Fun grounding issue that might help someone

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Potato Cakes

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I was testing a recent build of mine with the top cover of the enclosure off and it worked perfectly and was dead quite. I put the lid back on and didn't think about it. When I actually put it to use, there was a 60Hz hum. So I took the lid off to see what was going on but it was quiet again. Turns out, since the toroidal transformer being used barely fits in there, the bolt had worn into the paint of the top cover and the bolt had continuity on the top and bottom. I put some electrical tape over the top of the transformer and now it's quiet with the cover in place. Not sure I understand why, but the problem seems fixed.

Just thought I'd share. Maybe that would help someone. Maybe not.

Thanks!

Paul
 
Potato Cakes said:
I was testing a recent build of mine with the top cover of the enclosure off and it worked perfectly and was dead quite. I put the lid back on and didn't think about it. When I actually put it to use, there was a 60Hz hum. So I took the lid off to see what was going on but it was quiet again. Turns out, since the toroidal transformer being used barely fits in there, the bolt had worn into the paint of the top cover and the bolt had continuity on the top and bottom. I put some electrical tape over the top of the transformer and now it's quiet with the cover in place. Not sure I understand why, but the problem seems fixed.

Just thought I'd share. Maybe that would help someone. Maybe not.

Thanks!

Paul
Click to expand...
Thank you for bringing this; it should be made a sticky. It's well-known of designers and often caveated by xfmr manufacturers, but most DIYers (and some designers too) are not aware (or forgot or are too lazy busy to RTFM).
 
> the bolt had continuity on the top and bottom.

The case becomes a One Turn Short on the transformer. Done very well with a large core, the case will melt. Meanwhile the high current stresses the transformer and throws line hum at your circuit.

Tape will NOT last. Trim the bolt, dimple the cover, use fiber washers on the bolt, use tire-grade rubber (heater hose), whatever it takes so this will never happen again.
 
Completely agree PRR.
Also many toroids have a built-in primary fuse that will usually pop and render the transformer useless when this happens.
 

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