OVERVOLTAGE PROTECTION - CROWBAR CIRCUIT
Because excess voltage can break down junctions of transistors and diodes, said parts being in extremely limited supply, it is essential that power circuits in vintage systems like the IBM 1130 have a means of detecting any deviation upwards in voltage and cutting it off very rapidly.
The method used in these systems is to short across the input to a power supply when the output voltage exceeds some threshold, this short circuit causing the circuit breaker to trip very rapidly. This is termed a crowbar circuit, alluding to the method used with electrified railways to quickly kill power by tossing a metal bar across the power rails to force fuses to blow.
IBM's power supplies are built to use an SMS card that is set with a zener diode at some threshold above the nominal supply voltage. For example, it might be set at 7.2V to give a bit of margin for power supply swings while crisply acting on any larger excursion. The zener diode sets a reference voltage on one of a pair of transistors, the other side of the differential pair is sensing the supply voltage.
When the supply side exceeds the reference side, the differential pair swings over to the side that is connected to a SCR (Silicon Controlled Rectifier). The SCR will latch on and remain conducting until all power is removed from the system.
Because the power supply being protected is high capacity and protected by a 30A circuit breaker, the SCR must be able to absorb more than that for the time it takes for the quick trip breaker to flip off. The IBM SMS card actually uses a pair of SCRs - a smaller one that will conduct immediately based on the relatively sensitive differential transistor pair, which in turn will feed enough current to trigger a major 80A rated SCR.
THE FAILURE IN THE 6V REGULATED SUPPLY CAUSED HIGH VOLTAGE SWINGS
What I observed in the power supply on the bench was that with the relatively high input at 16.5V, the supply would be unstable, the output voltage surging up over 7.5 volts regularly. This in turn would cause the overvoltage card to latch the SCRs on in an attempt to trip the break.
Since my measly 5A bench supply couldn't deliver the high current needed to trip the breaker, we just went into current limiting mode and the input voltage sagged. When hooked to the real IBM 1130 supply of 16.5V, with its ability to feed serious current, the breaker tripped almost instantly.
TRIPPING BREAKERS RATHER THAN DESTROYING ELECTRONIC COMPONENTS
Tripping a breaker in milliseconds, while lowering the output voltage to nearly zero in much less time than that, means that sensitive junctions don't have time for heat effects to damage their junctions. This keeps a power supply malfunction from destroying hundreds of SLT cards.
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