An electronic circuit has been designed as a prototype of a device that determines whether critical electrical systems have been compromised because of pyrolysis of polyimide-insulated wires. This circuit can be modified to prevent further pyrolysis and to check for indications of pyrolysis for a wide variety of load resistances and power supplies. Circuits like this one could be beneficial in spacecraft, in military and commercial aircraft, and in the nuclear power industry where polyimide-insulated wires are used.
The device is divided into two main sections: a pyrolysis-detection circuit and a potential-pyrolysis-indication circuit. The heart of the pyrolysis-detection circuit is an instrumentation amplifier that monitors the voltage drop across a shunt resistor as current flows through the resistor to a load resistance. If polyimide-insulated wire in series with the load resistance begins to pyrolyze, the current through the shunt resistor begins to increase. If the current exceeds a preset value (in the present design, 1.50 A, corresponding to a pyrolyzed-polyimide resistance value of approximately 35 Ω), then the instrumentation amplifier puts out a signal with a potential of approximately 1.10 V to the inverting input pin of a comparator. The output potential is set to this level by adjusting a gain resistor while 1.50 A of current is flowing through the shunt resistor. When the comparator (which is configured with a threshold voltage of approximately 850 mV) is triggered, a transistor is turned off, thereby disabling a solid-state relay and interrupting the flow of electrical current to the load and the pyrolyzing wire.
After the pyrolysis event has passed, the circuit then tests the pyrolyzed wires and adjacent wiring to determine whether any wires are hazardous. This involves routing the output from a precise 5-mA current source through the load and the low resistance of the pyrolyzed wire. A potential unsafe pyrolysis condition is detected by use of an operational amplifier configured as a comparator with a trigger threshold voltage that corresponds to a critical pyrolysis resistance of approximately 35 Ω. After the threshold voltage has been reached, a transistor becomes turned on, causing illumination of a light-emitting-diode display to indicate the potential unsafe pyrolysis condition in the wiring under scrutiny.
This work was done by Timothy E. Roth of Allied Signal Technical Service Corp. for Johnson Space Center. MSC-22717