Fluke Biomedical 956A-201-M2 - Manual (Page 67)

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R2 is a 1000:1 voltage divider that provides feedback to control the high voltage. U1-B is used as a buffer
between the 1000:1 divider and the 1000:1 output signal. This output signal is also utilized by U1-A to
regulate the high voltage and is sent to the HV test jack. The SHUTDOWN signal (provided by the
controller) when low has no effect on the high voltage. When the SHUTDOWN signal goes high, Q3
conducts causing Q1 to turn off. This action forces the oscillator to turn off, effectively shutting down the
high voltage.
Short circuit protection is provided by the positive temperature coefficient thermistor (PTC). The PTC
resistance in normal operation is nominally 5 ohms. When the high voltage output is shorted, the control
circuitry U1-attempts to maintain regulation by increasing the base drive for transistor Q1. Excessive
current flows through the PTC, causing the internal temperature to increase. As the temperature
increases, the PTC resistance also increases dramatically. The effect is that the control voltage to the
oscillator is decreased to a minimum level. The response of the PTC is approximately ten seconds.
Removal of the short circuit condition results in restoration of the high voltage to the preset level.