attempt has been made to provide controlled output voltage by using a combination of transistor and zener diode. Since the transistor is kept turned on in all conditions a fuse having fast destruction time compared to that of the transistor is provided to protect the transistor from over current heating in fault conditions. Zener diode used in the circuit is only for limiting the output voltage level when the transistor fails.
U.S Patent No.5,050,060 illustrates an intrinsically safe power supply unit that attempts to provide high output power and improved voltage regulation at the output terminal by using a post regulator after the barrier resistors. In this technique current limiting resistor passes the rated current in all conditions and dissipates proportionate wattage. Moreover, the post regulator passes all of the rated current to maintain output voltage regulation when load gets disconnected from the output terminal and there will be wastage of power. So this type of intrinsically safe power supply circuit can not be a right choice for battery powered applications. Zener diode used in the circuit is exclusively for control purposes and the actual voltage regulation is via the transistors.
Hence, it is clear from the hitherto known prior art that no attempt has yet been made to use zener diode in intrinsically safe power supply unit as voltage regulator to provide significant current and stable output voltage due to the drawbacks associated with zener regulated power supply circuit.
Strategy for designing a zener regulated power supply circuit using zener diode and series current limiting resistor is to design for a maximum power dissipated in the zener diode when the regulator is open circuited. The logic of this design strategy is to protect the zener diode. The value of series current limiting resistor limits the current so that the zener diode is not burned out if the load gets disconnected. So the serious drawback in situations where we require significant output voltage and current is that the series current limiting resistor always pass the total current and must dissipate proportionate wattage even when no output is required. In another situation when load is not connected, all of this current will pass through the zener and dissipate high power. So for high power applications, a suitable zener diode capable of dissipating high power has to be selected which is expensive. Moreover, the rating of zener diode in zener regulated intrinsically safe power supply circuit should be such that it will be capable of carrying 1.5 times the current circulated at normal condition in the particular conditions of mounting and in the ambient temperature specified (CI. No. 9.2.2 of IS: 5780-1980). So, intrinsically safe zener regulated power supply is normally restricted to low power applications only.
The main object of the present invention is to provide an intrinsically safe zener regulated power supply unit useful in hazardous locations.
Another object of the present invention is to provide an intrinsically safe zener regulated power supply unit useful to provide significant current and stable output voltage to electronic instruments placed in hazardous locations, such as under ground mines.
Still another object of the present invention is to provide an intrinsically safe zener regulated power supply unit, which enables to provide stable output voltage at varying load currents.
Yet another object of the present invention is to provide an intrinsically safe zener regulated power supply unit which enables reduction in power dissipation in the series current limiting resistor and zener regulator when load is disconnected from power supply unit.
Still yet another object of the present invention is to provide an intrinsically safe zener regulated power supply unit, which enables to limit the energy flow to the hazardous area when over current is sensed due to external fault.
A further object of the present invention is to provide an intrinsically safe zener regulated power supply unit which enables to maintain output voltage under regulation even when the input unregulated voltage changes due to the fluctuations in main power supply voltage.
Still further object of the present invention is to provide an intrinsically safe zener regulated power supply unit which also enables to limit excess current flow to zener regulator by melting fuse connected in series with Zener regulator thereby limits output voltage.
In the present invention there is provided an intrinsically safe zener regulated power supply unit useful to provide significant current and stable output voltage to electronic instrument placed in the hazardous location. Combination of bipolar transistor and resistor, connected in parallel with bipolar transistor, is connected in series with zener regulator to control the current flow in zener regulator. Control circuit controls the conduction of bipolar transistor to achieve controlled current flow in zener regulator at varied load conditions. When load is not connected to the power supply unit or the zener regulator is open circuited control circuit turns off bipolar transistor and the resistor connected in parallel with bipolar transistor keeps zener regulator under conduction. Since the value of the resistor connected in parallel with bipolar transistor is high, energy dissipation in zener regulator as well as in current limiting resistor is maintained at lower
