# How to test a transformer?

• posted on August 30, 2003, 3:30 am
I have a small transformer that I would like to test out with a voltmeter. What should a look for as to resistance? It as rated at 118V 60CY 3.8 Amps. Thanking someone in advance for the help.
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• posted on August 30, 2003, 1:30 pm

Resistance is almost irrelevant. Why do you want to test it?
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http://inquisitor.i.am/ | mailto: snipped-for-privacy@i.am | Ian Stirling.
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• posted on August 30, 2003, 3:18 pm
| IanStirling.

You do not know the secondary voltage, so anything you do is guesswork. You can try the following:
If you have another transformer that puts out 10 or so volts, connect the secondary of the unknown transformer to the 10 volts and measure the primary voltage. If it is reasonable (120 volts or so), attach a 25 watt 120 volt light bulb to the primary to see if it will work.
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• posted on August 31, 2003, 2:16 am

Well, if you are just trying to determine the turns ratio, then you connect your 10 volts from the KNOWN transformer to what you hope and believe in the PRIMARY. You expect about 1 volt out.
To a first approximation the DC resistance is in proportion to the SQUARE of the turns ratio. The higher voltage winding should have MUCH higher DC resistance. (Note that I didn't say the PRIMARY had higher DC resistance. It is quite possible that your transformer was designed as a step UP transformer.

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• posted on August 30, 2003, 2:52 pm

can do other than do a simple continuity check of the primary and secondary winding(s). The continuity test however will not tell you if any of the windings are shorted. Kevin
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• posted on August 30, 2003, 6:20 pm

Without manufacturer's specs you cannot tell, because what matters is impedence (micoHenrys?) not ohms. For example I connected an ohm meter to the AC side of a power brick rated 120V~0.3A 60Hz and it measured .000 ohms, but it works fine. So you cannot necessarily tell with an ohm meter if there is a short, only if there is no connection at all.
PS: you forgot to tell us its output rating.
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• posted on August 31, 2003, 2:08 am
Thanks to all who responded to my question. The transformer is used in a circular flourescent lamp-magnifying glass combo that is about 40 or so years old. The lamp does not light up and the bulb has been checked OK. That leaves the switch (a special switch for flourescent lighting) and the transformer suspect. I guessing that the transformer acts as the ballast for the bulb but I may be wrong. I believe that the output of the transformer is 22 watts. I'm trying to repair this lamp for an individual that is losing his sight.
wrote:

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• posted on August 31, 2003, 4:03 am

It's not a transformer, it's a ballast. By "special switch", do you mean the starter, which is typically a small cylindrical object which fits into a socket, approximately 2cm in diameter by 4cm long.
Does anything happen when you turn the light on?
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http://inquisitor.i.am/ | mailto: snipped-for-privacy@i.am | Ian Stirling.
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• posted on September 1, 2003, 1:19 am
Hi Ian, It is not the typical starter switch that you describe as a cylindrical object. The manufacturer's name on the switch is Gaynor-Andrews of Bridgeport, CT. I did a "Google" and found a Edwin Gaynor Corp.in Stratford, CT. If you go to www.egaynor.com/wd1170.htm , you will find a similar type of switch except for the fact that the old one is a two button switch versus the new one is a rocker. These are made expressly for flourescent lighting. I'm going to contact the manufacturer and see if they can help me. Thank you for your interest.
| IanStirling.

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• posted on September 1, 2003, 1:59 am
By special switch, do you mean the starter? 40 years old sounds like it would have a starter. Yes the transformer is the ballast.
AJW wrote: