# Power transformers

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#### m85476585

##### Well-Known Member
I have some old 240V-120V step-down transformers, and I am wondering if I could connect two in series to get 120V-30V? The first one would go from 120V to 60V, then the second one would go from 60V to 30V. Is there any reason why this wouldn't work?

I'm trying to make an adjustable power supply, and this would be the power source.

I wouldn't do it. When you halve the voltage you double the current. Therefore, the current in your second transformer could be twice what it is rated for. Power dissipation is a function of current squared times resistance. Twice the current is four times the power loss in the secondary coil of your second transformer. And I just noticed that neither is rated for 120V on the primary side. Don't do this. Those ratings are there for a reason.

I could be wrong, but it is too late in the evening to be thinking straight.

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I think what your saying is you want to connect the primary of the second transformer to the secondary of the first transformer. Well in theory this would work. What is your load? (in Amps) and watts. Your load determines the current draw through the transformers. And what is your source amps? and watts?
If you just want a 30 volt AC supply there are much simpler and efficient ways to make such a supply.

CornyL

I have some old 240V-120V step-down transformers, and I am wondering if I could connect two in series to get 120V-30V? The first one would go from 120V to 60V, then the second one would go from 60V to 30V. Is there any reason why this wouldn't work?

I'm trying to make an adjustable power supply, and this would be the power source.

Absolutely. Are there better ways of doing this such as with a single transformer - sure. As the other poster said, just make sure the transformers are rated for the power (VA) you are running with. Now running the other way (higher voltage), is usually a problem since you start running into core saturation issues, but for what you propose, i don't see a problem other than just being a bit overly complicated and bulky.

If you just want a 30 volt AC supply there are much simpler and efficient ways to make such a supply.

CornyL

I'm open to any suggestions. I would like around 30V and a good amount of current, maybe 5A or more. DC or AC doesn't matter since I can just rectify an AC supply.

If you can deal with 24VAC, you can use an industrial control transformer, or an outdoor landscape lighting transformer. Both available with plenty of available current.

If you look into the outdoor lighting transformer, double check the output, a lot of them are 12AC outputs.

Radio Shack, Newark Electronics, Digi-key and others have them. Check out these from Honeywell

I'm open to any suggestions. I would like around 30V and a good amount of current, maybe 5A or more. DC or AC doesn't matter since I can just rectify an AC supply.

Lower voltage shouldn't be a problem at all, but higher current will. In your case, if your transformers are nominally rated at least at 300VA (first stage) and 600VA (second stage), you know they can handle the 2.5A and 5A that you want to draw. Therefore you need at least 900VA 'worth of' transformer to get your desired 150VA. If your existing transformers are big enough and you don't mind the size and weight, you should be fine.

Reinhard

If you look into the outdoor lighting transformer, double check the output, a lot of them are 12AC outputs.

Usually only the smaller units are straight 12VAC. Larger units are generally 24VCT, with the 12V light strings connected from either end of the winding to the CT.

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