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Microwave Oven Capacitor Bank
Microwave Oven Capacitor Bank

One of the "big HV projects" is blowing stuff up with high voltage capacitors. Now for some really good capacitors, you'll need to dish out a few hundred dollars, if you can even find them for sale. So I was quite surprised when I saw Steve Ward had used common microwave oven capacitors in a capacitor bank. These are the capacitors with 2kV AC, 1µF ratings at 50Hz, making them pretty much unsuitable for any thing interesting. However, Steve found by experimenting that they can withstand up to 10kV DC for a limited time, and have remarkably low internal terminal inductance. So suddenly once useless capacitors appear suitable for some low energy pulse discharge experiments.

MOC bank

Before we continue I hope I don't need to warn you of how dangerous this is. If you don't experience with BOTH electronics and high voltage, then steer clear. One mistake and your heart WILL stop and you lose a lot of flesh. This project has been planned for years. About two years ago I bought 20-something microwave oven capacitors, since I had a hard time finding them. A year after that I cleaned, soldered them together, and made the HV switch. Finally this year I made a small voltage doubler to rectify the "Big-Mofo" transformer, so I could charge the bank.

The capacitor bank itself consists of 20 capacitors averaging 1µF each, and capable of withstanding 8-10kV for a short duration, according to the finds of Steve Ward. The measured capacitance of the bank is 21,8µF, and the estimated energy in each shot is about 700 joules, give or take 100j. I couldn't read the final bank voltage with much accuracy, and I don't know how much it would have sunk while the switch closed. Bank voltage was measured using a 50µA ammeter, with a 200M resistor giving 50µA at 10kV.

HV Switch  can crusher setup

The HV switch was the only critical part of the project, and it serves two purposes. First it switches the bank into the load, and second it switches the charging circuit off of the bank. Leaving the charging circuit connected may result in failure of the rectifiers once the bank voltage reverses (which it will). When switching a 700 joule bank using a spark gap, any electrode surfaces are going to vaporize, and if they happen to be in contact they'll just weld together. So what's required is a heavy duty switch that won't make contact, but get very close, and is able to handle several 10s of thousands of amps. To solve this, I used a metal rat trap, and rebuilt it to slam a copper bar into another bar. The spacing could be easily adjusted using the rubber stopper. The real beauty is that the charging circuit in connected to the holding pin, which is disconnected from the capacitors once the mechanism has fired. The switch is also easy to trigger from a distance. After 10 shots there is some wear on the electrodes, but not enough to impair it's function. The solder holding the copper to the iron? bar is what's showing the most wear.

Worn busbar

Electromagnet can Crushing
The first thing I tried was electromagnetically crushing beer cans. The cans are wrapped in 5,5 or 6,5 turns of 18AWG wire.
Can in deathcoil  Pinched cans

Coin Shrinking
Steve Ward was able to crush some dimes using his setup, so I hoped I could crush some Norwegian 50-ørings, which are 97% copper. I didn't have much luck though, and even after several shots on the same coin there is almost no noticeable deformation. I'll have to investigate this further. The work coil became warm, and would pull in and bulge out a little as described on Bert Hickman's coin shrinking site. Ultimately my bank is too weak, but some degree of shrinkage should be possible.
Coin shrinker coil.

Can Crushing Video

Similar High Voltage Capacitor Banks:
Bert Hickman's Page
Microwave oven capacitors and a Heavy duty Maxwell setup at Steve Ward's page
Destructotron at Mike's Electric Stuff
TeslaDownUnder - Can Crushing
Kaizer Power Electronics - 333 Joule MOC capacitor bank


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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.

Creative Commons License This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

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