PSU for B+ and Filament grounding follies.

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daskew81

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Joined
Oct 11, 2018
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Before anyone asks, "why did you decide to do this rather than this??....", sorry, but it is what it is. Just hoping to keep the chatter down, and keep the thread productive.

I've attached a rough schematic of a PSU design I've been using for quite a few tube projects. I've never discovered any issues up until yesterday, when I used my scope and measured across the 12.6 AC that goes to the filament rectifier.

When I touched my ground clip from the scope, to presumably the "0v" line on the secondary, the transformer physically hummed as if I was shorting it. I'm guessing because the ground clip is physically earthed? Touching it to the 12.6V tap doesn't cause it.

Then I got to thinking about that connection between the cathode of C1 and C4. When connected, the circuit itself performs fine, with the exception of measuring across the 12.6 AC line. I dremeled the trace from the PCB, low and behold the issue is not present when probing. However, I do get a nasty 60hz hum from the audio circuit, which is at this point grounded to the cathode of C1.

Perhaps it's the way I've wired my secondary? Perhaps this transformer really isn't meant to do 12.6v the way I've implemented it? Any suggestions would be appreciated!
 

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I didn't, but maybe i should.

I noticed a discrepancy in the transformer datasheet vs the actual sticker on it. The datasheet shows the mains backwards from the actual sticker on the transformer. I did swap the mains, but it did not change the scenario. I sure it mattered anyways but I'll definitely try that.

Fwiw, in the original config, i have the c1/c4 connection at earth. If i break that connection, then earth the 0v winding on the transformer that goes to the rectifier, all issues solved. I'm not sure how that affects the safety of the circuit however.
 

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If the dc output 0V was also connected to mains earth - as it normally would be, then it is possible the ground clip of your scope is also connected to mains earth. In this case you have accidentally shorted the bottom and left connections of the bridge rectifier together. This means the diode connected between the top and the left of the bridge in your schematic is connected straight across the transformer. This will definitely make the transformer buzz a lot.

Cheers

ian
 
Thanks Ian! Yes i see that now. The ground clip is earthed, as well as the 0Vdc side. It makes sense.

As a general rule of thumb, I'm with the understanding that nothing from the AC portion of the transformer secondary, or AC points of the rectifiers should ever be earthed or chassis referenced, just leave them floating correct? Earth the 1st filter cap cathodes however? This is coming purely from a noise and design point of view.

I also understand that by earthing the 0v tap on the secondary why I'm able to properly measure everything. But do you think that would cause a safety concern?
 
Thanks Ian! Yes i see that now. The ground clip is earthed, as well as the 0Vdc side. It makes sense.

As a general rule of thumb, I'm with the understanding that nothing from the AC portion of the transformer secondary, or AC points of the rectifiers should ever be earthed or chassis referenced, just leave them floating correct?
Correct
Earth the 1st filter cap cathodes however? This is coming purely from a noise and design point of view.
Unfortunately 'earth' is a vague term and can mean different things under different circumstances to different people. I have attached my grounding101 document which gives you some basic rules.
I also understand that by earthing the 0v tap on the secondary why I'm able to properly measure everything. But do you think that would cause a safety concern?
Yes. When measuring floating connections you should use a floating instrument like a battery powered meter.

Cheers

Ian
 

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