What's ground on a transformer?

Question

I don't have a lot of experience with things that plug into the wall outlet. But if I wanted, say, a +/- 12 volt power supply, I understand I should begin with something like a 110V to 30V transformer with a center tap on the secondary, to get +15V and -15V. But I'm wondering, then, what to do with grounds? The ground wire from the wall should be connected to the chassis for safety. And the center tap would define the ground relative to +15V and -15V. But if that's also connected to the chassis, would that be about 15 volts above the neutral line and act like a short circuit?

And for my next question, if I don't have an appropriate transformer with a center tap, is it equivalent to connect two transformers in series with the connection between the secondaries acting as a center tap?

I appreciate any illumination.

Rich Grise · Accepted Answer

The earth ground is the chassis, yes. But the "ground" on the other side of the transformer doesn't need to be connected to anything at all - it's just power supply common. The transformer isolates it from the line, so you could ground the "-15" line, and have a +15 and +30 supply, and so on.

Usually, the circuit common is connected to frame ground, at one point, but as far as the line and neutral from the wall, they're isolated, so it doesn't matter what the neutral is relative to ground, although if it's more than a few millivolts, you might have a wiring problem.

Yes, but remember to put the primaries in parallel. :-)

Cheers! Rich

glhansen · Answer

That's counter-intuitive. I was thinking in terms of chopping a big transformer in half.

Oh, wait... if the primaries are in series, all I'd accomplish would be to change the total number of windings, but not the ratio of windings in primary to secondary.

Melodolic · Answer

If you put the primaries in series, you'll halve the voltage across eachcoil.-- Melodolic Spielberg

What's ground on a transformer?

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