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.
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
will 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.
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. 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 thousand 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 rubber stoppers. 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.
Crushing and Skrinking stuff
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.
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 much weaker than those which are usually used, but some degree of shrinkage should be
possible.
Can Crushing Video
Update 01.08.2014
While cleaning old projects I came quite close to sending this one to
the dump before thinking better of it. Steve Ward was able to skrink
coins and tear cans apart using less energy than this bank can contain,
so I should be able to do the same with my capacitors. After looking
over the construction with new eyes I saw some potential for
improvement. For one, the bank and switch could be redesigned for lower
inductance and resistance, simply by rearranging things and using solid
copper conductor. In the previous design multistrand wire and poor
solder connections were abundant. To remedy this I opted to integrate
one of the bus connections with the switch, to reduce the length of
conductor needed, and simulatnously the inductance of the connection.
The multistrand conductor was removed altogether, and replaced with
thick solid copper. The distance between the capacitor bank terminals
has been reduced as well. To reduce the contact resistance proper screw contacts have been soldered on.
Unlike with the previous capacitor bank, I decided to characterize this
one to get some numbers for simulation. The test setup consisted of a
SCR, current transformer, and external damping resistor. The HV switch
was closed with a washer to short circuit it (in the nominal closed
position it is spaced less than 1mm, too wide for 60V to jump across).
With this
simple setup all parameters of the capacitor bank can be determined.
The capacitance was measured using my LCR meter. The leakage
resistance, RL, can be determined by working backwards,
realizing the feed resistor and the leakage resistance create a voltage
divider. Once the capacitor bank has been charged to a known voltage,
it can be discharged through the SCR, and the resulting current
waveform captured on a storage scope. The frequency of the current
waveform will only depend on the capacitive and inductive components of
the bank, and with the capacitance known, the inductance can be worked
out using the formula for LC resonance frequency. The dampning of the
waveform will depend on the resistance of the circuit, which consists
of the conductor resistance, contact resistances, and also the external
damping resistance. By noting the peak current and using Ohm's Law, the
system resistance can be determined, and the bank resistance after
subtracting any external resistance. I've placed all the required formula in a spreadsheet for easy use.
Characterization schematic, and captured discharge waveform.
Difficulty reading the voltage of the bank, could instead use op-amps
and voltage dividers of defined ranges, with LED indication of each
voltage threshold. Would allow for consistent voltage between shots,
and hopefully avoid overvoltage situations. Also a means of automatically
stopping the charger once the main switch is thrown. As it stands, I need
to manually stop the charger, then throw the power switch. In this time,
the capacitor bank can drop several kV losing much potential energy!
Stronger capacitor charger, the previous model was just barely able
to bring the bank to full charge.
Determine how the number of turns is calculated. The inductance must
have some impact, of perhaps just the number of turns? Unsure if some
kind of inductance matching is needed, or if some sweet spot in terms
of current rise is present. The naive idea would be that many turns =
stronger magnetic field = more crushing force. Only draw back is how
much current the wire can carry. Maybe the coil resistance needs to
match the ESR of the bank, for maximum power transfer/impedance
matching? Simulation seems to show that a lower inductance gives a much
higher current peak, and shorter pulse duration. Prime area seems to be
2μH for this bank configuration.
Update 01.08.2014
This update is actually from 09.05.2022, as I saw while looking through old projects that I had performed more work than I had documented. It seems I got around to preparing a work coil for coin shrinking, but had problems charging the capacitor bank. The charge regulator circuit and SLR unit weren't able to charge the bank. The SLR driver would latch up after a few cycles, leaving the capacitor bank to discharge slowly. I then attempted to charge the capacitor bank without the charge regulator, not knowing that the SLR driver was more than capable of quickly pushing the voltage beyond 8kV. This resulted in the catastrophic failure of one of the capacitors in the bank. In hindsight obviously a bad idea, but things get rushed in the lab sometimes. I think using an optocoupler and a better synchronized enable/disable function in the SLR driver might have solved the issue with the latch-up. On the other hand the voltage detection circuit might have been what was latching up. Either way, putting some more time into testing and improving the relatively cheap regulator circuitry would have saved my capacitor bank. An important take away for next time!
After measuring 50-ørings again using a digital caliper, some shrinkage was in fact detected in the coin from the original experiments! The stock 50-ørings measured 18.37mm +/- 0.03mm in diameter. The coin which had been through a few shots in the shrinker measures 18.09mm. Not much, but enough to show that the principle worked.
What happens when the entire bank discharges into one capacitor.
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.
This work is licensed under a
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
