Power Switch Pwb Assembly - SAC SS-800AVS Manual

2.2.10 Power Switch PWB Assembly
The Power Switch PWB Assembly allows current to pass through a
transistor switch Q4, Q5 to the final transistors of a transmitter.
The Power Switch PWB monitors the emitter current in each power
transistor, and the overall supply current to the final amplifier
stage. If an overall overcurrent condition is sensed, the transistor
series switch Q4, Q5 is opened, removing power immediately from the
final transistor collectors. The supply line from the Power Switch
PWB to the final collectors is then shorted to ground by an SCR
crowbar circuit. After about one second, power is reapplied to the
final transistors. When the circuit is activated, an LED on the
assembly will light indicating that power is being removed from the
final transistors.
Overall high-current conditions are sensed by QI. If current through
RI, R2 exceeds approximately 25 Amps, Q1 begins to conduct, pulling
pin 3 on Ul above the reference level set by R24 (approximately +3
VDC). Likewise, if any final transistor emitter current exceeds
3 Amps, the input signals to P2 from the transistor emitters will
exceed the reference level. The output from comparator U1 will
then go high, charging C12 and causing the U2 output to go high,
followed by the U3 output going high. As the U2 output goes high
the series transistor switch Q4, Q5 stops conducting. As the U3
output goes high, SCR Q9 is fired, discharging any charge remaining
in the final amplifiers to ground. To return power to the final
stage, the process is reversed.
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