How to test a microwave transformer

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.

[uraclint]

Like so many others, I commend your 'no bullshit, no waffling and no self inflation" method of delivery. Thank you for that. subscribing.

[thesadboxman]

Clearest explanation I've come across so far!

[tomh.]

Clear, , to the point with no b.s chit chat simple as that but too many other utubers just don't get it. Thank you and thumb up for your video.
Actually I have noticed other of your videos have the same format. Excellent

[Gluluman]

very short, clear and to the point ... which it was a simple to grasp for none technical people

[jaykk]

my high voltage was reading 198ohms .

I have no idea what kind of microwave it was, as I got it from a second hand store free, I saw just a bottom plate with a transformer bolted on it, against the wall, asked what's it off he said a microwave asked if he was throwing it away, as I was actually looking for broken fan motors at the time.

and it's 240V .

[reginodelacruz]

What if 0.00 ohms secondary windings connecting to magnetron, is it ok?
Watching from saudi arabia

[mattjenson]

What happens if you plug that into the wall with a spliced 120v cord?

[southerner]

Good explanations for checking DC resistance of windings, but I have seen cases in transformers where an inter-winding short would only appear with the kind of high AC voltage swings seen in actual operation. A DMM will not reveal these.

[DeborahVinti]

Could I have your opinion.Great video BTW Thank You. My Over the stove microwave blows the breaker after 15 secs. On the transformer, Primary leads read 0.8 ohm, terminals to magnetron read 0.4 ohm however High voltage terminal to ground reads 194 ohm ( over the 150 stated in video) Is this the problem or is the transformer good?

[mrbassman]

From the white secondary to the body, I get 127 ohms, but feom the red secondary to the body there is nothing. This means the transformer is bad. Is that correct?
My microwave stopped heating. The first thing I noticed when i took the shell off was the connector burned (its plastic charred) on the white primary wire.

[chrishadjipetris]

So you're saying that the one terminal of the secondary winding touches the core of the transformer, which is attached to the metal case of the microwave. Isn't it dangerous, especially in case the ground wire is disconnected for any reason?

[LeruoSenau]

How do we check if the magnetron is working?

[Wayne-pn]

Not sure but do you have to test both secondary wires to body

[MrDunk]

Shouldn’t there be also be a check for windings shorting to the transformer body? Perhaps using an insulation tester [megger]?

[Rickzabor]

Great video. Will subscribe. I have a GE microwave that has components looking like what you show in the video. All of the transformer reading were within your ranges but 2 wires that were attached to the low voltage side of the transformer burnt off at the connector. I don't see any evidence of windings being charred. Should the microwave transformer still be replaced? the microwave is 10 years old and I"ve already replaced or fixed 2 door switches. Transformers are about $140 and a new microwave about $500. Is it time for a new microwave?

[quintinplaza]

the high voltage side on mine shows 320 ohms, That is probbally very bad

[jimmybonges]

Good day Mr.
I would like to ask you regarding the microwave Transformer.
I had taken out the transformer from the microwave unit and i was doing measurement at the primary, secondary and hv wire winding with digital and analog miulimeter, all are good,
My question is:
Why the transformer makes loud humming noise when I was connected the primary to 220volt ac, is this a symptom that the transformer is going damaged?
Your reply very much appreciated thanks🤝

[denzilharris]

Need a little assistance.
Got 0.5 ohms on the low voltage from pin to pin
Got 0.5 ohms from the two low voltage wires.
I only have one wire coming out of the high voltage side. How would I check that side exactly. Checking from the high volt to the low volt wire the meter reads OL

[Wayne-pn]

So 8 ohms across primary is no good and would that cause blowing a fuse every time you hit start?

[ACommenterOnYouTube]

So my microwave blows the 20A fuse IMMEDIATELY after hitting start.
Opened it up, found the blown fuse, replaced it, thinking we are good, set the time, hit start and the fuse blows instantly again.
I found the schematics inside with troubleshooting instructions, it tells me the HVT is grounded.
So i checked for a ground fault on that leg that goes back to the 20A fuse but NO ground on it, its clean.
I checked the HV side and got .2 ohms across the 2 wires.
I checked the wire that goes back to that fuse for a ground on it, nothing, its clean.
So how is this a faulty HVT when all readings show good and no ground faults on the transformer itself NOR the 120V feed wire .. ?