value. Whenever load is connected to the power supply unit, control circuit turns on bipolar transistor and controls its conduction to provide the required current to the load and maintains zener regulator current more or less unchanged. Hence the power supply unit according to the present invention overcomes the drawbacks associated with zener regulated power supply circuit. The present invention is also provided with means to monitor the excess current flowing in the circuit during fault condition and acts if current flowing exceeds specific limit to signal control circuit to turn off the bipolar transistor thereby limits the energy which would otherwise be delivered to hazardous area. To improve safety two bipolar transistors are connected in series to ensure the safe operation of power supply unit when one transistor fails. In the event of failure of both the bipolar transistors the excess current, delivered to the zener regulator, is limited by fuse. The rating of bipolar transistor is such that it can dissipate sufficient heat to maintain output voltage regulation if the unregulated voltage at the input changes due to fluctuations in main power supply voltage.
The intrinsically safe zener regulated power supply unit of the present invention includes bipolar transistors, resistor connected in parallel with bipolar transistors and control circuit. Combination of bipolar transistors and resistor is connected in series with current limiting resistor and the control circuit provided controls the operation of bipolar transistor to control current flow to zener regulator and thereby to improve output voltage regulation. Bipolar transistor is turned off by control circuit, when output terminals get disconnected from load, and the zener regulator is kept in conduction by the resistor connected in parallel with bipolar transistors. Due to the higher value of the said resistor, minimum current will flow in the circuit. So the power consumption of series current limiting resistor and zener regulator are maintained at a lower value. Thus it becomes possible to avoid unwanted wastage of power which is common in the conventional zener regulated power supply circuit when no load is connected to the power supply unit. When load is connected to power supply unit control circuit turns on the bipolar transistor and controls its conduction to supply required current to the
load and thus helps to check the changes in zener regulator current at varied load condition. As a result stable output voltage is obtained at varied load condition which is not practically possible in conventional zener regulated power supply circuit. It also provides means to monitor over current condition in the circuit. Whenever current beyond normal operating current, due to external fault, tries to flow in the circuit the said means trigger control circuit to reduce bias current of bipolar transistor and thus controls the conduction of bipolar transistor. Finally, bipolar transistor when current drawn due to external fault crosses a specific limit is brought into its blocking state and the fault current is supplied through resistor connected in parallel with bipolar transistors. Thus the power supply unit of the present invention restricts current flow to external fault. In normal operating condition output voltage is regulated by combination of control circuit, bipolar transistor and zener regulator but in the event of failure of bipolar transistor output voltage is limited by zener regulator and fuse.
In figures 1 and 2 of the drawings accompanying this specification are detailed the various parts constituting the intrinsically safe zener regulated power supply unit of the present invention. In figure 1 of the drawings is shown a block diagram of the various parts constituting the intrinsically safe zener regulated power supply unit of the present invention is depicted. Figure 2 represents a circuit diagram of a preferred embodiment of the present invention.
The intrinsically safe zener regulated power supply unit, useful to provide significant current and stable output voltage to electronic instrument placed in hazardous locations, of the present invention comprises bipolar transistors (12a, 12b), a resistor (11) connected in parallel with bipolar transistor (12a, 12b), current limiting resistor (10), filter capacitor (2) with a series resistor (1), control circuit (13a, 13b) to control the conduction of bipolar transistor (12a, 12b), fuse and zener regulator (7,8,9).
