How to determine primary & secondary of a transformer

[kokuhorice]

I have a transformer which doesn't have absolutely any labels or markings.
The only thing I know is that the primary voltage is 250Volts.
How do I figure out which one is primary and which one is secondary? (using multimeter or anything else)

[Pano]

If it's a step down, the the secondary will have a lower DC resistance.
You can also feed one side from a little 6V transformer and measure the AC on the other winding.

[kokuhorice]

Originally Posted by Pano

If it's a step down, the the secondary will have a lower DC resistance.

Thanks.
What is the theory behind this?

It's a step down transformer and non center tapped.

[Frank Berry]

A step down transformer has fewer turns on the secondary than on the primary.
Fewer turns = lower DC resistance.

[kokuhorice]

Thanks a lot Pano & Frank Berry.
I was able to figure out the primary & secondary of the transformer.
The primary had 2.5 KOhms and the secondary had 80 Ohms.
It turned out to be a 9V transformer which I used in one my projects.

[Tony]

^that's a mighty small traffo....

[AndrewT]

Yes, probably <6VA and maybe as low as 1 or 2VA.
Could the 80r secondary indicate that the transformer has been designed to survive a short circuit on the output?

[kokuhorice]

Sorry, the secondary had 18.4 Ohms.