Transformers convert voltage for use in appliances. High-voltage or "step-up" transformers turn lower voltages into higher intensities. Appliances like microwaves make use of high-voltage transformers. Because they pose a risk of shock, the transformer must be disconnected, and the capacitors bled, before testing. With a readily available resistance tester, though, the whole process becomes simple, quick and very informative. Knowing how the transformer operates helps understand what the results of the tests mean. It will determine whether the transformer itself is defective, or if it is operating properly.
Disconnect the transformer from its power source.
Allow power to drain from the capacitors. If the unit uses high-voltage capacitors without drain resistors, it may be necessary to short-circuit the capacitors. Otherwise, just allow the resistors to drain power from the capacitors on their own.
Test that the unit has no power with the voltmeter.
Disconnect the high-voltage lead from its tap on the transformer proper. This may involve simply unplugging the wire, or it may require that a set screw be removed. If there are multiple high-voltage taps, disconnect each of them now.
Set the ohmmeter to its most sensitive. Test for resistance between the terminal just disconnected and the ground. The meter should display somewhere between about fifty to about seventy ohms (50-70 Ω) of resistance between these two points. Significant variance from this range indicates a defective transformer.
Disconnect the leads to the input terminals on the transformer, following the same procedure as for the high-voltage outputs.
Test with the ohmmeter between each input terminal. The meter should display a very low reading (close to zero ohms (0 Ω)) between these terminals. Too much resistance between these terminals indicates a defect in the transformer.
Test between each of the input terminals and the ground. The ohmmeter should show infinite ohms (∞ Ω), indicating no connection at all between these points. Any finite resistance here indicates a short circuit.
Disconnect the low-voltage output leads, and test for resistance between those terminals. The ohmmeter should display a low, finite reading (less than one Ω) , as with the input terminals. Too much resistance there indicates a problem with the transformer.
Finally, test between the low-voltage output terminals and the ground. The meter should display infinite ohms (∞ Ω), indicating no connection at all. Again, any connection here indicates a short circuit.
If all the resistance readings seem correct, clean off the terminals and reconnect the leads, then reconnect the transformer. The tests indicate that any problems with the system lie elsewhere.