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Old 30th Jul 2007, 1:32 pm   #1
Radio_Dave
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Default Testing an output transformer

I have a push pull output transformer which had an o/c primary winding. Luckily the break was near the surface and I was able to solder it.

I measured the DC resistance, either side of the centre tap and there's a difference of about 30 ohms. I know this doesn't mean alot, but because something caused the break I want to test it properly... but I'm not sure how.

If I apply a small AC voltage to the secondary should I get an identical voltage reading, either side of the centre tap on the primary or is there an acceptable variance?

Thanks
David
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Old 30th Jul 2007, 1:48 pm   #2
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Default Re: Testing an output transformer

We had a thread on tranny testing a while back. ISTR the idea was to apply some DC, break the circuit and see if there was enough inductive kick back to light up a neon. This will find shorted turns.

To check ratios, apply a low AC voltage to a winding and measure across all the others. Take care when doing this because if you put 6V on a heater winding you'll get a nasty surprise if you touch the primary Same goes for an output tranny putting volts on the speaker side.

The resistance imbalance on your output tranny is most likely due to one half of the winding being inside the other and so has a shorter length of wire. Not many are bifilar wound. I don't know what a realistic balance accuracy would be but I'd be surprised to find even a 5% error.
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Old 30th Jul 2007, 1:58 pm   #3
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Default Re: Testing an output transformer

As I expect you know the difference in resistance between the two windings is probably down to the fact that the outer winding has longer windings. It's not likely that a winding would go higher in value, but it could go lower due to short circuited turns.

A method of testing for short circuited turns using a neon has been posted in the forums, but it's only really applicable to mains transformers.

Applying an AC voltage to the secondary and measuring the voltages in each half of the primary is as good a method of testing as any, but you'll just have to see if the results "look right". I can't quote a tolerance.

I have a DMM which measures inductance, and I've wondered whether measuring the inductance of windings might show up short circuit turns in the measured winding or those coupled to it. Unfortunately I don't have a transformer with short circuited turns to play with.
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Old 30th Jul 2007, 2:03 pm   #4
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Default Re: Testing an output transformer

Well... I've just applied 1.5v to the secondary and there's 46.8v on one side of the primary and 47.4v on the other. I guess this make a difference of about 1%

Is it safe to assume there's no shorted turns?

Thanks
David
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Old 30th Jul 2007, 2:13 pm   #5
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Default Re: Testing an output transformer

Quote:
Originally Posted by Station X View Post
A method of testing for short circuited turns using a neon has been posted in the forums, but it's only really applicable to mains transformers.
Provided you are gentle so as not to stress the insulation I don't see why the method shouldn't be OK for output transformers too.

Dave, your 1% match suggests that everything is OK. The ratio, about 30:1, sounds plausible too.
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Old 30th Jul 2007, 2:44 pm   #6
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Default Re: Testing an output transformer

Here's the thread about testing transformers:-

https://www.vintage-radio.net/forum/...ad.php?t=14317
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Old 5th Aug 2007, 5:49 pm   #7
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Default Re: Testing an output transformer

Quote:
Originally Posted by Radio_Dave View Post
Well... I've just applied 1.5v to the secondary and there's 46.8v on one side of the primary and 47.4v on the other. I guess this make a difference of about 1%
I've yet to measure a p-p transformer where both sides of the primary are exactly the same - a difference of 1% seems fine to me.
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Old 5th Aug 2007, 6:21 pm   #8
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Default Re: Testing an output transformer

Thanks for the replies

While the transformer is out I might as well work out the turns ratio, in case it fails in the future.

I know how to do it with a single ended transformer, but I'm not sure how to do a push pull one. Do I apply the AC voltage across the whole primary winding and disregard the centre tap?

Thanks
David
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Old 5th Aug 2007, 6:36 pm   #9
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Default Re: Testing an output transformer

David,
If you apply voltage across the whole of the primary you should get the the total primary turns from the voltage ratio. Divide this by two to find where the centre tap should be .
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Old 21st Jan 2011, 11:21 pm   #10
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Default Re: Testing an output transformer

an inductance meter will "mostly" show up shorted turns well..
it effects the inductance on all windings to some extent, and the faulty winding more so.
the exception being a winding with poor insulation that breaks down "under load"
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Old 29th Apr 2013, 1:24 pm   #11
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Default Re: Testing an output transformer

One can take a coil and "ring" it. The waveform will be damped considerably
by even one shorted turn.
They make some test equipment for measuring inductance that also has a built in
ringer function. I have seen circuits one can cobble together and show the ringing
on an oscilliscope. That is quite fun to do and experiment around with shorting out
turns on a coil or transformer to see the effect it has on the waveform.
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Old 29th Apr 2013, 5:01 pm   #12
Herald1360
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Default Re: Testing an output transformer

Hi FrankB- I think that's been covered here:

https://www.vintage-radio.net/forum/...ad.php?t=14317

at any rate for other types of transformer......
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