So, what is the correct way to test a transformer?

[NewYorkDave]

You want to know the correct way to test a transformer for various parameters, or JUST how to test DC resistance of the windings?

If you do use an ohmmeter to check DCR, you may need to degauss the core afterwards. This can be done by injecting a low frequency signal, slowly increasing the amplitude until the transformer saturates, then slowly decreasing the amplitude back down to zero. If your ohmmeter has manual ranging, select the highest resistance range (lowest current).

A better way to check the winding resistance would probably be to do an AC impedance measurement, but at a very low frequency, below the audio band. The result would be higher than the true DC resistance, but probably not by much.

 

[mathtracks]

http://www.rhombus-ind.com/app-note/degauss.pdf

a brief description of transformer polarization.this is news to me.
never realized you could polorize a tranny with a dmm.
are there any other ways to degauss a tranny when this happens?

 

[CJ]

It's really a minor issue. I mean, checking windings for continuity has been going on in all the transformer houses for decades. But, one experiment is worth 1000 bad theories, so I did some testing. Here are some specs for 7 different meters. 5 hand held and 2 bench models. I measured how much voltage they put out and how much current when set on ohms:

Fluke 45 Bench Meter: 1.54 volts @ 810 microamps
Fluke 8010A Bench Meter: 0.990 volts @ 803 microamps

Tektronix DMM249: 0.60 volts at 278 microamps
Tektronix TK1: 0.523 volts @ 500 microamps
Triplett 3303: 0.300 volts @ 270 microamps
Circuitmate DM25L 0.285 volts @ 647 mivcroamps
Soar 3510: 1.267 volts @ 313 microamps

Then I grabbed a Cinemag 610 input tranny and measured the inductance.
7670 millihenries primary. This did not change after using one of the bench meters and one of the handheld meters.

Same thing with the secondary.

I don't have one of the old Simpson/Triplett analog meters with me but you might want to steer clear of those for ohming out a x-former. Not to many people use those anyway.

Bottom Line: Much ado about nothing. Gapped outputs will be even more stable when ohming them out.

As for degaussing, just playing music thru the transformer will slowly degauss them.

Interesting note: I have heard from two sources that say break in a new transformer for 100 hours before you expect it to perform at it's best. One source was Kevin C at K and K, Kevin noting that this is particulary true for amorphous cores.
The other source was Mike at Magnequest. These guys really know their stuff, so I believe it. Breaking them in will rid the core of any residuals.

cj

 

[flyingfader]

[quote author="mathtracks"]http://www.rhombus-ind.com/app-note/degauss.pdf

a brief description of transformer polarization.this is news to me.
never realized you could polorize a tranny with a dmm.
are there any other ways to degauss a tranny when this happens?[/quote]

Hi,
I've found this in our Studer 900 manual
http://site.voila.fr/flyingfader/audio/studer/demag_studer.jpg

regards.

Fly