CONSTANT CURRENT POWER SUPPLY SYSTEM WITH REDUNDANCY FOR RESISTANCE TEMPERATURE DETECTOR

Abstract: A constant current redundant power supply/signal con­ ditioner system for a resistance temperature detector in­ cludes power supply/signal conditioner modules having diode circuits connected across the output terminals of each of the power supply/signal conditioner modules to provide an alternative current path when the corresponding power supply/signal conditioner module is removed or is not conductive due to a failure. Each of the modules also includes an overvoltage protection circuit for preventing the voltage output by the constant current source from exceed­ ing a predetermined value and a signal conditioner which outputs a signal based on the temperature detected by the resistance temperature detector.

Description: BACKGROUND OF THE INVENTION Field of the Invention : The present invention generally relates to power supply and signal conditioning modules used in connection with a resistance temperature detector (RTD); and more particularly, to a system having redundant power supply and signal conditioning modules used in combination with a single resistance temperature detector in a pressurized light water nuclear power system. Description of the Related Art A conventional power supply and signal conditioner module for a resistance temperature detector (RTD) 12 is illustrated in Fig. 1. The power supplied by the module 10 is produced by a constant current source 14 connected to a +15 volt power source, not shown. Since surge withstand testing is commonly performed in the control systems of nuclear power systems, a surge withstand circuit 16 is provided across the output terminals 18 and 20 of the power supply portion of module 10. A similar surge withstand circuit 22 is provided across input terminals 24 and 26 to provide protection for an input signal conditioner 28. The conventional constant current source 14 includes a precision reference 30, such as an AD2710H manufactured by Analog Devices, which provides a constant voltage of, for example, 10.0 volts when connected to a +15 volt power supply and ground. Resistors 32 and 36 act as a voltage divider to produce a control voltage V₁. An operational amplifier 38 receives the voltage V₁ and a voltage V₂, generated by current through a feedback resistor 40 connected between the second output terminal 20 and ground. The operational amplifier 38 is powered by the ±15 volt power supply and outputs a constant current to the RTD 12 via the surge withstand circuit 16 and the first output terminal 18. The constant current returns from the RTD 12 via the second output terminal 20 and surge withstand circuit 16 to flow through the feedback resistor 40, causing the voltage drop V₂ by which the operational amplifier 38 is controlled. The surge withstand circuits 16 and 22, as described above, are commonly used in control systems for nuclear power systems, but are not required by power supply and signal conditioner modules for resistance temperature detectors when surge withstand tests are not performed. An example of the surge withstand circuit 16 for the conventional power supply portion of module 10 is illustrated in Fig. 2. The circuit in Fig. 2 includes capacitors 42, 44, 46 and 48 connected across the output terminals 18 and 20. A resistor 50 and 52 is connected to each of the output terminals 18 and 20 and a fuse 54 is connected to one of the resistors, in this case resistor 50. A bipolar zener diode 56 is connected in parallel with capacitor 48. The bipolar zener diode 56 may be a Tranzorb diode manufactured by General Semiconductor Industries, Inc. The capacitors 42-48, in the surge withstand circuit 16, are typically 0.1/uF capacitors except for capacitor 48 which is a 1/uF capacitor. After emerging from the surge withstand circuit 16, the constant current output from the operational amplifier 38 passes through the first output terminal 18, through the RTD 12 and back to the output terminal 20, causing a voltage drop V₃ across the RTD 12. The voltage drop V₃ across the RTD 12 is sensed by the input signal conditioner 28 and varies depending on the temperature of the resistor comprising the RTD 12. A typical resistance temperature detector 12 can be obtained from RdF Corporation and pressurized light water nuclear power systems typically use model number 21204. The input signal conditioner 28 as illustrated in Fig. 3 comprises an input buffer 58 and a filter 60. The input buffer 58 and filter 60 each comprise operational amplifiers 62, resistors 64 and capacitors 66. The power supply and signal conditioner described above are usually provided as a single module which may be disconnected at cable connectors 68 (Fig. 1) for repair or replacement. However, when the module 10 is disconnected, there is no longer either a power supply or an input signal conditioner 28 connected to the RTD 12. It is possible to provide redundant input signal conditioners by simply connecting the input terminals 24 and 26 of multiple modules 10 in parallel, however there is no known system which provides redundant power supplies. SUMMARY OF THE INVENTION The present invention, which is defined in claim 1, provides a redundant power supply and signal conditioner modular system for a resistance temperature detector, which can be connected in series with other power supplies and which supplies power only when a power failure is sensed in the series circuit, in which only one power supply puts out current at any one time, in which a power supply and signal conditioner module can be removed for testing or maintenance and another power supply and signal conditioner module will automatically take over the function of detecting temperature via a resistance temperature