12VAC to 24VAC transformer needed

The method I suggest below will require a slightly larger transformer than one designed for your purpose, but it would only leave about half the copper unused. For a one-off application, that may be better than trying to find an unusual transformer.

If you can find any transformer with a 24 VAC center-tapped secondary rated for your load current (which appears to be a bit above 2 A), then you can drive the center-tap and one end with your 12 VAC and take 24 VAC from the ends. Be sure to cover the primary lead ends as they will (likely) have a higher voltage. This will function as an auto-transformer, with about

2 A flowing thru each half of the 24 V winding for a total of 4 A applied to the center-tap. If the center-tap is brought out as a single wire, you need be sure it is good for 4 A. (If the secondary is split, as is common, the wires should be large enough for the rated current.)

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--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

Thanks Larry -- seems like an obvious solution once you mentioned it :-) and MUCH better than finding a something special.

Hi Ed ... that was exactly the approach that I tried before (with the 'travel' transformer)... it just didn't give me enough current on the output -- presumably it had a low number of turns. I think Larry (above) gave me something to run with but thanks for the feedback!

The number of turns defines the voltage ratio. For a 110 to 220 transformer, the ratio is 1:2. This is also true of a 12 to 24 transformer.

I'm guessing your source is not keeping up it's part of the bargain, and drooping under load. That is, unless there is something sinister about the travel transformer that we don't know about.

You may not know that you'll have to supply a bit more than twice the current at 12V than the 24V device needs. A transformer will keep constant volts * amps (except for losses). Thus, you'll need to supply at least 4.2A at 12V to keep up with your 50W motor.

Larry's suggestion also discards any isolation between the load and the source. This may not be an issue for you, but then again, it may.

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Regards,
  Bob Monsen

the ratio is 1:2. This is also true of a 12 to 24

drooping under load. That is, unless there is something

at 12V than the 24V device needs. A transformer will

least 4.2A at 12V to keep up with your 50W motor.

Losses are a big issue when a transformer is used at a fraction of its design voltage. The OP's 2.1 A load, if taken from a 110 to 220 transformer secondary, will require a 460 VA transformer. The fact that the load is only 50 VA means a much larger transformer would be required, about 9 X. By using the secondary of a

50 VA transformer as I suggested, there is much less iron and copper put to waste. (Copper is wasted due to having 9 X as many turns as are needed for the core. Iron is wasted to hold all that extra copper.) There is some waste with my suggestion too, but not so much. A dedicated 12 to 24 VAC autotransformer could be built without the wasted copper of a 110 VAC primary. But that waste nominally only about half the copper.

source. This may not be an issue for you, but then again,

When the OP stated "isolation isn't an issue so I could go with an Autotransformer", I took him at his word, assuming he understood the loss of isolation.

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--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

Elegant. I like it.

But if the OP desires isolation, he might also want to get two transformers of at least 70VA rating (12VAC * 6A or more secondary,

24VAC * 3A or more secondary) and backfeed the first transformer to supply the second like this (view in fixed font or M$ Notepad): ` ` | ` T1 T2 ` o------. ,-----|-----. ,-------o ` 12VAC In )|( )|( 24VAC Out ` )|( | )|( ` o------' '-----------' '-------o ` Sec. Pri. | Pri. Sec. ` ` | ` ` T1: Pri. 120VAC Sec. 12VAC 6A ` ` T2: Pri. 120VAC Sec. 24VAC 3A

created by Andy=B4s ASCII-Circuit v1.24.140803 Beta

T1 is set up backward (the 12VAC is supplying the secondary of the transformer, and the 120VAC primary is the output). This is called backfeeding a transformer. The output of T1 is used to supply T2, which outputs a standard 24VAC.

Good luck Chris

A 110 to 220 transformer will work with 12 vac on its primary to give you 24 vac output on its secondary. Ed

Your solution not only saves on the very real "technical" losses, it also avoids the "wallet" losses incurred with the 110:220 transformer my post mentions. I just looked at the prices for the kind of transformer I had in mind - ouch. We're talking the 100 dollar neighborhood. Should be able to get something at 24V ct around 5 amps for around $25.

Sounds like your idea is a winner.

Ed

But to supply the OPs load the 110 to 220 volt transformer will need to be rated at about 500 va to have heavy enough wire to cope with the 2 amps needed.

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John G

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