Re: How to determine transformer current

I have a number of power transformers I have picked up and am looking

> to use one for a specific application. Most are completely unmarked. > It is generally easy enough to determine the pinnings through > continuity and resistance checks, likewise by connecting it to a > source, the voltages are easily determined. What I am struggling with > is how to determine the current it is designed or rated for, any > suggestions? > > Thanks, > > Sean Mathias

If you are able to determine the rated voltage, one method I use to identify the approximate current capability is to supply the transformer with the rated primary voltage, and load the secondary until I get a voltage drop of approximately 5%. I then leave the transformer operating at that load level for an extended period of time checking frequently for over heating. This method isn't fool proof, but has worked for me in every instance so far.


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Reply to
Louis Bybee
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When dealing with unknown xformers use extreme caution. Apply a low voltage with a variac and check the secondaries voltage. Years ago I found a unit at work that appeared to be a high current step down type. I applied line voltage and when I measured the secondary I drew an arc and the meter went poof. Turned out I had a microwave oven unit that I knew nothing about at the time. I still break out in a cold sweat when I think about it.


Reply to
Grumpy OM

Usually reckon 25Watts per pound weight, (open frame or toroid).

Reply to
John Jardine

The VA capacity of the whole transformer is determined by the iron size and stack size.

The ampere max capacity is determined by the wire size of each winding to the VA capacity of the trans. Sometimes the wire size can be seen.

Reply to
Gym Bob

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