Ways of Measuring a Transformer

Thread Starter


Joined Apr 12, 2021
i have a bunch of old transformers, different sizes and different shapes, but there aren't a lot of identifying marks on them. i've seen some youtube tutorials that talk about how to differentiate primary and secondary sides of a transformer. i don't know how to determine the winding ratio, though. is the only way to do this by sending a low-voltage AC signal through it and documenting the step up or step down on the other side?
when measuring the resistance across terminals, what does that information tell you exactly?


Joined Sep 9, 2010
The lower resistance side is typically the lower voltage side, since you want more current and less length (turns) on that side.


Joined Oct 2, 2009
Welcome to AAC!

If you are to assume that these are step down transformers then the DC resistance measurement should tell you which is primary and which is secondary. The primary winding would have higher resistance.

Firstly, check to make sure that there is no conductivity between windings and laminations.
Next, check that there is no conductivity between two windings (primary and secondary).
If there are multiple taps on a winding then you have to figure it out by making a note of the DC resistance between taps.

A low voltage AC on the primary is one way to test for voltage ratio.

Finally, the size, weight and construction of the transformer will give you an estimate of the VA rating.
If you were to post photos and state the size and weight we could give you some estimates.


Joined May 3, 2013
You can also have a look at the Lamination Thickness. Thin ones, say < 1/32 " could mean an Audio Output Transformer. May be from those Vacuum Tube Days.

Thread Starter


Joined Apr 12, 2021
i should explain that i am able to identify which side is the primary and which is the secondary - based on the DC resistance measurement, as suggested.
i am curious if there is a way to measure the winding ratio without using a AC voltage source. i am specifically trying to pick out some 1k:8ohm transformers and i don't really have a way of measuring a 1000:8 voltage difference. i also don't have any good idea what to use as an AC voltage source.
subsequently, i was curious what the 1k and 8ohm mean, then, if that resistance isn't what is measured at the transformers terminals.


Joined Mar 14, 2008
i was curious what the 1k and 8ohm mean, then, if that resistance isn't what is measured at the transformers terminals.
It's an audio transformer.
Those impedance values are the relative impedance rating.
With a 8 ohm secondary speaker load, the primary impedance will be 1k ohm (likely for a tube amp).

You could likely use a square-wave oscillator, such as from a 555, running at about a 100Hz with a 1Vpk output to characterize the transformer turns ratio.


Joined May 3, 2013
No, you cannot get the turns ratio without AC voltage measurements.
You can use a simple 110/220 to 6 Volts output transformer and use that as an AC source. Just be careful since the more turns winding can produce 300 / 600 Volts.
Can we have some photographs of the transformers?


Joined Aug 7, 2020
A quick test you can do before attempting any more involved measurements:
Assuming that the primary and secondary windings occupy equal volumes on the bobbin, the ratio of the DC resistances of the windings will approximately the same as the ratio of the impedances (with a big emphasis on "approximately").
The power rating will be approximately the same as that of any other transformer of the same shape and weight.