A combination of zener diode (Z2) and resistors (R30, R31 & R32) of the control circuit (13b) is such as capable of setting the output voltage to control the current flow in zener regulator (7,8,9). Voltage drop across resistor (R32), when output voltage tends to increase due to decrease in load current, of the said combination also tends to increase to control the conduction of bipolar transistor (12b) through transistors (T6, T7 & T8) to control the zener regulator current to keep output voltage stable.
A combination of zener diode Z2 and resistors (R30, R31 & R32) is such as capable of turning bipolar transistor (12b) into off condition when zener regulators (7, 8 & 9) are open circuited. Voltage drop across R32, when zener regulators (7, 8 & 9) are open circuited, is raised to provide forward bias to the base of transistor T6. Conduction of transistor T6 makes transistor T7 on and thereby provides reverse bias to the base of transistor T8 and helps to turn off bipolar transistor (12b).
The over current sensing circuit (19b) consisting of optocoupler (26), resistors (R33, R34 & R35) and transistor T10 is such as capable of monitoring over current flowing through current limiting resistor R1. Voltage drop across current limiting resistor R1, when current flowing through it tends to cross the normal operating current due to external fault, becomes significant to activate the optocoupler (26) to make the transistor T10 on. Conduction of transistor T10 helps to provide back bias to the base of T9 and thereby to turn off the bipolar transistor (12b).
The resistor R19 is such as capable of maintaining low current flow, when bipolar transistor (12b) is turned off by control circuit (13b) due to open circuiting of zener regulator (7, 8 & 9), in the power supply circuit. Resistor R19 thereby helps to reduce power consumption of series current limiting resistor and zener regulator when output terminal is open circuited. The said resistor R19 also helps to limit the short circuit current in case of external fault.
The filter series resistor R2 and current limiting resistor R1 are such as capable of limiting the amount of energy to an external fault and zener regulator as well.
The control circuit (13a) is such as capable of controlling the operation of bipolar transistor (12a) in the event of damage of bipolar transistor (12b) to maintain safe operation of power supply unit.
The bipolar transistors (12a & 12b) are such as capable of dissipating sufficient heat without any damage to regulate output voltage when unregulated voltage at input terminals (3 & 4) varies due to fluctuations in supply voltage.
The rating of zener regulator (7, 8 & 9) are such as capable of carrying, without open circuiting, the current which would flow if both the bipolar transistors (12a & 12b) fail. Voltage at the output terminal is limited, when current flow exceeds specific limit, by zener regulator by allowing fuse to melt and thus fuse protects zener regulator against wide variations in unregulated input voltage.
Accordingly the present invention provides an intrinsically safe zener regulated power supply unit useful in hazardous locations, which comprises: a pair of bipolar transistors (12a, 12b) connected in parallel to a resistor (11), the said bipolar transistor pair (12a, 12b) being provided with a series current limiting resistor (SCLR) (10), a filter capacitor (2) with a series resistor (1), control circuits (13a, 13b) capable of controlling the conduction of bipolar transistor (12a, 12b), a plurality of zener regulators (7,8,9), over current sensing circuits (OCSC) (19a,19b)andafuse.
In an embodiment of the present invention, the bipolar transistors (12a & 12b) are such as capable of dissipating sufficient heat without any damage to regulate output voltage when unregulated voltage at input terminals (3 & 4) varies due to fluctuations in supply voltage.
In another embodiment of the present invention, the control circuit (13a) is such as capable of controlling the operation of bipolar transistor (12a) in the event of damage of bipolar transistor (12b) to maintain safe operation of power supply unit.
In yet another embodiment of the present invention, the combination of zener diode (Z2) and resistors (R30, R31 & R32) of the control circuit (13b) is such as capable of setting the output voltage to control the current flow in zener regulator (7,8,9).
In still another embodiment of the present invention, the said combination of zener diode Z2 and resistors (R30, R31 & R32) is such as capable of turning bipolar transistor (12b) into.off condition when zener regulators (7, 8 & 9) are open circuited.
In still yet another embodiment of the present invention the rating of zener regulator (7, 8 & 9) are such as capable of carrying, without open circuiting, the current which would flow if both the bipolar transistors (12a & 12b) fail.
In a further embodiment of the present invention, the over current sensing circuit (19b) is such as capable of monitoring over current flowing through current limiting resistor R1.
In a yet further embodiment of the present invention, the over current sensing circuit (19b) consists of optocoupler (26), resistors (R33, R34 & R35) and transistor T10.