detector connected to the power supply and signal conditioner system, and which provides protection against voltages higher than the power supply is designed to output. The invention in its broad form comprises a constant current power supply with redundancy, for a resistance temperature detector, wherein a voltage drop across the resistance temperature detector is a measure of temperature being monitored, comprising: constant current source means, operatively connectable to the resistance temperature detector, for supplying a constant current with an output voltage at a predetermined current output; overvoltage protection means, operatively connected to the current output of said constant current source means and the ground, for preventing the output voltage from exceeding a predetermined voltage; characterized by: a first diode operatively connected to the current output of said constant current source means and operatively connectable to the resistance temperature detector; and a second diode operatively connected to the ground and operatively connectable to the resistance temperature detector. A preferred embodiment described herein provides a redundant power supply and signal conditioner system for a resistance temperature detector comprising power supply and signal conditioner modules having output terminals and diode means connected across the output terminals of each of the power supply and signal conditioner modules. Each of the power supply and signal conditioner modules includes signal conditioner means for generating an output signal indicating a temperature detected by the resistance temperature detector, constant current source means for supplying a constant current with an output voltage at a current output to the resistance temperature detector via the output terminals, and overvoltage protection means for preventing the output voltage from exceeding a predetermined voltage. Each of the power supply and signal conditioner modules also includes a first diode connected between the output of the constant current source means and the first output terminal and a second diode connected between ground and the first output terminal. BRIEF DESCRIPTION OF THE DRAWINGS A more detailed understanding of the invention may be had from the following description of a preferred embodiment given by way of example and to be studied and understood in conjunction with the accompanying drawing wherein: Fig. 1 is a block circuit diagram of a conventional power supply 10, resistance temperature detector 12 and signal conditioner 28; Fig. 2 is a circuit diagram of the conventional surge withstand circuit 16 of Fig. 1 in a resistance temperature detector system; Fig. 3 is a circuit diagram of the conventional signal conditioner 28 of Fig. 1 for a resistance temperature detector; Fig. 4 is a block circuit diagram of a power supply and input signal conditioner system according to the present invention; Fig. 5 is a circuit diagram of an overvoltage protection circuit 84 of Fig. 4; and Figs. 6A and 6B are circuit diagrams of diode circuits 78A/B of Fig. 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, multiple power supply and signal conditioner modules 10 are connected in series as illustrated in Fig. 4. Each of the constant current sources 14 in the modules 10 in a system according to the present invention is designed to output a slightly different current with a difference of approximately one-tenth of one percent between the voltage V₁ supplied by the precision reference 30 and resistors 32 and 36. Assuming that the voltage V 1A is slightly greater than the voltage V 1B , while the voltages V 2A and V 2B are equal, operational amplifier 38A in module 10A will be driven positive while the operational amplifier 38B in module 10B will be driven negative. Thus, a current will flow through diode 70 to the output terminal 18 of module 10A, while the diode 70 in module 10B will block the flow of current to the operational amplifier 38B. The current from the first output terminal 18 of module 10A flows through the RTD 12, causing a voltage drop V₃ which can be sensed by the signal conditioners 28 in modules 10A and 10B. After flowing through the RTD 12, the current reaches screw terminal 76B. A diode circuit 78B between screw terminals 76B and 80B has a bias voltage from screw terminal 76B to 80B, i.e., from the second output terminal 20 to the first output terminal 18 of module 10B, which is higher than that caused by the current flowing through the surge withstand circuit 16, resistor 40B and diode 82 in module 10B. Therefore, the current flows through module 10B rather than through diode 78B and returns to the resistor 40A in the constant current source 14 in module 10A after passing through the surge withstand circuit 16. Whenever, in the specification and in the claims the terms "redundant" or "redundancy" will be used, it will have the meaning of system having at least two identical components, with one of them active and the remaining one in backup mode, with the remaining component activable upon failure of the active component. The redundant power supply and signal conditioner system illustrated in Fig. 4 is capable of surviving any single failure. If the constant current source 14 "fails high" by outputting a voltage higher than is allowed for by the system, an overvoltage protection circuit 84 grounds the output of the operational amplifier 38, as will be explained later with respect to Fig. 5. Therefore, if the constant current source 14 in module 10A, which is again assumed to output the highest voltage, "fails high", "fails low", or is removed from the system, the result is the same -- a current is no longer supplied to the system by the constant current source 14 in module 