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Old 19th April 2013, 08:01 PM   #1
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Default Testing a transformer without a data sheet

I recently acquired a transformer for which I can't locate a data sheet. I would like to test it but at his point I am unsure of the primaries. The transformer has no serial number but here is a picture of the specs:

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I measure the resistance in the primaries and find that if I assume that brown is the common, orange has 2 ohm and red has 4. I would assume that more copper has more resistance which would make the red the 230V lead and the orange the 115V lead. Am I far off here?

I suppose if I have that much right, it's safe to move forward with the bulb test. I've searched the forums a bit but I haven't found anything suggesting what wattage should be used to do that. Any advice?
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Old 19th April 2013, 09:10 PM   #2
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Judging by the approximate size I would guess 100VA. I think you are correct with your assumption of the primary winding. To prove it, power it up with no load and a 15Watt lamp in series with the primary. The lamp should go out.
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Old 19th April 2013, 09:13 PM   #3
DF96 is offline DF96  England
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You are probably right about the primary.

For a lamp limiter any normal domestic bulb power would do, as all you are doing is covering for a shorted turn.

The total transformer VA rating can be estimated from size and weight, so look at catalogues. Bear in mind that the two secondaries may have quite different ratings e.g. 36V at a few amps for power amp, and 20.8V at, say, 100mA for preamp. Secondary resistance measurements will help.

Yes, I would guess 100VA too.

Last edited by DF96; 19th April 2013 at 09:13 PM. Reason: add VA guess
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Old 19th April 2013, 09:20 PM   #4
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I should most likely contact the seller. It was sold as a 300VA transformer. It doesn't seem to indicate that anywhere on the sticker.

I'm editing now to acknowledge I just said something kind of dumb.... sorry!

Last edited by bramofon; 19th April 2013 at 09:29 PM. Reason: acknowledging brainfart
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Old 19th April 2013, 09:27 PM   #5
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What is the weight? Approx. 3 kg = 300 VA
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Old 19th April 2013, 09:32 PM   #6
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It's 3kg easy (just doing the heft test).
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Old 19th April 2013, 09:37 PM   #7
pilli is offline pilli  France
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You can get an idea of what's primary and what's secondary by putting a very low voltage (which is safe anyhow), say 6V AC, into any winding, and measure what comes out on the others.


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Old 20th April 2013, 05:00 PM   #8
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I hooked it all up today with the light bulb in series. I wired the primaries according to what I learned from previous testing. When I flipped the switch, no light came on so that went well. I tested the secondaries and they seem fine as well.

Thank you all for your input. You were very helpful.

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