How can we figure out which side of transformer is primary which one is secondary by measuring resistance ? I know that the side where input or supply is provided called primary from where the output is collected is called secondary side in both step-up step down transformersAnd, how can we troubleshoot a bad or a good transformer by measuring its resistance on both sides i.e. primary side secondary side ?thanks
Transformers have no definite primary and secondary, either winding can be used as a primary or secondary. Eg, a 120 volt to 12 volt transformer where the 120 volt winding is usually considered the primary, can be equally used with the 12 volt winding as the primary. This can really be seen in a 120 to 240 volt step up, which can be (and is) turned around and acts as a 240 to 120 step down. If the resistance measured is more than an ohm or two, the transformer has a problem. although some special transformers, such as flyback transformer, the high voltage winding can have a higher resistance. But it should never read open. .
I was wondering the same thing myself today
I agree with Billruss. Some digital ohmmeters have a resolution as low as 0.01 ohm which is typically less than the resistance of most windings on small and medium size transformers. The ohmmeter is useful to identify which leads go with which windings some transformers have several windings. Typically you can connect 1 volt to 6 volts ac to one of the medium resistance windings. Then measure the voltage on each of the other windings. This determines the turns ratio on a good transformer. Applying the low voltage to a different winding will show close to the same turns ratio, unless the transformer is defective. Neil
While you can try measuring winding resistance on both the primary and secondary sides of a transformer with a digital voltmeter or an older volt ohm meter, the value is so low a fraction of an ohm most meters can't measure it correctly. What you can do is look for different wire sizes, the step down transformer will have larger diameter wires on the secondary side of the transformer ( smaller AWG) than on the primary side (larger AWG). A step up transformer will have larger diameter wires (smaller AWG) on the primary than the secondary. Again it is almost impossible to use an ohm meter to determine if either the primary or secondary winding is shorted. This is most likely found by applying power to an unloaded transformer and measuring the excessive current draw.
