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Old 14th October 2011, 03:33 PM   #1
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Default How to determine primary & secondary of a transformer

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)
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Old 14th October 2011, 03:39 PM   #2
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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.
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Old 14th October 2011, 04:25 PM   #3
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Quote:
Originally Posted by Pano View Post
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.

Last edited by kokuhorice; 14th October 2011 at 04:28 PM.
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Old 14th October 2011, 04:33 PM   #4
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A step down transformer has fewer turns on the secondary than on the primary.
Fewer turns = lower DC resistance.
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Old 18th October 2011, 06:35 AM   #5
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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.
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Old 18th October 2011, 08:02 AM   #6
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^that's a mighty small traffo....
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Old 18th October 2011, 01:49 PM   #7
AndrewT is online now AndrewT  Scotland
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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?
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Last edited by AndrewT; 18th October 2011 at 01:52 PM.
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Old 19th October 2011, 06:15 AM   #8
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Sorry, the secondary had 18.4 Ohms.
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