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I'm setting up a microwave transformer for the purpose of high-voltage woodburning

I'm trying to validate the output voltage that I'm getting before proceeding with the project. I'm measuring the output voltage with a multimeter and three 1 MΩ resistors in series. I'm measuring the voltage across one resistor since I'm expecting up to around 700 V, which is 300 V shy of the 1000 V limit of my multimeter.

Measuring the resistance of the two white wires, and the red wire, the two white wires have near 0 Ω resistances (assumed that's the secondary winding), and the resistance from the white wires to the red wire is around 1.5 MΩ.

When measuring the voltage across each possible wire combination, I'm only getting a maximum output voltage of 4.2 V (1.4 V per resistor, so 4.2 V in total). I don't understand why I'm not getting 700 V. I wonder if either my transformer is damaged, or if my primary and secondary windings are incorrect.

For reference, my input voltage is 120 VAC at 60 Hz.

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    \$\begingroup\$ Well, if you had primary and secondary the wrong way round you would only register a few volts but, unless you are planning to exercise your right to prove Darwin's theorem on the survival of the fittest (negatively) I'd say you stop this right now and get some local expert help. \$\endgroup\$
    – Andy aka
    Commented Feb 27, 2022 at 16:25
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    \$\begingroup\$ Just looking at those bare conductors makes me feel nervous. \$\endgroup\$
    – Michael Harvey
    Commented Feb 27, 2022 at 16:45
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    \$\begingroup\$ (c) Looks like your using convention low-power low voltage resistors. I doubt they will withstand 700V. I'd limit any resistor's voltage to approx 100V max. So use more in series. Or, special high-voltage resistors are sold if that's what you really need. \$\endgroup\$
    – Rich S
    Commented Feb 27, 2022 at 19:24
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    \$\begingroup\$ (h) Do not work on it live, even if to probe it, or clip a wire. Always shut it off between every change, no matter what it is. Working on HV circuits is slow work, & not for the impatient. \$\endgroup\$
    – Rich S
    Commented Feb 27, 2022 at 19:46
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    \$\begingroup\$ For Lichtenberg figures, you typically need a much higher DC voltage, and don't need all that lethal current that MOTs can supply. Typical wood is also likely to be too conductive to get proper figures, you would need to dry it thoroughly and select appropriate species. Make a low stage count Cockcroft Walton mutiplier driven from an ignition coil - it stands a better chance of generating the figures you're after, and is much much safer. \$\endgroup\$
    – Neil_UK
    Commented Feb 28, 2022 at 9:16

2 Answers 2

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The two white wires are the 3 to 4 V 10 A heater winding, so 1/3rd of that sounds spot-on. The 2 kV output is the red wire with respect to the case, which should be grounded.

The few volts you're measuring on the red wire are due to capacitive coupling between the windings.

Please stop before you kill yourself.

You appear to be using standard 200 V resistors as your potential divider. Use a string of at least 20 of those in series (2 kV AC RMS is about 3 kV peak) as the hot end of your potential divider. Use a small resistor to ground to give you at least 100:1 voltage division ratio, and measure that, with respect to ground. Better still, use a proper high voltage resistor or probe. Better still, please stop before you kill yourself.

Please note that a microwave oven transformer output voltage is enough to jump through dry clothing (which mains can't do, which is why mains is relatively safe), and is high enough current to stop your heart in a moment (which a neon sign transformer, burner igniter or car ignition coil is unlikely to do, play with one of these if you want high voltage). Seeing that to operate your mains switch, you have to pass your hand under a loop of wire which for all you know has 2 kV on it gives me the shudders.

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    \$\begingroup\$ You can just see the tag on the cold end of the HV winding riveted to the case in your middle picture. It's like that for safety. It's so that all HV stuff is referenced to a ground. You will sometimes see references on the net to removing that connection to float the HV winding. Don't do that. The fact that you have used ordinary resistors, and hung your meter across the middle of the divider rather than the groundy end shows me that you don't have the experience to handle the needed safety precautions. Leave one end of the HV winding solidly grounded. \$\endgroup\$
    – Neil_UK
    Commented Feb 27, 2022 at 16:48
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    \$\begingroup\$ This is a suicide rig if I ever saw one. \$\endgroup\$
    – Michael Harvey
    Commented Feb 27, 2022 at 16:49
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    \$\begingroup\$ @Jay I think you should understand that you're way out of your depth, and a funeral is just one trivial accident away. You can burn wood with a car ignition coil. It may take longer, but you can f%%up with an iggy coil and live to tell the tale, you don't get a second chance with a microwave oven transformer, it's just the wrong tool in the wrong hands. I can't teach you how to be safe. You either take an electricians course, or do it the amateur way and get shocked multiple times to learn what you have to learn, but from safe sources like iggy coils, not from 300lb gorillas like MOTs. \$\endgroup\$
    – Neil_UK
    Commented Feb 27, 2022 at 17:49
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    \$\begingroup\$ @Jay There are some high voltage forums on the net, 4HV for instance, that might be better for getting into conversation with a lot of people that do this sort of thing safely, This site has a Q/A ethos which does not really lend itself to the conversations you probably need. \$\endgroup\$
    – Neil_UK
    Commented Feb 27, 2022 at 17:52
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    \$\begingroup\$ My father was a radio ham, and when I showed an interest in the topic, and in electricity generally, he told me about a person he knew who had died. He had built a transmitter with a 200 watt power amplifier stage where the output tube had an anode voltage of 1200 DC. While tuning-up he came into contact with the tiny zero-adjustment screw on the milliameter measuring the PA plate current (around 200 mA). The post mortem found the mark on the back of his hand where he must have lightly brushed the screw head. An MOT supplies 2 kV RMS approx, or 2.8 kV peak, and much higher wattage. \$\endgroup\$
    – Michael Harvey
    Commented Feb 27, 2022 at 23:03
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You could add a auto transformer in primary to reduce the voltage by half or one fourth to begin with. That should do the trick.always have some one around you with instructions in case you have an accident..

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    \$\begingroup\$ Rich S suggested using one tenth: easier to follow, less dangerous. \$\endgroup\$
    – greybeard
    Commented Mar 19, 2023 at 6:27
  • \$\begingroup\$ Yes, a small 12 VAC transformer on the primary should get about 220 VAC on the HV secondary if the transformer is rated 2200V. Another safer test method would be applying your 120 VAC to the HV secondary, and reading much lower voltage on the primary. Then you know the winding ratio. A GFCI mains device may provide another layer of protection. \$\endgroup\$
    – PStechPaul
    Commented Mar 19, 2023 at 7:37

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