How to test a microwave transformer

Fred's Appliance Academy
August 4, 2023
Microwave Training

Please be sure that the microwave is unplugged before testing the transformer. You must also properly discharge the capacitor after the unit has been unplugged. Please reference our video on discharging the capacitor.

So here's our microwave transformer, and this is responsible for driving the high voltage side of your microwave. And we have three different windings in here. We have our primary winding, which we can see down here is a little bit thicker gauge wire. It's kind of hard to see, but down in between the two large windings is our low voltage winding, and there's one wire here and an additional wire coming off the backside. And then the upper winding here, it's a little easier to see on this one. It's a much smaller wire. This is your high voltage winding, and this is going to be 2000 wraps of wire, which is going to give us about an equivalent of 2000 volts of output. And that comes out on this little wire here.
If you're familiar with power at all, you know that the power has to return to the source, and that is done through the actual body of the transformer itself. So voltage exits here. The neutral wire actually returns to the case of your transformer, and that's usually done through little tiny screw holes down here. So we're onto ohms. Let me see, ohms, OL. And then we're simply going to take our two leads. We're going to do the primary winding first. I'm going to put it on there and we should have five or less ohms. And here you can see we have 0.4, and that tells me that our primary winding is good.
Next, we're going to take our transformer and I'm just going to turn it to the side so we can see the wire coming off the back here as well as the wire in the front. And I'm going to go ahead and slip one meter lead right here, and then I'll take the additional and put it here. This is a much smaller winding and it should give us less than one Ohm always. If it's over one ohm, you would have something faulty inside of here. This transformer would need to get replaced.
So now we have the transformer facing us again, and we're going to test the high-voltage winding. We're going to put one meter lead there, and we're going to take the other meter lead and we're going to go down right inside that little screw hole there. And we should get between 35 to 150 ohms. And you can see right there we're at about 86.2. So that tells me that the high voltage winding on this transformer is working or should be working. I should mention that you also want to check for obvious signs of failure. If we have any burning or charring or discoloring of the wires or any part of the windings, this transformer has been compromised. It is going to need to get replaced. And that only goes for the front side, but for the backside as well. And this little wire right here, that's the return for the high voltage winding. So the output here and the return again goes to the chassis or body, whatever you want to call it.


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