2-12. SINGLE-PHASE SILICON-CONTROLLED RECTIFIER CONTACTOR CIRCUIT,
CIRCUIT FUNCTION DESCRIPTION 6.3
Refer to figure 2-10. From leads 151T and "N," 115vac is applied to the
transformer leads BLK and WHT. One secondary operates as a dc-power supply while
the other two secondaries develop the dc-gate voltages for SCR-1 and SCR-2. When
these SCRs are turned on, the ac voltage from the power circuit on lead AM passes
through the SCRs and becomes lead AMR to the high-voltage generator.
a. Across transformer leads RED and VIO is about 48vac center-tapped by the
GRN lead. This voltage is rectified, filtered, and regulated becoming a positive (+)
2Ovdc source to the impulse-timer circuit. From VIO lead, 12vac is also applied to the
b. The remaining two secondaries, leads BLU/BLU and GRY/GRY, are identical
in operation. The ac voltage is rectified by the diode bridges, capacitive filtered, and
controlled by optocouplers OC2, OC3, and transistors Q7 and Q8. When an exposure
is initiated, leads BAM/BAC from the impulse-timer circuit is high, causing the
optocoupler OC1 to conduct. A high on the lead GPS turns on the two optocouplers
OC2 and OC3. The optocouplers conduct the dc voltage to the base of transistors Q7
and Q8, biasing them on and passing a dc voltage from collector to emitter to the gates
of SCR-1 and SCR-2. This gate voltage is about 1.5vdc.
c. Since the duty cycle of the SCRs is 100 percent, both positive and negative
cycles pass through to lead AMR. When the timers terminate exposure, leads
BAM/BAC and GPS return low and the optocouplers and transistors are turned off,
removing the dc gate voltage to the SCRs. The SCRs continue to conduct until the next
zero crossover of incoming ac voltage on lead AM and then they inhibit.