In a still further embodiment of the present invention, the resistor R19 is such as capable of maintaining low current flow, when bipolar transistor (12b) is turned off by control circuit (13b) due to open circuiting of zener regulator (7, 8 & 9), in the power supply circuit.
In another embodiment of the present invention, the filter series resistor R2 and current limiting resistor R1 are such as capable of limiting the amount of energy to an external fault and zener regulator as well.
The embodiments, features and advantages of the present invention will be set forth in part in the description, and in part will come to be understood by those skilled in the art by the reference to the following description of the invention and referenced drawings.
In figure 1 and 2 of the drawings accompanying this specification are detailed the various parts constituting the intrinsically safe zener regulated power supply unit of the present invention. The detailed description with reference to the drawing is given below.
Figure 1, which is a simplified block diagram of the embodiment of the present invention, shows an intrinsically safe zener regulated power supply unit. The said power supply unit meets the requirements to limit the voltage and current on its intrinsically safe output side to a value, which is incapable of causing explosion. Bipolar transistors (12a & 12b), resistor (11) connected in parallel with bipolar transistor (12a & 12b) and control circuit (13a & 13b) play an important role in the operation of the power supply circuit of the present invention. Firstly, bipolar transistor (12b) act as a switch and remain in off condition when load gets disconnected from output terminal or output terminal is open circuited. In this situation resistor (11), connected in parallel with bipolar transistors (12a & 12b), acts to keep the power supply in operation by allowing minimum amount of current to flow in the circuit. Thus less energy is dissipated in the series current limiting resistor (10) and zener regulators (7, 8 & 9). Secondly, whenever load is connected to power supply unit, control circuit (13b) turn on bipolar transistor (12b) and controls its conduction to provide required amount of current at varied load conditions without significant variation in zener regulator (7, 8 & 9) current. If
bipolar transistor (12b) is damaged, control circuit (13a) will act to control the operation of bipolar transistor (12a) for safe operation of power supply unit.
Series current limiting resistor (10) primarily serves to limit maximum current escapes from power supply unit during external fault. But over current sensing circuit (19a & 19b) monitors voltage drop across current limiting resistor (10) and are activated, whenever current flowing through current limiting resistor (10) tends to cross the normal operating current due to external fault, to signal control circuit (13a & 13b) to turn off bipolar transistors (12a & 12b). Thus the energy delivered to the external fault is significantly reduced. Zener (7, 8 & 9), in the power supply circuit of the present invention, serves as regulator for regulating output voltage in normal operating condition and in the event of failure of bipolar transistors (12a & 12b) output voltage is limited by the said zener regulators (7, 8 & 9) and fuse. Thus the power supply unit of the present invention fulfils the requirements of intrinsic safety by limiting current and voltage on their intrinsically safe output side to avoid danger of explosion during fault condition. The said power supply unit along with step down transformer and rectifier circuit, not shown in the fig. (1), are enclosed in a flame proof housing so that it can be used in hazardous locations to provide intrinsically safe power to electronic instruments.
Figure 2 represents circuit diagram of a presently preferred embodiment of the bipolar transistors (12a & 12b), resistor R19 connected in parallel with bipolar transistors (12a & 12b), control circuit (13a & 13b), over current sensing circuit (19a & 19b), series current limiting resistor JR1, filter capacitor C and zener regulators (7, 8 & 9) of the power supply unit of the present invention. Control circuit (13b) contains a combination of zener diode Z2 and resistors (R30, R31 & R32). The said combination is used, when no load is connected to the output terminals, to set the output voltage such that very small current passes through zener regulators (7, 8 & 9). In this condition voltage drop across resistor R32 is sufficient enough to turn on transistor T6 and thus to provide a forward bias to
