I took apart an old microwave oven just to see what happens with the circuit inside it. The overall idea is actually pretty simple, but I'm having a hard time figuring out some parts of the circuit. Here's a picture of the main circuit viewed from behind the LCD control panel:

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At this point, I have figured out that the door switch must be closed for anything to work in the microwave, for safety purposes, obviously.

I have disconnected the magnetron and its transformer from the circuit. I found there are two wires leading to the high-voltage transformer for the magnetron. My main question is, when I'm probing the two wires, (white and red off to the left), I put the GND probe on the common ground (the metal piece to the right), and the positive probe to the wires. But each time I try this, both wires show 120V AC. Does this make sense? I'm thinking that one should be +120V and one should be ground. However, when I probe the two wires (positive probe on red wire, GND probe on white), I get 0V AC. How is the input to AC transformers defined? Both are hot or one hot and one GND? I feel like it should be a full circuit, with positive and ground to the input terminals of the transformer, and a higher positive and ground out from the output terminals.

(All this is done with the power plugged in and I've set the time on the LCD and have started it.)

  • \$\begingroup\$ Are you probing with a multimeter or an oscilloscope? \$\endgroup\$
    – Adam Haun
    Dec 29, 2014 at 4:30
  • \$\begingroup\$ I'm probing with a multimeter. \$\endgroup\$ Dec 29, 2014 at 6:27

2 Answers 2


It's quite possible there is an RC snubber or MOV across the switching device which passes enough current that your meter shows 120VAC. I can't quite make it out from your haywire setup, but it looks to be a relay with spade terminals for the power. It could also be a triac, likely mounted on a heat sink.

In such a case, you won't see the switch functioning unless there is a credible load in place of the transformer. Something like a light bulb would work.

Be extremely careful, even with the lethal high voltage, and potentially dangerous magnetron excised, you've got potentially lethal exposed mains voltage.

  • \$\begingroup\$ Do you mean the black box on the bottom left corner of the PCB board is a triac or snubber or triac? I have also figured out that the transformer is a single phase transformer, but I'm not sure if that's related to anything. \$\endgroup\$ Dec 30, 2014 at 1:02
  • \$\begingroup\$ I can't really tell from your photo, but it looks to me like a relay, something like this one. \$\endgroup\$ Dec 30, 2014 at 3:46
  • \$\begingroup\$ It is a relay, part number DU1PU: ebay.com/itm/… I found that when the switch sitting next to the fuse is held down, (I'm guessing closed), there is 120V at the red wire of the transformer, and 2-4V at the white wire. The white wire is a common ground, I'm pretty sure. But what still confuses me is how when this switch is released (I'm guessing open), 120V goes through both the white and red wires. Any idea why this is? \$\endgroup\$ Dec 30, 2014 at 5:02

The little thing on the bottom left of the board with the red & black wires with spade lugs is the relay that switches the HV transformer. One of the door switches is in also series with it in case the electronic shutdown fails. When the relay is picked up, there should be mains voltage on both wires. The neutral (if you have active & neutral in America), will be connected straight to the transformer, and if the neutral is earthed at the supply, like in Australia, will read 0V. The black thing above the relay might be a MOV (metal oxide varistor). In case you are not that familiar with electricity, that is a voltage dependant resistor, which acts like a symmetrical zener diode to reduce sparking & burning of the contacts by clipping the high voltage spikes generated by the transformer primary when switched off.


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