Transformer help

I have a transformer from a dead microwave. Its schematic diagram shows a primary coil, a secondary coil, and a third one named F. The primary (115 volts) appears to be two or three turns around the core, insulated all the way, with both ends sticking out. This primary winding is sandwiched between the other two coils, both of which have many windings. One of these coils has two connectors sticking out of the windings. The other, with very fine wire, has one connector on the windings, and the other connector is attached to the frame(why?), with the other end of the fine wire attached to it.

How can I measure the voltage of the outputs? I have a multitester, but if I connect it across the outputs won't I just trip a breaker with no load on the output?

There is a series of numbers on the transformer's sticker but I have no idea how to translate those into something meaningful. (YEC DPC OBJY2 YN-685)

Thanks for any help.


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Ted, I think you are wrong about which coil is the primary. I am assuming that your transformer is from a microwave that does not have a inverter in it .

For a conventional micrewave over transformer, the coil that only has a few windings is for the filament of the magnetron ( F for filament ). The coil with the very fine wire is the high voltage and the other winding with maybe one or two hundred turns is the primary.

Since microwave oven transformers produce high voltages with the ability to supply relatively lots of current, they are dangerous to test with the full 115 volts on the primary. I would use a much lower voltage on the primary( maybe 5 volts AC ) and then measure the secondary voltage. And figure what the secondary voltage is. If you put 115 volts on the primary, you are likely to fry your multimeter.

If you put 5 volts on the primary, that is about 1/23 of 115 volts. So if you measure 50 volts on the secondary, the transformer will have

1150 volts on the secondary when connected to 115 volts on the primary.

Be careful. Dan

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Get 2 MOTs. Measure the voltage of the F winding and connect it to the primary of the second MOT.It should be less than 5 volts under load, IIRC.Then measure the output of the second MOT HV winding using a meter capable of measuring up to 10KV. Just to be safe.

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Since you need to ask this question you do not know enough about electricity to be playing with LETHAL transformers. DO NOT RISK YOUR LIFE.

Reply to
John G

This would be a good question for or sci.electronics.basic. I would warn you about the off-topic political posts on, but they're only the _second_ worst newsgroup in that regard that I subscribe to.

The HV secondary has one connector attached to the frame because the frame is grounded and the microwave is either operating on a single-phase rectifier circuit, or with a voltage-doubling rectifier circuit (probably single-phase, they're incredibly cheap).

Get a copy of the ARRL handbook, it'll have some good pointers on HV safety.

What are you trying to do that you need 1200 volts?

Reply to
Tim Wescott


Nothing really. Your comments, and others', made me decide to be not such a packrat and toss the thing.

Thanks for you input.


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It's a small world; I just pulled that *exact* same model tranny out of a microwave this morning (ISTR it coming out of the Kenmore I dismantled), down here in Eugene.


Reply to
Jon Danniken

Whoa! DON"T PANIC !!

Microwave oven transformers are kind of neat things. The primary and secondary are loosely coupled because they are not wound one on top of the other. That makes it easy to take off just the high voltage secondary.( the one with many turns of fine wire ). Once you do that most of the danger is gone. Not that 115 volt house power is danger free, but it is not the same as about 1200 volts at close to an amp. While you are at it take off that filament winding.

Now you can wind a few turns of heavy wire and have a source of low voltage.


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What a coincidence. I just pulled your transformer out of the trash can ! JUst what I needed Thanks

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"John G" wrote in news:cwZId.30$IT2.2317

It's his life, let him risk it!

The ultimate result of shielding men from the effects of folly is to fill the world with fools. ? Herbert Spencer (1891)

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H.C. Minh

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