the base of transistor T7. Conduction of transistor T7 keeps the voltage at the junction (27) well below the voltage required to forward bias transistor T8 and thus helps to turn off bipolar transistor (12b). When load is connected to the power supply unit the voltage drop across resistor R32 is reduced and reverse bias the transistor T6, which in turn helps to generate reverse bias to the base of transistor T7 to turn it into off condition. Now voltage at the junction (27) is raised to a value to turn on transistor T8 and finally the bipolar transistor (12b) is turned on to supply current to load. Control circuit (13b) controls the conduction of bipolar transistor (12b) in such a way that it supplies current drawn only by the load and thus helps to maintain the zener regulator (7, 8 & 9) current more or less unchanged at varied load conditions. Thus voltage at output terminals (5 & 6) is not influenced by dynamic impedance effects and device temperature coefficient characteristics which are inherent with zener. So it becomes possible to achieve a very stable output voltage at varied load conditions. In the event of damage of bipolar transistor (12b) control circuit (13a) will act to control the operation of bipolar transistor (12a) for safe operation of power supply unit.
Over current sensing circuit (19b) comprising optocoupler (26), resistor (R33, R34 & R35) and transistor T10 monitors the voltage drop across current limiting resistor R1. Voltage drop across current limiting resistor R1, when load attempts to draw excess current from power supply unit at fault conditions is raised to activate optocoupler (26) to provide forward bias to transistor T10. Which in turn helps to reduce bias current of bipolar transistor (12b) and the voltage at output terminals (5, 6) is reduced. Finally, bipolar transistor (12b) is completely turned off when current drawn by external fault crosses a specific value which reduces output voltage further to make it incapable of providing any bias current to bipolar transistor (12b). Over current sensing circuit (19a) consisting of optocoupler (25), resistors (R16, R17 & R18) and transistor T5 also acts simultaneously as above to reduce bias current to bipolar transistor (12a). Once the bipolar transistors (12a & 12b) are turned off resistor R19 connected between emitter and collector
of bipolar transistors (12a & 12b) acts to supply the fault current. Since the value of resistor R19 is high fault current is significantly reduced and voltage across output terminals (5 & 6) becomes very low. So until the fault is cleared the voltage at the output terminal will not rise and the bipolar transistors (12a & 12b) will remain in off condition. In the event of damage of bipolar transistors (12a & 12b) short circuit current is limited by fuse.
Control circuit (13b) along with resistor R19 and over current circuitry (19b) control the operation of bipolar transistor (12b) as above, when bipolar transistor (12a) fails, to maintain safe operation of power supply unit.
In the event of failure of bipolar transistors (12a & 12b) zener regulator (7, 8 & 9) becomes more conductive and more current passes through fuse () which is finally burned to de-energize the power supply unit. Thus the output voltage is limited to zener regulator (7, 8 & 9) voltage. Capacitor C filters unwanted ripple and resistor R2 limits energy delivered by capacitor C to safe output terminals.
In the present invention the intrinsically safe zener regulated power supply unit includes bipolar transistors and a resistor connected in parallel with bipolar transistors and control circuit. The combination of zener diode and resistors in the said control circuit helps to set the output voltage to control the current flow in zener regulator. Whenever the output voltage tends to increase due to changes in load current, voltage drop across resistors of the said combination also changes accordingly and helps to control the conduction of bipolar transistor to stabilize the output voltage at the preset value. Similarly, when zener regulator is open circuited tendency to increase the output voltage will immediately increase voltage drop across the said resistors and helps to turn off the bipolar transistor. Since the bipolar transistor is turned off resistor connected in parallel with bipolar transistors acts to limit current flow in the circuit and thus reduces the power consumption of series current limiting resistor and zener regulators.
In the present invention intrinsically safe zener regulated power supply unit includes over current sensing circuit to monitor current flowing in the circuit. The over current sensing circuit comprises phototransistor optocoupler, resistors and transistor. Voltage drop across series current limiting resistor, when current flowing through it tends to exceed the normal operating current due to external fault, will be significant to forward bias the emitter of the optocoupler and enable the detector of the said optocoupler to turn on transistor. Which in turn enables to turn off the bipolar transistor to limit energy flow to external fault.
In the present invention the intrinsically safe zener regulated power supply unit includes filter capacitor and filter series current limiting resistor to provide smooth output voltage. Filter series current limiting resistor helps to restrict the energy delivered from filter capacitor to output terminals to a safe value.
In the present invention the intrinsically safe zener regulated power supply unit includes bipolar transistors which are capable of dissipating sufficient heat such as the output voltage remains within regulation during changes in unregulated input voltage due to fluctuations in main supply voltage.
