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How to test a transformers maximum output current?

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toli:
Hello Guys,

I have two transformers I took out of an older Sansui amp. I've measured the output voltages with no problem using my DMM. Now I would like to see what the maximum current of each rail is. In the lab at the university we've used a variable transformer to feed the transformer we were testing. What else can I do to test the maximum current of the transformer?

Unfortunately I wasn't able to find info of the transformer online.

djsb:
One way would be to connect a high wattage (50W or more) 10 Ohm wirewound resistor (or several in series) load across the secondary and then measure the voltage across the resistor.

http://cgi.ebay.co.uk/RESISTOR-10-OHMS-5-50-WATT-ALCLAD-1PC_W0QQitemZ350330181524QQcategoryZ39997QQcmdZViewItemQQ_trksidZp3286.m7QQ_trkparmsZalgo%3DLVI%26itu%3DUCI%26otn%3D3%26po%3DLVI%26ps%3D63%26clkid%3D6730205628487125926

At the same time you could fix a temperature probe to the transformer and check that the temperature stays stable. If the transformer is being overloaded it will start to get hot very quickly.

David.

toli:
I dont have a 50W resistor, but I guess I can find a way to burn so much heat. Is there no other way to do so? I'm not to sure as to what temperatures are the SOA of such a transformer, and how well it will dissipate that heat (can't it just heat a part of the transformer and not all of it?). This transformer has 4 secondaries.

Simon:
I asked one of my college proffs this question and he told me to connect the primary to 10% it's rated voltage and then read the amperage on the secondary with a meter (a short circuit) I don't know how accurate this advice was though, the idea of temp sensing is a good one, at the end of the day it is about how hot it gets, as long as it stays under a safe temp that hopefully someone here can tell us I would think you can pull all the amps you want. It would be good to make a testing load for this purpose that can control the current draw. The other thing will be internal resistance of the transformer, if overloaded the output voltage will drop, but at that point your looking at seriously overloaded

Zero999:

--- Quote ---I asked one of my college proffs this question and he told me to connect the primary to 10% it's rated voltage and then read the amperage on the secondary with a meter (a short circuit)
--- End quote ---
Are you sure he meant to short circuit the secondary?

Sounds like a bad idea.

Perhaps he meant increase the load until the voltage drops by 10%

That's not a very accurate method, the regulation factor depends on the size of the transformer and its construction, larger transformers generally have a better regulation factor than smaller transformers.

I think the most accurate method is to weigh it and compare its mass to that of transformers with a similar power rating and minimum frequency - e.g. two 100VA transformer should have a similar mass. Another option is to measure the primary winding resistance and compare it with other transformers of the same construction, primary voltage and frequency, higher power transformers will have a lower resistance. One thing to also note is the construction, toroidal transformers will be lighter than E core transformers and will have a lower primary resistance and if it's potted, it'll be heavier so only compare like with like.  Look in component supplier catalogues for datasheets of transformers which will give specifications such as mass and winding resistance.

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