# basics of transformer

Discussion in 'Electronic Basics' started by Abstract Dissonance, Jan 30, 2006.

1. ### Abstract DissonanceGuest

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? and the .25A means that it can handle at most .25A
across its secondary coil(since its a step down from 120VAC the max current
in its primary should be smaller... ~80/120*.25?).

Also should I assume these are all RMS values? When I put my variac up to
100VAC and hook it up to my transformer I get about 40 volts across the hole
secondary(hence 20V's across from the center tap). But this means that at
the mains(117VAC?) I'll be getting more than 40VAC across the full
secondary? Is this ok to have the transformer have a little more than its
voltage rating?

Thanks,
Jon

2. ### Abstract DissonanceGuest

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).

something like

V+ -+- V1o
|
---+
---
|
+---G
|
---+
---
|
V- -+- V2o

Thanks,
Jon

3. ### John FieldsGuest

---
40VCT means that with the rated voltage into the primary and the
rated load on the secondary, the secondary voltage will be 40V
across the entire secondary. (20V from the center tap to either
end.)
---
---
Yes. It's supposed to because of the fact that since there will be
resistive losses in the primary and secondary windings when the
transformer's secondary is fully loaded, there will be an inevitable
rise in voltage across the primary when it's less than fully loaded.
The difference between the no-load and full load voltage is known as
the regulation of the transformer, and for small transformers like
you have is typically 30%

4. ### Abstract DissonanceGuest

cool...

Thanks a lot,
Jon

5. ### Jasen BettsGuest

No, the secpondary is 40 volts end-to end with a tap in the centre.

sometimes it's written 20-0-20
under load the voltage will "sag" down from 43V to about 40V
hopefully that 'G' is connected to the centre tap.

Bye.
Jasen

6. ### Pooh BearGuest

No it means 40 Volts 'centre tapped'. e.g. 0-20-40 or 20-0-20 if you prefer

Graham

7. ### Rich GriseGuest

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