In the present invention the intrinsically safe zener regulated power supply unit includes zener regulator to regulate output voltage. Since the zener regulator, in normal operating condition, is kept in conduction mode with low zener current it helps to suppress the unwanted ripple generated due to the switching of bipolar transistor. Further, rating of zener regulator is such as capable of carrying, without open circuiting, the current which would flow in the case of damage of bipolar transistors and open circuiting of output terminals. In this circumstance if the unregulated voltage at the input terminals of the power supply unit varies due to the fluctuations in mains supply voltage, fuse limits excess energy flow to zener regulator and protects it against wide variations in input voltage.
The novelty of the intrinsically safe zener regulated power supply unit, of the present invention, resides in the capability in controlling the zener regulator current at varied load conditions to ensure stable output voltage, thus making it capable of providing significant current and stable output voltage to electronic instruments placed in hazardous locations, such as underground mines. In the power supply unit of the present invention zener regulator current, when output terminals get disconnected from load or zener regulator is open circuited, is maintained at a minimum value to minimize the power consumption of series current limiting resistor and zener regulator. Further, it provides means to limit fault current, when over current is sensed due to external fault, to ensure that less energy is delivered to external fault. Zener regulator further acts to limit output voltage, when zener regulator is open circuited and unregulated voltage at the input terminals of the power supply unit varies widely due to fluctuations in mains supply voltage, by allowing fuse to melt and thus power supply unit is de-energized in fault condition.
The novelty of intrinsically safe zener regulated power supply unit has been realized by the non-obvious inventive steps such as incorporation of bipolar transistors and resistor, connected in parallel with bipolar transistors, and control circuit in the present power supply circuit. Control circuit controls the operation of bipolar transistor which is connected in series with series current limiting resistor to maintain zener regulator current at lower value without significant variation as set by the combination of zener diode and resistors in the said control circuit during varied load condition. When the voltage across the output terminals tends to increase due to the decrease in load current, voltage drop across resistors of the said combination of control circuit is increased and it helps to control the conduction of bipolar transistor to stabilize the output voltage at the preset value. Thus the changes in zener regulator current is controlled even when load current varies due to changes in load resistance and it becomes possible keep the output voltage stable. Similarly when output terminals get disconnected from load the said control circuit turn off the bipolar transistor and then the resistor
connected in parallel with bipolar transistor acts to maintain current in the circuit at lower value. So, the power consumption of series current limiting resistor and zener regulators are significantly controlled when output terminal is open circuited. In the hitherto known prior art, where zener is placed in parallel across a load to be regulated, the current through the zener is increased due to the decrease in load current and it reaches the maximum value when the zener regulator is open circuited. The changes in zener current influence the zener voltage due to dynamic impedance effects. Further, the zener voltage is also influenced by device temperature coefficient characteristic inherent with zener, particularly when applied power is significant relative to full rating or heat sinking is marginal. Moreover, heat is unnecessarily dissipated, which is nothing but wastage of power, in the series current limiting resistor and zener regulator when output terminals get disconnected from load. In present invention the over current sensing circuit monitors the current flowing through series current limiting resistor. In case of external fault, when excess current beyond the normal operating current is attempted to be drawn from the power supply unit, over current sensing circuit acts to reduce bias current and finally to turn off bipolar transistor. Thus the energy delivered to the external fault is significantly reduced by over current sensing circuit. In the event of failure of bipolar transistors over current is limited by fuse. Similarly, zener regulator is capable of dissipating sufficient heat, without open circuiting, if bipolar transistors fail and output terminal gets disconnected from load. Further increase in zener regulator current due to variations in unregulated voltage at the input terminals of power supply unit allows fuse to melt and thus limits the output voltage without any damage to zener regulator.
