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Re: [TCML] coin shrinking

On 10/27/13 10:37 AM, Bert Hickman wrote:
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

I use about 25-30 uF charged to 20kV  (5 kJ or thereabouts..)

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.

I use work coils of 10 turns of AWG10 magnet wire, formed around a broomstick. I expect them to not survive.

My cabling is about 2 uH total.
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.

This!!! YOU must have a way to discharge it. Misfires happen. Rebound happens. I have a big 300 ohm non-inductive resistor that is connected across the cap with a SPDT HV relay. When there's no juice on the relay coil, the resistor is across the cap bank.

AND I use a pair of grounding hooks. Removing a hook from the holster opens a microswitch in series with the safety relay (and other things too), so the cap is discharged. 300 ohms limits the current to about 60-70 amps, which is still a fairly loud bang when the relay closes. I've thought about adding some inductance to the discharge circuit to get the current down to around 10 amps.

I use a pneumatically actuated switch for the actual discharge: a couple bolts with one on the end of an air cylinder with several inch throw. The whole switch is behind a shield, so the actuation is via air pressure and a plastic tube.

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,

I run my work coil inside a hollow concrete block with concrete bricks above and below. That seems to contain the fragments fairly well. (A real concrete block.. not cinderbloock CMU, btw).

 the "bang" from
the gap and exploding coil are deafening,

Yes.. louder than a shotgun. It *will* attract the attention of the neighbors.

 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.

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