On 05/02/2016 06:17 PM, andy pugh wrote: > > Today things went a bit strange, blowing the breaker and then the > discharge resistor. This was with the PSU powered up, but not turned > on. > > It turns out that I have an unanticipated failure mode, if both input > SSRs fail closed-circuit. > > I have tested the SSRs on the bench, running a light bulb, and they > both light the bulb with nothing connected to the control terminals > (And with the control terminals shorted together). OK, so the SSRs failed shorted. That is their usual failure mechanism. You should very conservatively derate the SSRs in many circumstances, but especially where capacitive inrush surge is possible. > So, I am wondering if there is a problem running a rectifier input > with SSRs. I found something in an Omron document suggesting that > turn-off might be unreliable, but that doesn't seem to be the issue > here, the relays are now incapable of turning off a (filament) light > bulb. > > Did I get unlucky, or are SSRs a bad choice for this application? > Do you know what the ACTUAL inrush surge is? Also, is it possible that a momentary power dip caused a surge while the SSRs were in the turned-on mode? Possibly, there was a voltage spike while the SSRs were off, and it caused them to catastrophically break down. You might think of getting some MOV surge suppressors to put across the input mains. Also, maybe get SSRs rated for higher input voltage.
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