TM 5-6675-323-14
c . W h e n the ac voltage enters the control board, it is applied to C1 and R1
which provide a slight delay in the input voltage, which is applied to the timing
circuit composed of capacitor C2 and resistors R4, R3, R5, and external INCREASE
p o t e n t i o m e t e r 1R1.
(Note that resistor R6 is not used in this application.)
d. As the delayed ac voltage is applied across capacitor C2, the capacitor
b e g i n s to charge at a rate depending upon the setting of INCREASE potentiometer 1R1.
The voltage across C2 also appears across rectifier CR1 and trigger diode CR2. When
t h e trigger diode reaches the breakover voltage of 43 5 V, it conducts to complete
the path across rectifier bridge CR1. This forms a closed loop circuit through
capacitor C2, the primary of pulse transformer T1, and rectifier CR1. Current
flows until capacitor C2 is discharged. The discharge time is very fast and a short
duration pulse is generated, shaped by capacitor C3.
e. The pulse current flowing through the primary of pulse transformer T1
induces a voltage across the appropriate secondary which is applied through diode
C R 3 or CR4 to the gates of SCR Q2 or Q1, respectively. When either SCR is
triggered, it allows the rest of the ac half-wave to pass to external grid lamp
transformer 1T1 through pin B. The SCR will continue to conduct until the ac half-
wave reaches the zero crossing point, at which time it turns off.
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