Re: basics of transformer

On Mon, 30 Jan 2006 14:58:28 -0600, Abstract Dissonance wrote:

>>I have a 40VCT .25A transformer and I'm wondering what those numbers
>>mean..
>>
>> does the 40VCT mean 40VAC from the center tap? If so across its full
>> secondary its 80VAC?
No, as others have said, it's 40 end-to-end, 20-0-20.
and the .25A means that it can handle at most .25A
>> across its secondary coil

No, this is a misnomer, the current isn't _across_ the coil, it's
_through_ the coil. It's the voltage that's across it. Voltage is
pressure, current is flow. :-)

> Oh, at at those settings(40VAC across full secondary) the output off my
> caps is ~30VDC "half way" and ~60VDC full (I'm using a split bridge so I
> can supply 2).

Yes, the no-load output voltage is the peak of the input, which for a
sine wave, is 1.414 times the RMS.

>
> something like
>
> ,-----|>|-----+- V1o = V+
> | |
> ( ---+
> ( ---
> ( |
> >-------------+---G = 0V
> ( |
> ( ---+
> ( ---
> | |
> `-----|<|-----+- V2o = V-
>
Yes, as Jasen said, connect that G to the center-tap.
You could also get just the +30, by turning the bottom diode around,
lose the bottom capacitor, and tie the two diodes' anodes together.
That's called a "full-wave center-tap". What you have here is
stacked half-wave rectifiers. If you want to double the power-
handling capability of that, and get the DC bias out of the secondary,
you can add two diodes like this:

> ,---+-|>|--+--------+- V1o = V+
- | | | |
> | +-|<|--|---, |
> ( | | ---+
> ( | | ---
> ( | | |
> >----------|---|----+---G = 0V
> ( | | |
> ( | | ---+
> ( +-|>|--' | ---
> | | | |
> `---+-|<|------+----+- V2o = V-

That's just a squared-off full-wave bridge, with 0V from the
center-tap, just like you had, but exploits the other half-
cycle. :-)

We've determined that it's safe to turn the variac up to 120V, and
measure the no-load output there; then, if you wanted to , you could
plug the tranny right into a the fused, switched mains. Then, if
you want, you could put various loads on the output and see what
happens to the voltage and current. For a transformer this size,
I might even use a .1A fuse, lessee, 40V at .25A is 10 watts, which
is like .083A at 120V. :-)

Cheers!
Rich

.

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