240V Control Transformer issue

Discussion in 'Electronic Basics' started by none none, Feb 9, 2007.

1. none noneGuest

i have a Control Transformer with 240V on the primary, with 24VAC out the
secondary. My question is what would happen if i try to use this as a power
transformer using only 110/120V on the primary?

3. JamieGuest

you get 12 volts

4. none noneGuest

Sweet.
I got this transformer from Fry's and intend to make a power supply with a
1-12V 1-3 amp spec using digital volt & ammmeter for the displays and have
it variable current and voltage. Seems much more fun than buying one.

5. EeyoreGuest

You'll get 11/12V out.

Graham

6. JamieGuest

Well lets see now..
your average peaks should be around 16.9 volts at the
filter caps.
Don't expect to source must current when looking for
12 volts.
I don't know what kind of regulator you plan on using
but!, you should have some more voltage than that if
you plan on a linear type.
I suppose you could make you're self a smps (switch mode power
supply), with proper regulation you could most likely get it stabilized
some what in the ripple region.

7. Rich GriseGuest

With enough capacitation, 16V is quite comfortable headroom for a 12V
regulator, if it's designed right.

I must point out, however, that spec'ing it at 1.2-12V would make the
design considerably simpller, if you're going to use an LM317.

Good Luck!
Rich

8. ehsjrGuest

Good point. To get 1V out, he could adjust it to
around 1.6 and put a diode in series. And that ~16.9
volts is AC peak before rectification, not the voltage
across the cap, so the 16 volts you mentioned is the
correct approximation for a single diode drop. (A
typical 1N540x will drop about .9V at 3A)

But he's not out of the woods. The LM317 won't handle
3A. He could use an LM350 instead. Still, the 16 volts
won't be there if he uses a bridge. It will be ~ 15.1
volts at 3A. Add in line voltage variation, transformer
sag, and ripple, and he'll have problems.

I think for most cases the supply would work - but most cases
are at well under the max current and with the line voltage
closer to 120 than to 110. For a solid 12V design, he needs
a different transformer.

Ed