Making an isolation transformer from two transformers

Question

I've been reading up on isolation transformers and was wondering if I can set one up. This question is somewhat related to [1] but without the centre-tap.

I have two 230V->12V AC transformers. Having read [2], I understand that I can have the incoming mains go into the first transformer, and have the secondary part be connected to the second transformer's secondary. Then the second transformer's 'primary' is connected to the unit in question.

In theory, I feel that it should work. However, while the transformers have the label of '230V' on the primary and '12VAC' on the secondary, I don't know the current rating. Also the size of the two transformers are different. I salvaged them from an old lighting system at work which powered a series of halogen 'spotlights' (the small round halogen lights that shines downwards... I don't know the right terminology for this).

Is this do-able?

Thanks

[1] - Connecting two center-tap transformers for isolation transformer

[2] - current transformer in reverse

Answer 1

Yes, it is doable. There's a slight problem that if you were to take two identical step-down transformers and connect them the way you're planning, the output voltage would be less than the input voltage. For good quality off the shelf transformers I would expect maybe a 5 or 10% drop -- for salvaged transformers, I couldn't say.

You can judge the power handling capability of a transformer by what the light that you took it out of was rated at. If the light was 10W, the transformer is 10W. Another way to get an educated guess is to weigh them, and compare with transformers of known weight and power capability.

I'd connect things up so that the smaller transformer is on the output side, because some energy will be lost on the input side -- you want to dump that heat into the bigger transformer.

I'd also make sure as I was using them that they're not getting too hot. If the overloading is mild, the transformer will just quietly overheat. After making sure you won't get electrocuted, feel the transformer as you're using it, starting from cold. If you can put your hand on it and hold it there, it's fine. If it gets too warm to touch, it's too hot. (Note that you don't want to just let it work for an hour and then check without being very careful -- if you haven't touched it for a while, approach it like your finger will go "tssss!" when you touch it).

Answer 2

As TimWescott says, it's doable, but as you realize, the limiting factor is how much power the transformers can handle.

A systematic way to approach the problem is to get some 12 volt incandescent bulbs, and start hooking them up to the secondary. Then monitor the transformer temp. A rule of thumb (no pun intended) is that 60 degrees C is the "can't hold it for more than a fraction of a second" threshold, and it's probably a good limiting condition. Automotive 12v bulbs are cheap and easy to get.

Start with one bulb, wait a half-hour and see if the transformer is too hot. If not, add another bulb in parallel and repeat. At the limit you can figure how much power the transformer can handle simply by summing the power of the bulbs. Subtract about 10% and this is the maximum power of the isolated load.