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Transformers Amp Ratings?

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5/30/1997 4:56 PM
Mike Oriente	Transformers Amp Ratings? I found a bunch of transformers all differant sizes and outputs, however none of them have an amp rating printed on them.How can I use a DMM to find out what they are?In the past I've taken DC mA readings, on already completed circuits, by putting my meter in series, not sure how to do it with just a lone transformer.

6/2/1997 8:21 AM

Transformers don't really have an amp rating as such. They have a temperature rise rating, and the manufacturers obligingly tell us at what amperage the transformer will hit it's temperature limit.

This seems like splitting hairs, but it's really not. For a transformer with one output winding, the ratings are pretty much directly rated, bot if there's more than that, you can usually trade power between windings to a great extent, until the individual winding gets too hot.

I guess that another way to look at it is that each winding has an amperage limit, determined by wire size, and a temperature limit, determined by both the current in that winding and by the power through the whole transformer.

So - how do you tell? You load up the transformer with a test load, let it temperature stabilize (can be hours for a big transformer) and measure the temperature rise, by measuring the change in resistance of the copper wire inside. Copper wire resistance rises with temperature, and you can read this directly.

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