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Re: Cap Safety Question
> Original Poster: Adam <psycho-at-tradewind-dot-net>
>
> I've heard that it's a good idea to put a high value resistor across the
> capacitor, so that the capacitor will discharge safely and calmly very
> quickly after the coil is turned off, and also to prevent against
> "ghost" charges.
Yes.... good idea
>
> How do I determine the value of the resistor (both Ohm and Watt), so
> that maximum safety and maximum performance are both achieved?
The electrical code way is to figure out what will discharge to less than
50 Volts within 1 minute. So, you can calculate the required RC time
constant as: RC = 60/ln(V/50), or a TC of about 10 seconds for 15 kV. If
your cap is .01 uF, the resistor can be as big as 1000 Meg, to "meet code".
You can see that for small caps, the leakage resistance is probably enough
to discharge it.
For a bigger cap though, (say 10 uF), you need a correspondingly smaller
resistor (100 K for the same voltage). That 100K resistor is going to
dissipate a fair amount of power (2.2 kW -at- 15kV)
>
> And, of course, where can I get these resistors and how much should I
> expect to pay for them?
Surplus is the only cheap source. HV resistors are expensive ($100) brand
new.
You can get a whole batch of 2 Watt resistors (500-1kV each) and string
them in series, trading your time for the money.
or build a water resistor (http://home.earthlink-dot-net/~jimlux/hv/rwater.htm)