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Old 12th September 2010, 02:51 AM   #1
ArtG is offline ArtG  United States
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Default Xfrmr secondary phasing

I've pulled an isolation transformer from my "junkbox", and wish to use it for bench testing of various equipment. I purchased it many years ago, but have lost the wiring scheme. The primary is easy to identify (two black wires wired to an AC plug). If appears to have two secondaries, each delivering 117 V. I'm thinking that for maximum current capability, the secondaries need to be paralleled, but I'm wondering how to determine the correct phase. I can think of one way to do that, but I don't wish to try it! Can anyone give me a "tip" on this? The two secondaries are color coded red-yellow, and green-black/white tr. Additionally, there is a white wire that may be a ground, as it shows no continuity to other wires. I remember seeing the procedure, years ago, but cannot locate the information.
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Old 12th September 2010, 03:47 AM   #2
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Dirt simple would be a light duty low voltage ac supply as from an unrectified wall wart or other small transformer. Anything that can supply an Amp would be good. Drive the primary with it and parallel the secondaries until you get sparks. When you do, they're wired out of phase. Try not to touch the conductors when you do that, just to play it safe. Make the primary connections first and then plug in the supply transformer. Over about 20 volts a decent connection to 60Hz can be alarming.

If you have a meter you can use the same supply and when wiring the secondaries in series note which way gives you voltage and which doesn't. When they don't give voltage, you can parallel them without drawing any current, because they are in phase.

Those "tricks" only work for windings of the same voltage, of course. If you have a meter you can check that too.

Last edited by Andrew Eckhardt; 12th September 2010 at 04:08 AM.
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Old 12th September 2010, 07:28 AM   #3
Elvee is offline Elvee  Belgium
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Using a voltmeter is the simplest solution, but if you want to make the test without connecting it to the mains, a special continuity checker can provide a solution:
http://electronicdesign.com/article/...icks-up-i.aspx
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Old 12th September 2010, 09:43 PM   #4
ArtG is offline ArtG  United States
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Andrew; Thanks for the AC wallwart idea! I've figured it out. It's along the same line as my original "brute force" idea but I didn't think of limiting the voltage and current with a wallwart. ("Yes, dear, I know the lights are out--it's probably just a fuse--I'll go check") I'd considered my variac, but that still passes a lot of current. (6A)

Elvee-Great looking little tester! I don't have time right now to build it though. I get sidetracked too easily as it is.
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Old 13th September 2010, 02:15 PM   #5
AndrewT is offline AndrewT  Scotland
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this is exactly when the mains bulb tester comes off the shelf to safely plug your unknown transformer via the bulb to the mains.

You can miss-wire any which way and all that happens is the bulb shines.
If the bulb stays off, it tells you that little current is being drawn. Now measure your voltages.
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Old 13th September 2010, 02:57 PM   #6
AndrewT is offline AndrewT  Scotland
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Thanks Elvee.
That circuit is now in my Audiotest folder.
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