Got two isolation transformers

Hi all,

I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell one off I thought I'd best check whether there are any circumstances in electronic repair when it's really handy to actually have two of them and/ or can they be hooked up together to provide 1KVA supply? thanks, cd.

Reply to
Cursitor Doom
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Maybe. Maybe. Beware.......

Reply to
Sjouke Burry

** Where there are two technician at work?
** If the output voltages are *identical*, you can wire them in parallel and double the current rating.

To check this, with both trannys connected to the AC supply, link the secondary wingings at one end and measure the voltage across the remaining wires. It should be either 480V or close to 0. Reverse the connection if it is 480.

Close to 0 = about 0.5V max.

To safely parallel the pair, the primaries must be wired to the SAME AC plug - cos using two plugs means one will have exposed live pins.

... Phil

Reply to
Phil Allison

well, and the %Z must match too if you want to get real serious, otherwise one transformer may still be doing more work as it takes more of the load.

Reply to
Cydrome Leader

Thanks, Phil. Best I hang on to the pair of 'em then, just in case...

Reply to
Cursitor Doom

** Both transformers are stated to be rated at 500VA so the input and output voltages should match exactly at that load to comply with the 240:240 spec.

So the extra condition that the off load voltages also match nails the voltage regulation characteristic for resistive loads.

Ideally, the trannys should be of the same design, ie both e-core, c-core or both toroidal.

... Phil

Reply to
Phil Allison

Simple test. Parallel the primaries. Measure the input POWER (loss). Parallel the secondaries. Measure the input power again. All that power is heat in the transformers. The increase is due to voltage mismatch across the impedance. Notice, I didn't say measure current. I said measure REAL power. That should give you some idea of how long before the smoke gets out.

Pass that test? Move on to load tests and measure the currents in each secondary winding. Easy way to do that is run the individual secondaries in opposite directions thru your current probe. Result should be zero if the load is shared equally.

Reply to
mike

** Really ???

** That process looks like the HARD way and needs test gear most do not have.

... Phil

Reply to
Phil Allison

Well, the default test is to just parallel them and wait for smoke. No smoke, you're good to go. Smoke, you're done.

Your choice. You can get some indication of the input power by putting an incandescent light bulb in series with the input.

I'd argue that there are a lot of reasons not to just hang two transformers in parallel.

If I had two transformers with a nameplate that claimed they were each 500W and isolation, I'd not even think about paralleling them without making some measurements.

Advising someone who'd ask that kind of question to do it is irresponsible.

Reply to
mike

** No it isn't.
** Absolutely idiotic "straw man" pile of bullshit.
** Then you'd be arguing with yourself.

** Tell me, did you read my posts at all ??

Rhetorical question, the answer is obvious.

** It would be.

But has not happened here.

... Phil

Reply to
Phil Allison

Whoa, whoa wait a minute now, there's an idea. Isn't that what they teach you in electronics school? Always work with a helper, so if you're injured then, maybe at least SOME ONE will know to call for help.

Reply to
mogulah

All too often not possible, I'm afraid.

Reply to
Cursitor Doom

Testing polarity is easy. Wire the secondaries in series. You will either get the sum or difference. Identical windings will be zero. Wire them to get the sum. You now know the phasing.

Two identical secondaries can be paralleled.

Reply to
Ron D.

You talk about primary and secondary but there are no such markings on the trannies. I guess given the 1:1 windings ratio the sec/prim aspect simply doesn't matter?

Reply to
Cursitor Doom

Actually in most cases it does matter. For one there will be loss, so that will be copensated. Also per a discussion over in SED a while back, there is soe other differennce between primary and secondary that has to do with the flux or something.

But it doesn't ALWAYS matter. One way to tell would be to try it both ways and see which way works better, like less current drain with no load or higher output voltage. Invariably, the wrong way won't work as well some how.

Reply to
jurb6006

** So called 1:1 transformers normally have non identical windings.

There is a primary and a secondary winding, with the secondary voltage somewhat higher than the primary under no load conditions, coming down to match primary with the rated load attached.

Wired back to front, the output voltage will be low by around 10 to 30% under load.

... Phil

Reply to
Phil Allison

Typically the primary is the winding closest to the core for power applications. You can sometimes determine that physically.

Reply to
Ron D.

OK, thanks for all the tips, gents. I shall carry out some tests!

Reply to
Cursitor Doom

Well, well. You're right, Phil. Just like you said. thanks!

Reply to
Cursitor Doom

And when you run them the right way they are considerably more efficient.

Reply to
jurb6006

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