TM 5-6675-324-14
(2) The thermistor is located underneath the 1 amp and reflector assembly
where it senses heat output of the exposure lamp. Any change in the heat output of
the exposure lamp causes a resistance change in the thermistor which alters the
drive motor speed proportionally to the lamp output.
(3) The unijunction transistor produces positive going pulses under the
control of the time constant, speed control potentiometer, and the thermistor. The
pulses are fed to the gate of the silicone-controlled rectifier (SCR), resistor R1,
and sets the gate operating level. The SCR is gated as determined by the speed
control circuit, and being in series with the drive motor, varies the power supplied
to it. Diodes D8 and D9 are blocking diodes and prevent reverse voltages from
reaching the SCR.
4-9
