I'm using a microwave oven transformer for some high voltage projects including testing it out with some voltage doubles. I ran into a rather curious problem I dont seem to understand and I'm just hoping some one can explain to me what is happening. So I built the double like this: enter image description here

When I connect the high voltage AC to the correct spots (in between the caps and in-between the diodes) It appears to double the voltage and the length the spark could jump. I wanted to verify this was the case and so i took the lead connected to the diodes and held it just off the other end of the bottom capacitor (as in the pic). As expected, the spark was half the length. However this is the odd part. My capacitors are ceramic disks rated for 10'000 volts and at most the double produces about 5000 volts. However after arcing for just a few second that spark jumped from the upper most lead on the top capacitor the the bottom most lead on the lower capacity. Thats a gap of about inch! Way more than 5000 volts! So I'm just wondering why this happened? I didn't get to do more testing because the cap that wasnt being used (the top one) broke down and failed, but the bottom one is fine. Can any one explain that happened here?

Edit: I added my own picture of what i was working to be more helpful. The "little spark" is where i was testing to see if the distance/voltage actually increased (and it did, in the configuration shown the spark is half the distance than in the proper configuration). This big "whats going on" spark is, well, the whole question. Also, the "little spark" was much closer than pictured. it was about 1mm but the big spark is just as shown.

  • \$\begingroup\$ Show us a diagram or photograph of your actual circuit, not somebody else's! You can safely assume that most of us here know what a generic voltage doubler looks like. \$\endgroup\$ – Dave Tweed Dec 21 '17 at 14:29
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    \$\begingroup\$ How did you measure it was more than 5kV? \$\endgroup\$ – PlasmaHH Dec 21 '17 at 14:59
  • \$\begingroup\$ probably because you sharp point where you soldered. That generates corona and from there everything goes downhill (or up hill if you want sparks. \$\endgroup\$ – Oldfart Dec 21 '17 at 16:07
  • \$\begingroup\$ I edited the original post with my own picture. I dont have a way to measure voltages above 1kv so its all guess work at what the values are. \$\endgroup\$ – Andrew Dec 22 '17 at 1:13

Sustained arc length is not a reliable measure of voltage- once it starts you can draw the arc out. That's how the classic V-shaped Jacob's ladder works- the heat from the arc draws it up until the arc stops, then it starts again at the bottom. Photo credit from this site.

enter image description here

You can get a reasonably good measure of voltage by using smooth hemispherical electrodes and observing when the arc starts. Sharp points result in a lower breakdown voltage because they increase the electric field gradient.

Don't kill yourself- the currents and voltages from an MOT are potentially lethal. As you probably know, the normal circuit in a microwave oven uses a doubler.

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    \$\begingroup\$ I know what you mean. When i was testing i was using the pointed cut ends of the wire and i could see the corona and it would arc much farther than i expected. After i bent the wires back i got results closer to what was expected. About 1mm in the configuration in the picture and in the proper configuration i got about 2-3 mm \$\endgroup\$ – Andrew Dec 22 '17 at 1:15

Very important notes to take:

  1. There is no formula for struck arc length of high voltage.

  2. The voltage you have is deadly.

  3. Voltage doublers takes away the power factor very quickly. Which means the possible arc length will NOT be at its maximum.
  4. Never touch wires even when the device is unplugged. Discharge the capacitor.
  5. Use a qualified insulator during testing. Also known as a chicken stick.

Please be very very careful at ALL times.

The pointer the wire or conductor that you are using is then more likely the arc will escape through the surrounding air and connect to the end wire.

Here’s a quick way to measure high voltages without expensive equipment. Find a small AC to AC Transformer from 22.2 volts to 107 volts. You can find these in audio equipment at a thrift store. Start at a small voltage hook the correct leads up to the correct connectors. Make sure you are using a low voltage. 22.2 volts is what I used to measure the output voltage of my microwave oven transformer (MOT)

I then took the input voltage divided it by 22.2. Then after that I took the output voltage of the MOT during testing and I multiplied it by the number I got when I took the input voltage and multiplied it by 22.2. Then in the end you will have the output voltage of the MOT.


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