10A. When this occurs, the voltage V 2B drops below that of voltage V 1B and the operational amplifier 38 in module 10B is turned on, producing a voltage within 0.1% of that previously produced by the constant current source 14 in module 10A. These results are easily provided by using an overvoltage protection circuit such as an MPC2005 manufactured by Motorola Inc. or a circuit such as that illustrated in Fig. 5 using a silicon controlled rectifier 86 controlled by zener diode 88 connected in series with a resistor 90. Very little current flows through the circuit illustrated in Fig. 5 until the voltage output by the constant current source 14 is sufficiently high to break down the zener diode 88. When the breakdown occurs, a positive voltage is applied to the gate electrode of the silicon controlled rectifier 86 which then turns on and remains on, routing all current supplied by the current limited operational amplifier 38 through an LED 91 which provides a visual indication of a "high" failure. Conventional silicon controlled rectifiers are turned off by stopping the current flowing therethrough, therefore if the cause of the excessively high voltage is corrected, there must be a break in the current in order for the silicon controlled rectifier 86 to permit the constant current source 14 to again supply current to the RTD 12. A failed power supply and signal conditioner module 10 can be removed from the system without affecting the operation of the remaining components. Assuming the above described failure to produce a current in module 10A has occurred and module 10A is removed for repair or replacement, the current output by the constant current source 14 in module 10B will continue to be supplied to the RTD 12. The current will flow from the first output terminal 18 (Fig. 4) in module 10B to the screw terminal 76A, and since the. usual current path through module 10A is not available, the current will flow through diode 78A to screw terminal 80A and return to module 10B via the RTD 12. In order to provide the operation described above, the diode circuits 78A and 78B must have a bias voltage from screw terminals 76 to 80 which is higher than that caused by the current flowing through a nonoperational power supply module 10. Conventional constant current source 14 and surge withstand circuit 16 combined with diode 72 will cause a voltage drop of approximately 3.0 volts across output terminals 20 and 18. Thus, as illustrated in Fig. 6A, each of the diode circuits 78 may comprise a group of series connected diodes 92 with a total forward bias voltage drop of greater than 3.0 volts. Alternatively, as illustrated in Fig. 6B, each of the diode circuits 78 may comprise a high power, reverse bias diode 94, such as a Tranzorb diode and a forward bias diode 96, having a combined breakdown voltage higher than 3.0 volts. IDENTIFICATION OF REFERENCE NUMERALS USED IN THE DRAWINGS LEGEND REF. NO. FIGURE SURGE WITHSTAND CIRCUIT 16 1 SURGE WITHSTAND CIRCUIT 16 4 SURGE WITHSTAND CIRCUIT 22 1 SURGE WITHSTAND CIRCUIT 22 4 INPUT SIGNAL CONDITIONER 28 1 INPUT SIGNAL CONDITIONER 28 4 OVERVOLTAGE PROTECTION CIRCUIT 84 4

Claims: A constant current power supply system (10A,10B) with redundancy, for a resistance temperature detector (12), wherein a voltage drop across the resistance is a measure of temperature being monitored, comprising : constant current source means (14) operatively connectable to the resistance temperature detector, for supplying constant current with an output voltage at a predetermined current output; overvoltage protection means (84), operatively connected to the current output of said constant current source means and the ground, for preventing the output voltage from exceeding a predetermined voltage; CHARACTERIZED BY: a first diode (70) for permitting the constant current flow therethrough while the power supply is active, said first diode being operatively connected to the current output of said constant current source means (14) and operatively connectable to said resistance temperature detector (12); and a second diode (82) for providing a current path when the power supply is inactive, said second diode being operatively connected to the ground and operatively connectable to the resistance temperature detector (12). The constant current power supply system according to claim 1, characterized in that said first diode (70) is having an anode connected to said current output of said constant current source means (14) and a cathode connected to said resistance temperature detector (12), said second diode (82) having an anode connected to the ground and a cathode connected to said resistance temperature detector, said constant current source means comprising: voltage supply means (30) for supplying a control voltage; an operational amplifier (38A,38B) having an input operatively connected to said voltage supply means and an output thereof connected to said overvoltage protection means (84) and to the anode of said first diode (70); and a feedback resistor (40A,40B) operatively connected to a further input of said operational amplifier and to said second diode (82), and operatively connectable to said resistance temperature detector (12), for driving said operational amplifier inactive when current flows through the second diode. The constant current power supply system according to claim 2, characterized in that it further comprises a surge withstand circuit (16) operatively connected to said first (70) and said second (82) diodes and said feedback resistor (40A,40B), and operatively connectable to said resistance temperature detector (12).

Inventors: SUTHERLAND, JAMES FRANKLIN
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: G05F1
IPC codes: G01K7, G05F1, G01R27, G21C17