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Re: [TCML] coin shrinking
Hi Mark,
Unfortunately, electrolytic caps probably won't work very well. Making
an efficient coin shrinker is similar to making an efficient Tesla Coil
- its a balancing act. For a variety of technical reasons, the "sweet
spot" for coin shrinkers falls in the range of 2.5 - 10 kJ of bank
energy, 5 - 20 kV of initial bank voltage, and 20 - 300 uF of bank
capacitance.
Because of the huge forces involved in coin shrinking, it is impossible
to keep the work coil from being destroyed. The best you can do is delay
its destruction until AFTER the coin has been shrunk by taking advantage
of the inertia of the winding and robustness of the insulation system.
We've found (experimentally) that we get best results using a
single-layer 10-turn close-wound coil made using 200C double-build
polyamide-imide magnet wire. Such a coil will "survive" (although
significantly changing shape) for about 50 - 70 microseconds when
subjected to the 30 - 60 kA sinusoidal current peak required for coin
shrinking. It gets squeezed axially, and stretched radially until the
copper is pulled apart and adjacent turns short out.
Maximum shrinking force occurs at the first current peak and is a
function of the square of the peak current. So, the first 1/4 cycle
current peak should occur in less than 50-70 microseconds (from above).
I'd suggest aiming for 25 - 60 microseconds in a new design. This limits
the period of oscillation for the work coil and bank capacitance to
about 100 - 240 microseconds, corresponding to a minimum natural
frequency in the range of about 4 - 10 kHz. For typical "coin size" work
coils, the coil inductance is in the range of 1.4 - 3.5 uH. Wiring,
switch, and capacitor bank inductance may add another 1 - 2 uH, which
limits the "sweet region" for bank capacitance to the 20-300 uF range
mentioned above.
For a given bank energy, using a smaller capacitance at higher bank
voltages may cause premature flashover/failure of the work coil, often
long before the first current peak is reached. An interesting
side-effect is that the perimeter of the coin often becomes markedly
thicker than the interior ("torroided"). Following is a torroided
quarter shrunk by Texas shrinkers Bill Emery and Phil Rembold using a
higher voltage bank:
http://www.capturedlightning.com/frames/gallery/hv1b.jpg
Using larger bank capacitance at lower voltages may permit the work coil
to self-destruct from magnetic forces long before the first current peak
is reached. Again, the coin stops shrinking prematurely and remaining
bank energy instead goes into the coil explosion and plasma.
Although The Geek Group uses electrolytic capacitors for exploding wires
and can crushing, their coin crusher also uses low-inductance film-foil
metal-cased energy discharge capacitors. It might be possible to come up
with a series-parallel combination of electrolytic caps that would work.
As you have mentioned, hockey puck diodes will help protect your caps
from possible voltage reversals that will almost certainly occur when
the coil disintegrates. Make sure that you have at least TWO ways to
safely discharge your capacitor bank through large wirewound or (better
yet) solid pulse-rated ceramic resistors.
Remember that a coin shrinker can kill or maim you in several nasty ways
- DON'T LET IT! Small coil fragments may reach 5000 fps, the "bang" from
the gap and exploding coil are deafening, and the energy lurking in the
capacitor bank can remove pieces of you if you cross it. The bank can
retain significant energy even after the coin has been shrunk. Plan your
design carefully, use adequate blast shielding, use numerous safety
interlocks, and always make sure the bank is shorted when not in use.
You may not get any second chances when you build this device.
Good luck and play safely,
Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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World's source for "Captured Lightning" Lichtenberg Figure sculptures,
magnetically "shrunken" coins, and scarce/out of print technical books
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Mark X2 wrote:
Hi guys,
recently I've purchased 50 6800uF 350V electrolytic capacitors and I was
wondering if I could use these in a capacitor bank to shink coins. I know
the reverse voltage may pose a problem, but I know there are ways around
this. Tips or ideas are very welcome.
Regards,
Mark
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