How to Test a High-Voltage Transformer from a Microwave

Testing a high voltage transformer from a microwave with an analog meter.

Note: Before testing any components in a microwave, be sure the unit is unplugged and the high voltage capacitor has been discharged.

The transformer consists of three windings. Your primary winding with the thicker wires, your secondary high voltage winding with the thinner wires, and your secondary low voltage winding that is between the primary and secondary windings.

To test the transformer, start with the primary winding, looking for less than five ohms. I suggest you use R times one on the meter and calibrate. Place your meter leads across both terminals looking for less than five ohms. You’ll also want to check each terminal to ground. The best place to find ground is where the mounting holes, where the screws are on the transformer. Checking to make sure you have no reading. If you ever reading between ground and one of the terminals, the winding is shorted. If you get no reading across both terminals, the winding will be open.

Testing our secondary high voltage winding, we will want to go ahead and turn the transformer around looking for the single wire by itself, or a spade terminal that maybe there. And we will want to go ahead and set our meter to R times 10 and calibrate. We will place one-meter lead on the high voltage secondary winding and the other meter lead to ground, looking for 35 to 150 ohms. If you get no reading, your winding is open. If you get less than one, your winding has shorted.

To test our last winding, the secondary low voltage, which goes to the magnetron, set your meter back to R times one and calibrate. Place your meter leads across the secondary low voltage winding and we should see less than one ohm. Again, if we get no reading, the winding is open, and if we get any reading to ground, the winding has shorted.