testing for the ratios on old opt's

[aardvarkash10]

problem - I have a sackful (well, a dozen pairs) of old se and pp opt's. Look similar to the small hammond and edcor open-frame jobbies, and mostly recovered from old consoles and the like.

To be really useful, it would be good to know their electrical specs - winding and inductance ratio, max DC current, etc.

I figured, hook the secondary side to a 50hz low voltage supply and measure what comes off the primaries to get a ratio. So, using a 230v-12v 4A halogen lighting transformer (a real one, not a switch mode) I have proceeded.

First few seemed ok - small pp units with between 30:1 and 60:1 voltage ratios @ 50hz.

BUT - I'm getting distinctly odd readings on some of the se units, and I'm beginning to doubt my reasoning.

Am I missing something here? Do I have a problem because I am using a 50hz cycle and the trannies aren't able to deal with that?

And more to the point, how whould YOU do this, in the absence of a reliable signal generator or 'scope?

[Steerpike]

SE transformers have an airgap in the core, which will make them behave 'oddly' as they near saturation. You'd need to energise them at lower voltages. 12V into the seondary might be a bit much for them.

[Ty_Bower]

You're on the right track. I usually stick the 12 volt supply on the primary side instead. You'll only get a few tenths of a volt out the secondary, but most meters are good enough to provide useful data. Here's some notes on one I measured:

http://www.diytube.com/phpBB2/viewto...t=3180&start=1

[Stixx]

Funny...
I just did that on an old Saba SE-OPT from one of their "famous" radios.
I am planning to use them in a headphone amp. Putting around 9V
from a small heater transformer on one of the windings gave me a clue on the winding ratio
(what will be something like a 6K:300R impedance ratio).

PS. Of course i have two of them...

[Michael Koster]

Originally posted by Steerpike
SE transformers have an airgap in the core, which will make them behave 'oddly' as they near saturation. You'd need to energise them at lower voltages. 12V into the seondary might be a bit much for them.

Agreed. Applying 12V RMS to the 8 ohm output of an OPT generates
the equivalent signal voltage of an amplifier operating at 18watts
output power. This would take a pretty big SE OPT not to saturate.
Plus, your step down transformer might put out more than 12V when
lightly loaded as in your test. The formula is E^2/R where E is the
RMS signal voltage and R is the design Zout of the transformer.
12^2/8 = 18 watts. 50 Hz should be fine for most OPTs at lower
voltages. I think using ~12V on the primary is a great suggestion
if that's all you have. Measure the actual primary voltage with your
meter, of course.

Cheers,

Michael

[aardvarkash10]

saturation, as I suspected...

THanks for the great tips!