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Old 20th December 2001, 10:40 PM   #11
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Default Transformer Testing

While we are discussing transformers, does anyone have a good procedure to test an unknown transformer for the correct VA?

I have a large selection of transformers I have collected from microwaves, TV's, kitchen appliances, old hifi gear and more. I have some very big transformers, one in particular that came from an old refrigerator, a 1500VA which is a 230 to 115 by 13 amps, this I know because it is marked on the housing, I am going use this to build a big power amp someday. But most of them have no indication on the transformer itself, and I can only guess from the specs on the original appliance.

I have tried resistor loads and etc, to get rough readings, but if there is a more precise method I would like to know about it.

Surf, Sun & Sound
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Old 21st December 2001, 12:20 AM   #12
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Default here we go

GRollins, thanks for all the info. I'm working off the original Zen plans, so it should be drawing 2A. One more question--each channel draws 2 amps, right? Is this transformer a little too small for both channels?

Just got some big heatsinks from Allelectronics, and am about to get elbow-deep in this....

Cheers,
Chris
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Old 21st December 2001, 12:22 AM   #13
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'Bout the only method I've ever heard of is to load it down until the rail sags--hardly scientific, I know, but...
At least with the big one, you'll have a hard time doing that. You'll also need to arrange a center tap or some kind of floating ground if you decide to go for a bipolar power supply.
(...unless you run it backwards to get a 350V rail for a nice tube amp...shhh! don't tell anyone...Oh, Paul, uh, howya doin'? What? Me? No, sir! I ain't pushin' no tube ideas over here! Not me, sir!)
You could also use it to make a really obnoxiously huge Zen and be the envy of the diy clan.

Grey
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Old 21st December 2001, 12:28 AM   #14
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Ouch! Two channels?
Yeah, that sounds like it's gonna get really hot unless you cut back on the bias. To do so you'd use larger values for the sense resistor in the current source. But I'm not sure (assuming that all that figgurin' I did above is anywhere near correct) that you'd be able to bias the circuit(s) high enough to be worthwhile. I hadn't thought the thing through as far as two channels--I was worried about getting enough current for one. Let's wait and see if someone catches me in a math error that will save the day.

Grey
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Old 21st December 2001, 12:56 AM   #15
paulb is offline paulb  Canada
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I believe the gent did say Zen, no tubes here nosirree.
I would calculate it by assuming you can't exceed the VA rating of the transformer, and you can't exceed the current rating of any individual winding. Keep your eye on that second criteria.
The 115V winding would then be limited to 138VA / 115V, or 1.2A. The 375V windings would be limited to 110 mA, and you could use one or both in series. Feeding 115V in, you're limited to 115V * 110mA, or 12.65VA. Assuming 90% efficiency, you can only get 11.4VA out the other side.
The turns ratio would be 115/375 = 0.31 or 0.155 depending on whether you use one or both 375V windings. So my calcs say:
18VAC or 36VAC at 11.4VA; which using the standard 1.4 / 2 rule would give:
25VDC, 6 watts, 250 mA; or 50VDC, 6 watts, 125 mA.
The problem is that 110mA rating on the 375V winding. You could push it a bit, but not too far.
Hmmm. How about a nice tube amp? They're all the rage these days. Course, for the cost of the tubes, you could buy a transformer...
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Old 26th December 2001, 04:01 AM   #16
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Default Cheap Transformers

you might try http://www.apexjr.com/Clearance.htm

they have a 120 v primary 48VCT 7A + 28VCT 3A secondary transformer for $9.95. They also have a 240v primary version that you could run on 120v if you need less voltage. I am about to order a few for my 5 W SOZ. BG Micro has 8200uF 50V caps for $1 http://bgmicro.com. I am also college student with no money so I know how it feels, but I think I will end up with a $60 5W x2 SOZ because of all my surplus parts.

They have to be better than my stack of modified computer power supplies powering my SOZ. (Modified so they could operate in series.)

Darrell Harmon
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