Converter In-Rush Thermistors etc, UPDATE -4
UPDATE on 17 Nov, Update 2 on 22 Nov, Update 3 on 16 Jan 14, Update 4 on 19 Oct 14. --------------- In a recent thread there was mention of how you can wreck your gizmo (in my case a converter) ...
Forum Discussion
I think this is being made way more complicated that it actually is.
If this is a half bridge topology, those caps are not connected in parallel. They're in series. And with a 120vac input, each of them is being charged on alternating half cycles. If we close the switch at either peak we impress 170v (or 169v with a diode drop in there) on one of those caps. So worst case with a cold thermistor-- they're fully discharged, we have 3 ohms of resistance in the circuit, and we flip the switch on a peak -- then at the moment the switch is thrown those caps are going to look like a short and we'll see (169/3)= about 56 amps. At any other point in the cycle, or with any charge already present on the caps, it's going to be less.
But say the thermistor is hot and its resistance is now negligible. So we have 1 ohm of circuit resistance and could see a peak of 169 amps, but in reality something is going to give before that happens. A diode, PCB trace, or thermistor. Whatever the weakest link is.
If this is a half bridge topology, those caps are not connected in parallel. They're in series. And with a 120vac input, each of them is being charged on alternating half cycles. If we close the switch at either peak we impress 170v (or 169v with a diode drop in there) on one of those caps. So worst case with a cold thermistor-- they're fully discharged, we have 3 ohms of resistance in the circuit, and we flip the switch on a peak -- then at the moment the switch is thrown those caps are going to look like a short and we'll see (169/3)= about 56 amps. At any other point in the cycle, or with any charge already present on the caps, it's going to be less.
But say the thermistor is hot and its resistance is now negligible. So we have 1 ohm of circuit resistance and could see a peak of 169 amps, but in reality something is going to give before that happens. A diode, PCB trace, or thermistor. Whatever the weakest link is.
