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What's ground on a transformer?

G

Greg Hansen

Jan 1, 1970
0
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.
 
R

Rich Grise

Jan 1, 1970
0
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?

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.
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?

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

Cheers!
Rich
 
Rich said:
On Thu, 11 May 2006 12:24:24 -0500, Greg Hansen wrote:


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

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.
 
M

Melodolic

Jan 1, 1970
0
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.

If you put the primaries in series, you'll halve the voltage across each
coil.
 
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