Electronics > Repair

Help sought in locating short circuit [Sorted]

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max.wwwang:

--- Quote from: fzabkar on November 14, 2024, 10:32:36 pm ---Perhaps one of the rectifier diodes is leaky? If so, then the associated electrolytic filter capacitor would look like a short circuit to AC, but not DC.

--- End quote ---

Thanks. I checked the full bridge diodes (without obvious anomalies) but didn’t have a careful and close analysis of the possible failure modes and corresponding behaviour (I didn’t suspect them that much).

Certainly that’s a possibility. By leaking do you mean they conduct to someone degree under reverse voltage? If so then certainly a bad one will cause AC short circuit without involving the filter cap (hence I don’t quite understand what you mean by “capacitor would look like a short circuit to AC”).

I will have another close look.

fzabkar:

--- Quote from: max.wwwang on November 14, 2024, 10:49:11 pm ---By leaking do you mean they conduct to someone degree under reverse voltage? If so then certainly a bad one will cause AC short circuit without involving the filter cap (hence I don’t quite understand what you mean by “capacitor would look like a short circuit to AC”).
--- End quote ---

I have had cases where a leaky diode in a bridge rectifier resulted in very large ripple and unstable behaviour. The diode doesn't need to be a dead short, especially if the capacitor is very large.

To answer your question, yes, "leaky" means that there is some conduction in the reverse direction.

Russ_A:
Yeah I get that it isn't directly applicable to your case since it appears the power supply and the control board share a PCB. If the problem is with the power supply maybe you can find a way to disable it and then bypass it with something off the shelf. Just throwing an idea out there.

marhuum:
Mere try / effort;
obtain thermal imaging, snapshoot or record a (time range on) the moments it gets voltage supplied while getting short so illustrating the current flow on the intended parts or areas

max.wwwang:

--- Quote from: Russ_A on November 14, 2024, 11:33:15 pm ---Yeah I get that it isn't directly applicable to your case since it appears the power supply and the control board share a PCB. If the problem is with the power supply maybe you can find a way to disable it and then bypass it with something off the shelf. Just throwing an idea out there.

--- End quote ---

Correct. The power supply here is hard to break (except perhaps the diode). Most likely the problem sits elsewhere (thought I will look closely into the supply diodes as suggested).

Your idea is welcome and certainly appreciated.

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