transformer maximum current

I'm in the need of a power supply and just found two old transformers of the right voltage in an old box. What I don't know about them is the maximum continuous current they can source. Not knowing also any data about the two windings turns count, could I get at least a rough value by measuring the secondary winding wire diameter? Thanks.

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asdf wrote in news:iiBKo.8978$

The wire diameter won't help you much if you don't know the turns count. (Or one turn and turns ratio, it's the same thing). Even though copper is expensive, specifying exact thickness for current won't be done without taking into account the temperature derating and cooling in whatever mount is used, so for safety it's usually easier and cheaper to be a bit generous with the copper.

If you have a multimeter, and not much else, you can find out empirically if you also have a heating element from a bar fire, the kind with a coil of resistive wire in a quartz glass tube. (Or any kind of bare resistive wire so long as it's made for a larger voltage than your transformer secondary winding.)

A transformer for a given voltage should put out more when unloaded, so connect one wire to one end of the heater wire, and bring the other to a point some way from it along the wire, and keep trying new contact points toward the connected end until you see the real voltage equal to the nominal voltage for the transformer. You might also be ok with a voltage drop to 90% of nominal voltage for more current if you have cooling air around the transformer but any lower and you'll be overdoing it. Read the current you get at the chosen best point, and leave it running to see how warm the transformer gets. If it ever gets too hot to keep your hand on it in comfort, it's too hot.

Those observations might sound unscientific but I based them on observing transformers inside existing equipment during the years I had no money or test gear, so if you do this you'll likely get results as good as by calculation. Calculation isn't easy either, especially without knowing a lot more about the transformers, so you might as well try it experimentally into a variable load as I described.

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** Just measure the primary resistance - it is a very good guide to the VA rating of any mains tranny.

For a 230 VAC tranny, 60 ohms = 50VA and 8 ohms = 225 VA.

Once you know the VA, the secondary current rating is simply VA / voltage.

..... Phil

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Phil Allison

where do these numbers come from?

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Cydrome Leader

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