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Old 20th March 2003, 04:26 PM   #1
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Default How do I determine what this Transformer is?

I picked up this transformer at the surplus store for $9.95.... they had an entire box of them. It was the biggest transformer that they had, but I'm curious what do I have to do to measure what the specs on it are? I'm pretty electronics noob, so let me know what tools I need and measurements I should take.

thanks,
rob
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Old 20th March 2003, 04:37 PM   #2
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Does it have any writing on the label or on its body? If not, you'll need a meter to measure its taps. How much does it weigh? Multiply its weight by 30 to approximate its VA rating. That's where I would start.
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Old 20th March 2003, 08:04 PM   #3
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Quote:
Originally posted by nania
Multiply its weight by 30 to approximate its VA rating.
Weight in pounds?

wotan... do you have a Variac? I use one of those to figure out power transfomers.

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Old 20th March 2003, 08:15 PM   #4
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Planet 10

Yes, that advice was for weight in pounds. I thought he was from the US so that was my suggestion. I suppose I should have been more specific since kilos times 30 would be too conservative an estimate
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Old 21st March 2003, 05:04 PM   #5
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It weighs about 6lbs.

I was hoping to use it for a 2channel gainclone, but I could always pick up more of these.... now looking at it it has a %.95 price tag on it, and the shop probably had 100 of them. Could I use something like this?

Thanks, Rob.



This is all that is written on it...
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Old 21st March 2003, 05:34 PM   #6
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Well... if you don't even know if it has a 120 volt primary, proceed with caution. But in general, here's what I do:

Use an accurate ohmeter to figure out the windings. Beware of the connection and lead resistance of your DMM. The higher resistance winding will (hopefully) be the primary. Some transformers have a double primary designed for 120/240 operation.

If what you think is the primary is center tapped, that's not a good sign. Hopefully the secondary IS center tapped if you're building a push-pull amplifier. On some transformers you also have to figure out the phase (direction) of the windings. This can be done using say an audio signal generator (which don't usually mind driving an effective short circuit) set to 60hz and experimenting with series/parallel configurations of windings until you get the desired results.

Once you figure out the windings, connect 120 volts ac (or some lower voltage from another transformer if you're being cautious) to the primary and measure the no-load secondary voltages. This will give you the turns ratio of the transformer.

With the transformer running on 120 volts, load the secondary with a power resistor (or combination of them) that will result in about a 0.1 amp to 1.0 amp AC RMS current. Note the voltage reading on the secondary with this load (make it brief if your resistors cannot handle that much power). It's safe to briefly overload the resistors by a factor of 4X if they're large ceramic power types (and they should be). P = V**2/R (V squared divided by the resistance).

Repeat the above test with a load the produces a current from 1.5 amps to 3 amps. Subtract the two readings and divide it by the difference in current:

R = (V1 - V2)/(I2 - I1)

The resulting value is the effective series resistance of your transformer and can be used to calculate its voltage drop at any given load as follows:

Vout = Vnoload - I*Rseries

You then multiply the Vout value by 1.414 to get your DC supply rail voltage.

What this test doesn't tell you is when the transformer will start to go into saturation (overload). Some people estimate the total VA capacity by weight as suggested above, and others use a formula based on the wire gauge of the windings (which can be hard to figure out with some transformers). It can also sometimes be estimated from Rseries but I don't have those formulas handy.
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