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?
240V Control Transformer issue
Feb 09, 2007
7 Replies
You\'d get 12VAC out of the secondary at the same current it was
rated for with 24VAC out of it, so it would only be able to deliver
half the power into the load that it could with 24VAC out.
you get 12 volts
"I\'m never wrong, once i thought i was, but was mistaken"
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Sweet. I got this transformer from Fry's and intend to make a power supply with a
1-12V 1-3 amp spec us>
You'll get 11/12V out.
Graham
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.
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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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
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
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