[TCML] crusher capacitor design

[Bert Hickman]

Ken wrote:
> List,
> 
> I have managed to acquire 72 X EPCOS 500V- capacitors. I want to make a 
> coin crusher using these capacitors. Would it be best to wire these in 
> sets of three of 24 in series for 12000 volts? I first tried it with 
> three sets of 24 in parallel for 1500 volts for a great bang, but no 
> crush. Is the high voltage needed in order to make it crush?
> 
> Here is a link to my other 6000 joule coin crusher that I would like to 
> parallel these new capacitors with for a total of 20,000 joules.
> 
> http://kenscoil.com/coincrusher.htm
> 
> Thanks!
> 
> Ken

Hi Ken,

For good results, use a capacitor bank voltage of at least 5 - 6 kV in 
order to overcome coil resistance and capacitor ESR. Higher voltages 
will also work, but bank voltages above 20 kV may cause premature 
turn-turn flashovers on the work coil. Capacitor internal construction 
is important. You want caps with low ESR and low ESL, and the caps will 
need to handle the expected peak current in order to survive thermal 
stresses and magnetic forces. The "sweet spot" for minimal coin 
distortion seems to be between 3000 - 8,000 joules (depending on the 
coin), 100-300 uF, and bank voltages of about 7.5 - 15 kV.

Putting groups of caps in parallel reduces bank ESR, ESL, and peak 
current/cap (all good), but you may need to connect more groups in 
series to also reach the necessary bank voltage. Some caps may not 
suitable for this application. Which EPCOS caps are you trying to use?

Since this is very off topic on the TCML, please contact me directly if 
you need any further information or would like to discuss further.

Bert
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