The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention.
The results of the experiments such as variation in voltage across intrinsically safe output terminals at varied load conditions and current limiting characteristic of the power supply unit according to present invention are detailed below.
Example 1
Intrinsically safe power supply unit in accordance with the present specification was made to provide 12V DC output voltage. 13V DC was applied as input voltage to the power supply circuit and the value of the series current limiting resistor used was 3.9 ohms, 5W. Rating of zener used to regulate output voltage was 12V, 5W with tolerance of 5%. In conventional zener regulated power supply circuit zener current changes for regulation of output voltage against both variations in the input voltage from a unregulated power supply or variations in the load resistance. So in zener regulated power supply circuit using only series current limiting resistor and zener, output voltage should vary between 11.4V and 12.6V as per specifications of the zener mentioned above depending upon zener current and ambient temperature. Zener regulated power supply unit according to the present invention incorporates unique combination of bipolar transistors, resistor connected in parallel with bipolar transistors and control circuit. Control circuit controls changes in zener current against the variations in the load resistance by controlling the conduction of bipolar transistor.
Figure 3, of the drawings accompanying this specification is a graph of the output voltage characteristic versus load current of the power supply unit configured as above. Combination of zener diode and resistors of control circuit of the said power supply unit was selected with values such that the output voltage was set at 12.00V when no load was connected to the output terminals. From the graph it is found that output voltage is maintained at 12.00V for load current up to 150mA. With further increase in load current voltage at output terminal starts decreasing and when load current reaches around 200mA over current sensing circuit starts functioning. Since the over current sensing circuit is activated bias current of
bipolar transistor starts decreasing due to the decrease in potential at the junction 29 of control circuit (13b). As a result output voltage is decreased. Decrease in output voltage further lowers the potential at junction 29 and the conduction of bipolar transistor is further decreased. When load current reaches around 226mA, output voltage starts decreasing more rapidly due to the fast decrease in bias current of the bipolar transistor. When the bipolar transistor is brought into its blocking state output voltage is decreased to a very low value and load current is supplied through resistor R19. In this situation the magnitude of load current depends on the value of resistor R19 used. Thus by selecting a suitable value of resistor R19 it is possible to maintain the fault current at lower value.
Example 2
In another situation with input voltage maintained at 13V, maximum current in the above power supply circuit when load is not connected to the output terminals and the bipolar transistor is short circuited will be around 256mA. This current is less than the rated maximum current (395mA) of zener regulator according to data sheet. Single zener used as regulator, is thus capable of carrying, without open circuiting, the current which would flow when bipolar transistor fails and output is open circuited. So two zeners, when connected at output terminals as per the requirements of intrinsic safety, will be able to carry sufficient current, without open circuiting, to melt the fuse provided in the circuit to limit current to protect zener regulator. Addition of more zeners at the output terminal will further improve safety and reliability of the power supply circuit. Zeners used as regulator thus help to limit output voltage too to restrict energy flow to external fault. In the event of failure of bipolar transistor fuse also acts to limit fault current.
As those skilled in the art will realize that value of the components of the present zener regulated power supply unit may be changed to make it capable of
providing any other required voltage and current. But for its use in hazardous locations the magnitude.of voltage and current has to be limited to a specific value which will pass the prescribed test conditions of intrinsic safety accepted by national or other testing authority. The present invention is an ideal one to design power supply unit for required voltage and current for battery powered application. Present invention may also be used to provide improved voltage regulation and significant current to electronic instrument used in non-hazardous area.
The main advantages of the intrinsically safe zener regulated power supply unit of the present invention are:

