How to test a transformer?

[AJW]

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.

[Ian Stirling]

Resistance is almost irrelevant. Why do you want to test it?

[Kevin Ricks]

Without having the manufactures specifications or data there is not much you 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

[Paul A]

Stirling.

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.

[David Efflandt]

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.

[AJW]

[John Gilmer]

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.

[Ian Stirling]

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?

[AJW]

Stirling.

[George E. Cawthon]