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
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.
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.
project has been planned for years.
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
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
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.
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.
The first thing I tried was electromagnetically crushing beer cans. The
cans are wrapped in 5,5 or 6,5 turns of 18AWG wire.
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.
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
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
High Voltage Capacitor Banks:
and a Heavy
at Steve Ward's
at Mike's Electric Stuff
Power Electronics - 333 Joule MOC capacitor bank
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.
This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.
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