1. Permits to provide significant current to electronic instrument placed in hazardous locations.
2. Makes it possible to keep the output voltage steady at varied load conditions.
3. Enables to reduce power consumption of series current limiting resistor and zener regulator when output terminals get disconnected from load.
4. Makes it a suitable one for battery powered applications.
5. In case of short circuit at the output terminals minimum current flows into the fault thereby ensures safety and protects the components of power supply unit from damage.

We claim:
1. An intrinsically safe zener regulated power supply unit useful in hazardous locations, characterised by : a pair of bipolar transistors (12a, 12b) connected in parallel to a resistor (11), the said bipolar transistor pair (12a, 12b) being provided with a series current limiting resistor (SCLR) (10), a filter capacitor (2) with a series resistor (1) , control circuits (13a, 13b) for controlling the conduction of the said bipolar transistor (12a, 12b), an over current sensing circuits (OCSC) (19a, 19b) and a fuse, a plurality of zener regulators (7,8,9) being provided across the output.
2. An intrinsically safe zener regulated power supply unit, as claimed in claim 1, wherein the bipolar transistors (12a & 12b) are such so as to dissipate sufficient heat without any damage to regulate output voltage when unregulated voltage at input terminals (3 & 4) varies due to fluctuations in supply voltage.
3. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-2, wherein the control circuit (13a) is such so as to control the operation of bipolar transistor (12a) in the event of damage of bipolar transistor (12b) to maintain safe operation of power supply unit.
4. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-3, wherein the combination of zener diode (Z2) and resistors (R30, R31 & R32) of the control circuit (13b) is such so as to set the output voltage to control the current flow in zener regulator (7,8,9).
5. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-4, wherein the said combination of zener diode Z2 and resistors (R30, R31 & R32) is such so as to turn bipolar transistor (12b) into off condition when zener regulators (7, 8 & 9) are open circuited.
6. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-5, wherein the rating of zener regulator (7, 8 & 9) are such so as to carry current without open circuiting, the current which would flow if both the bipolar transistors (12a & 12b) fail.
7. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-6, wherein the over current sensing circuit (19b) is such so as to monitor over current flowing through current limiting resistor Rl.
8. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-7, wherein the over current sensing circuit (19b) consists of optocoupler (26), resistors (R33, R34 & R35) and transistor T10.
9. An intrinsically safe zener regulated power supply unit, as claimed in claim 1-8, wherein the resistor R19 is such so as to maintain low current flow, when bipolar transistor (12b) is turned off by control circuit (13b) due to open circuiting of zener regulator (7, 8 & 9), in the power supply circuit.
10. An intrinsically safe zener regulated power supply unit, as claimed in claim 1- 9, wherein the filter series resistor R2 & current limiting resistor Rl are such so as to limit the energy amount to zener regulator & also an external fault.

Documents:

1759-DEL-2004-Abstract-(16-11-2011).pdf

1759-del-2004-abstract.pdf

1759-DEL-2004-Claims-(16-11-2011).pdf

1759-del-2004-claims.pdf

1759-DEL-2004-Correspondence Others-(16-11-2011).pdf

1759-del-2004-correspondence-others.pdf

1759-del-2004-description (complete).pdf

1759-del-2004-drawings.pdf

1759-DEL-2004-Form-1-(16-11-2011).pdf

1759-del-2004-form-1.pdf

1759-del-2004-form-18.pdf

1759-del-2004-form-2.pdf

1759-DEL-2004-Form-3-(16-11-2011).pdf

1759-del-2004-form-3.pdf

1759-del-2004-form-5.pdf

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Patent Number

259407

Indian Patent Application Number

1759/DEL/2004

PG Journal Number

11/2014

Publication Date

14-Mar-2014

Grant Date

12-Mar-2014

Date of Filing

17-Sep-2004

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	RANJIT MAITY	CENTRAL MINING RESEARCH INSTITUTE, BARWA ROAD, DHANBAD-82601, JHARKHAND, INDIA.

PCT International Classification Number

G05F 1/20

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA