Abstract:
Power supply arrangement comprising a direct voltage monitoring circuit. A power supply arrangement comprising a direct voltage monitoring circuit for at least two different-polarity direct voltages connected to a common reference potential, and including a signal transmission element. The monitoring circuit arrangement has an enhanced accuracy of response with limited circuitry and cost. A series connection of a zener diode and a resistor is connected in shunt with each direct voltage. The junction point of each zener diode and resistor is connected to the base of a transistor. A first electrode of each transistor is connected to the signal transmission element and a second electrode.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a power supply arrangement comprising a monitoring circuit for no less than two different-polarity direct voltages connected to a common reference potential, and including a signal transmission element. 
     Power supply arrangements comprising monitoring circuits are necessary, for example, for protecting the connected users against overvoltages in the case of failure or, in the case of undervoltages avoiding the transfer of faulty data are transferred without such transfer being timely recognised. Consequently, the failure of a direct output voltage, i.e. the deviation from a predetermined set value, is to with certainty be indicated by means of a signal transmission element so that safety measures can be taken. If a failure occurs in a secondary circuit, especially in a clocked power supply arrangement, the direct output voltages are accordingly adjusted or switched off by means of adjusting elements provided in the primary circuit. If electric isolation of the secondary circuit and the control circuit operating, for example, in the primary circuit of a transformer is necessary, a signal transmission element with electrically isolated inputs and outputs, for example, an optocoupler, is used for transmitting the control signals of the monitoring circuit. 
     In DE-A 37 07 973 there is disclosed a power supply arrangement comprising an overvoltage protection circuit and a signal transmission element. In this arrangement a series connection of a zener diode and a diode is inserted between a reference potential and each direct output voltage which is positive or negative with respect to this reference potential. A signal transmission element, arranged as a light-emitting diode optocoupler is inserted between the zener diode and the diode of either one of the two series connections. If no overvoltages occur, the zener diode will be blocked and the light-emitting diode will not emit. If an overvoltage occurs in a direct output voltage, the zener diode concerned will conduct and power will flow through the light-emitting diode of the signal transmission element. The accuracy of response of the signal transmission element and thus of the overvoltage protective circuit is then determined by the tolerances of a plurality of different modules, i.e. more specifically, by the tolerances of the diode, the zener diode, the signal transmission element and a dropping resistor connected in series thereto and thus has insufficient accuracy of response when more stringent requirements are involved. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a power supply arrangement comprising a monitoring circuit of the type mentioned in the opening paragraph, in which the monitoring circuit has enhanced accuracy of response whereas, on the other hand, the circuitry remains simple. 
     This object is achieved by means of a power supply arrangement comprising a monitoring circuit of the type mentioned in the opening paragraph, in that a series connection of a zener diode and a resistor is shunted to each direct voltage, with the junction point of each zener diode and resistor being connected to the base of a transistor. A first electrode of each transistor is connected to the signal transmission element and a second electrode to the respective positive or negative pole of the direct voltages. 
     Other advantageous embodiments are contained in the dependent-claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the following the invention will be further explained with reference to the exemplary embodiments shown in the accompanying drawings in which: 
     FIG. 1 shows a power supply arrangement comprising an overvoltage monitoring circuit for two direct voltages of different polarities; 
     FIG. 2 shows a power supply arrangement comprising an overvoltage monitoring circuit for two positive and two negative direct voltages; and 
     FIG. 3 shows a power supply arrangement comprising an undervoltage monitoring circuit for two direct voltages of different polarities. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the exemplary embodiment shown in FIG. 1, a power supply arrangement 1 generates a direct voltage U1+ which is positive with respect to a reference potential OV and a direct voltage U1- which is negative with respect to this reference potential. The positive pole of the direct voltage U1+ is connected to the emitter and through a resistor R1 to the base of a PNP transistor T1. The base of the transistor T1 is connected to the cathode of a zener diode Z1, whose anode is connected to the reference potential OV. The anode of a diode D1 is connected to the reference potential OV and its cathode is connected to the collector of transistor T1. The negative pole of the direct voltage U1- is connected to the emitter and through a resistor R2 to the base of an NPN transistor T2. The base of the transistor T2 is connected to the anode of a zener diode Z2 whose cathode is connected to the reference potential OV. The cathode of a diode D2 is connected to the reference potential OV and its anode is connected to the collector of the transistor T2. Between the collector of the transistor T1 and the collector of the transistor T2 is a signal transmission element consisting of as an optocoupler 2 in series with a dropping resistor R3. A light-emitting diode 21 is arranged such that its cathode is connected to the collector of the transistor T2. The optocoupler 2 is connected to a control arrangement of the power supply arrangement 1 so that the direct voltages U1+, U1- of the power supply arrangement 1 are adjusted back or switched off completely by means of a photocurrent produced in a photodiode 22 when the light-emitting diode 21 is emitting. 
     If the power supply arrangement has a normal operation, i.e. if with a positive direct voltage U1+ no overvoltage occurs, a current flow will be created through resistor R1 across the reverse biased zener diode Z1. The resistor R1, shunted to the base-emitter path of the transistor T1, is dimensioned such that in an undisturbed condition a voltage drop is produced which is smaller than the base-emitter voltage necessary for conduction of the transistor. The transistor T1 is blocked in this fashion. If an overvoltage occurs in the positive direct voltage U1+, the transistor T1 will become conductive via resistor R1 after the base-emitter voltage of approximately 0.6 Volt necessary for conduction is exceeded. The collector current of the transistor T1 now flows through the dropping resistor R3, the light-emitting diode 21 of the optocoupler 2 and the diode D2 to the reference potential OV. The dropping resistor R3 is dimensioned such that the current flowing through the light-emitting diode 21 is sufficient for light emission and the photocurrent produced by the photodiode 22 adjusts the direct voltage U1+ back so that the current is not too high or, alternatively, switches it off. In an embodiment not shown in the Figure a resistor and/or a capacitor is shunted to the signal transmission element 2 to receive a residual current. In a power supply arrangement controlled by means of the signal pulse width, the reduction of the direct voltage U1+ may be realised, for example, by reducing the pulse width of the control signals. 
     Alternatively, if the overvoltage occurs in the negative direct voltage U1-, the current will flow from the reference potential through the diode D1, the dropping resistor R3, the light-emitting diode 21 of the optocoupler 2 and through the conducting transistor T2 to the direct voltage U1-. In a practical embodiment of the d.c. overvoltage monitoring circuit the value of the direct voltages U1+, U1- is +/-5 volts. The zener voltage of the zener diodes Z1, Z2 is thus dimensioned at 5.1 volts and the resistance of the resistors R1, R2 at 1 kOhm. The accuracy with which the overvoltage monitoring circuit responds is determined only by the tolerances of the zener diodes Z1, Z2 and the base-emitter paths of the transistors T1, T2 and is thus independent of the collector circuit of the transistors T1, T2 and thus is independent of the tolerances of the dropping resistor R3 as well as that of the signal transmission element 2. Consequently, the overvoltage monitoring circuit is preeminently suitable even with requirements as to enhanced accuracy of response. 
     In the exemplary embodiment shown in FIG. 2, two more direct voltages U2+, U2- are monitored in addition to the direct voltages U1+, U1- of the exemplary embodiment shown in FIG. 1. For this purpose, the positive pole of the direct voltage U2+ is connected to the emitter and through a resistor R11 to the base of a PNP transistor T11. The base of the transistor T11 is connected to the cathode of a zener diode Z11 whose anode is connected to the reference potential OV. Accordingly, the negative pole of the direct voltage U2- is connected to the emitter and through a resistor R12 to the base of an NPN transistor T12. The base of the transistor T12 is connected to the anode of a zener diode Z12 whose cathode is connected to the reference potential OV. The zener voltages of the zener diodes Z11, Z12 are selected in accordance with the voltage values of the direct voltages U2+, U2-. With the aid of the overvoltage monitoring circuit represented in FIG. 2 each of two direct voltages of different polarities may be monitored and with little additional cost of circuitry. 
     If in addition to the direct voltages U1+, U1-, U2+, U2- further direct voltages of different polarities are to be monitored, the circuit shown in FIG. 2 merely needs to be extended by an arrangement of a transistor, a resistor and a zener diode, while the signal transmission element 2 is used for all direct voltages. In an embodiment the signal transmission element 2 is arranged as an optocoupler having a thyristor output. 
     In the exemplary embodiment shown in FIG. 3, a power supply arrangement provides the direct voltage U1+ which is positive with respect to the reference potential OV and the direct voltage U1- which is negative with respect to this reference potential. The positive pole of the direct voltage U1+ is connected to the emitter and through the resistor R1 to the base of the transistor T1. The base of the transistor T1 is connected to the cathode of the zener diode Z1 whose anode is connected to the reference potential OV through a resistor R4. The negative pole of the direct voltage U1- is connected to the emitter and through the resistor R2 to the base of the transistor T2. The base of the transistor T2 is connected to the anode of the zener diode Z2 whose cathode is connected to the reference potential OV through a resistor R5. Between the collector of the transistor T1 and the collector of the transistor T2 the signal transmission element 2 is inserted via a dropping resistor R3. A light-emitting diode 21 is arranged such that its cathode is connected to the collector of the transistor T2. The optocoupler 2 is connected to a control circuit of the power supply arrangement 1 so that, when the light-emitting diode 21 does not emit, the direct voltages U1+, U1- of the power supply arrangement 1 are adjusted accordingly or switched off as a result of the photocurrent that is no longer generated in the photodiode 22. 
     The resistors R1, R2, R4, R5 and the zener diodes Z1, Z2 are dimensioned such that in normal operation, i.e. when no undervoltage occurs, the transistors T1, T2 are conductive. For this purpose the voltage drop at the resistors R1, R2 is to be larger than the base-emitter voltage of approximately 0.6 Volt necessary for conduction of the transistors T1, T2. The collector current of transistor T1 then flows through the dropping resistor R3 across the light-emitting diode 21 of the optocoupler 2 and the likewise conductive collector-emitter path of the transistor T2 to the negative pole of the direct voltage U1-. The dropping resistor R3 is dimensioned such that the current flowing through the light-emitting diode 21 is sufficient for light emission but, on the other hand, not too much power is dissipated. If an undervoltage occurs in the positive direct voltage U1+, the transistor T1 will be blocked at resistor R1 since the base-emitter voltage value necessary for its conduction is now too low. The total power will then flow to the reference potential OV through resistor R1 and the reverse biased zener diode Z1 and the resistor R4. After the transistor T1 is blocked, current will no longer flow through the light-emitting diode 21 so that there will be no photocurrent passing through the photodiode 22 either and the direct voltage U1+ may, for example, be increased or switched off. In a power supply arrangement controlled by means of signal pulse width, the increase of the direct voltage U1+ may be realised, for example, by increasing the pulse width of the control signals. If, alternatively, the undervoltage occurs at the output having the negative direct voltage U1-, the transistor T2 will be blocked so that in this case too current will no longer flow through the light-emitting diode 21. The total current will then flow from the reference potential OV via the resistor R5 through the reverse biased zener diode Z2 and the resistor R2 to the negative pole of the direct voltage U1-. 
     The accuracy with which the undervoltage monitoring circuit responds is determined merely by the tolerances of the zener diodes Z1, Z2, resistors R4, R5 and the base-emitter paths of the transistors T1, T2 in the exemplary embodiment shown in FIG. 3, and is therefore independent of the collector circuit of the transistors T1, T2 and thus is independent of the tolerances of the dropping resistor R3 as well as the signal transmission element 2. Therefore, the undervoltage monitoring circuit shown in FIG. 3 is preeminently suitable in the case of requirements as to enhanced accuracy of response.