Author Topic: 3kJ coin shrinking  (Read 127 times)

Offline davekni

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3kJ coin shrinking
« on: October 21, 2021, 05:48:49 AM »
My 2002 coin-shrinker is built into a corner of my garage semi-permanently.  Wired into my house through 1meg array of power resistors, avoiding exposure to lethal current when charging and triggering.  Cap is an ancient oil/paper pulse unit, 14uF at 20kV, which I run at ~20.5kV for ~3kJ energy.  Over 1500 coins shrank in its 19 year life, with a few repairs along the way.

Optimization is different depending on capacitance.  At 14uF, two layer coils are better than the more common 1-layer coils.  Magnet wire is terrible, immediately arcing between layers.  Even with wraps of tape between layers isn't enough.  Enamel cracks as wire stretches, and arc paths develop.  Thin stretchy insulation over stranded wire works best for me (radiation-crosslinked PVC).  Broken strands overlap, so conduct better as wire stretches.  Coil shrinks axially and expands radially as it explodes.  My optimum ended up as 2x9 (2 layers of 9 turns each) of 18AWG.  Started with 4x6. but 2x9 passes through optimum shape (wire close to quarter rim) as it explodes.

A few pictures for comparison.  I haven't made a video.

Original wood containment box, lasted about 15 shots before 2x6 boards split.



After 3 rebuilds, changed to 1/4" alominum cylinder.









Coil voltage at 5kV/div.  Pulse lasts about 17us.  This short pulse time maximizes shrinking, but does leave the outer rim thicker than the center.  Skin effect prevents coin current from penetrating to the coin center.  See above image.





David Knierim

Offline klugesmith

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Re: 3kJ coin shrinking
« Reply #1 on: October 21, 2021, 06:09:19 PM »
Applause!

I agree that coil needs to be matched to capacitor to control the frequency (pulse time).   When there is time to shrink coins with my 52 uF capacitor, I think I will need fewer turns than Dave and slightly more turns than Steve. 

Volts per turn could be a parameter where shrinker designs converge, given their naturally similar coil sizes.
« Last Edit: October 21, 2021, 06:20:57 PM by klugesmith »

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Re: 3kJ coin shrinking
« Reply #1 on: October 21, 2021, 06:09:19 PM »

 


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