Establishing the ratio of AF transformers

[lesmw0sec]

Just remember that if you are looking for the impedance change rather than the turns ratio, it is the square of the turns ratio = Z/Z.

Les.

[SurreyNick]

Originally Posted by Dr Wobble

I have a small battery powered AF sig gen you can have for a fiver Nick

That really is most kind of you Andy. Thank you and I would be delighted to accept.
Nick

[Radio Wrangler]

That's a good start, Nick.

There's plenty of test gear around that won't stretch anyone's means. It also isn't the sort of stuff to sit forever on a shelf. Once you learn to use it in a basic way, you'll find it makes things easier, and being able to see things more directly makes the operation of everything more obvious. You'll find yourself picking up more advanced tricks from all sorts of people.

For example, I rarely use a multimeter on the volts range, I use an oscilloscope because it's accurate enough in most cases, and sometimes seeing the trajectory of voltages changing can give strong clues to what's wrong. I can also spot spurious oscillatins and various things which would be invisible on a meter.

Collecting test gear can be as addictive as old radios

CHeers
David

[SurreyNick]

Originally Posted by lesmw0sec

if you are looking for the impedance change rather than the turns ratio, it is the square of the turns ratio = Z/Z. Les.

Thank you Les I shall bear that in mind.

Originally Posted by Radio Wrangler

Collecting test gear can be as addictive as old radios

I don't doubt it David. I am already fascinated by the very early test gear which I read about in the trade magazines, especially the DIY models.

Nick

[Ed_Dinning]

Hi Col, the lower frequency should have an approx. equal effect on both windings.
In any case we are only looking for an approx. figure to determine the turns ratio, which is usually an integral ratio, high accuracy is not that necessary.

Ed

[ukcol]

Thanks Ed.

[trobbins]

I can't see that you could possibly go wrong with using a 5VAC heater on the 'higher' DC resistance winding. As clarified earlier, any Fluke or even non true-rms meter should give good relative accuracy for turns ratio at 50-60Hz. Even for high turns ratio's the other windings should give a reasonable, albeit low, voltage measurement.

If you have a P-CT-P winding, then apply voltage to one half winding P-CT and check the voltage on each half-winding, and then swap over.

If you can borrow a megger then that is well worth it, although probably don't test above about 250V. At least if you still get more than 10 Meg then the insulation is still probably working. The Emmco Goldenvoice 10kPP:2.5k I have measured 50Meg so appears to be fine. More modern OT's will measure much higher Mohm levels.

If you're really keen, or worried about shorted turns, then measuring the magnetising current across a sense resistor, or putting the magnetising current and winding voltage on a dual trace CRO can be informative.

Spreadsheet may be helpful.
www.dalmura.com.au/projects/OT%20calcs.xls

[SurreyNick]

Thanks. With everyones help I'm feeling pretty confident I will be able to carry out the measurements
Much appreciated
Nick

[Radio Wrangler]

It's only a beginning!

Keep your eyes open for a reasonable scope and an RF sig gen so that you've got them ready and fixed so they're there before you need them.

David

[Diabolical Artificer]

Whilst looking for a suitably low voltage tfmr for Nick, I found quite a few tfmr's lifted from SMPSU's. I havn't had chance to test them yet in regards of what turn's ratio's they have.

Would they be suitable for a short term test as the one Nick means to carry out ?
AFAIK, they are designed to work at higher frequency's that mains 50 hz- problem?

I wondered as they have several taps/windings, Andy.

[Radio Wrangler]

Ferrite cored transformers in SMPS will be intended to work at 25kHz to hundreds of kHz and over 1 MHz today in some. They will be quite useless at 50Hz.

In some serious SMPS you may find a 50Hz iron transformer... just a small one to run things to start up the main beastie.

a LOT OF smps uses flyback topologies and gapped cores so they are really inductors with secondaries. There are push-pull and half-bridge (and full bridge) systems with ungapped transformers, but all ferrite SMPS transformers are specialised, and there cwsn be many output windings.

David

[antenna441]

I came across this site recently while searching for something else. There is a simple ratio tester described.

The Radio constructors Circuit Archive.

http://www.vintageradio.me.uk/radconnav/misc.circuits/

look for:- Transformer Ratio Analyser From Radio Constructor Nov1974

[PJL]

Nobody has mentioned using the heater voltage from another (AC mains) radio.