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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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