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Re: Bleed resistors for MMC caps

Original poster: "Mark Broker" <mbroker-at-thegeekgroup-dot-org> 

Terry Fritz did some testing a few years ago on the 10Meg, 1/2W, carbon 
comp resistors (the 10MH-ND from Digikey) and showed that they didn't break 
down for at least a few kV.  As I recall the resistor suffered from thermal 
issues long before voltage issues ;)  The results are buried in the pupman 
archives, but Terry may be able to post a link in shorter order than I....

Basically most people seem to use just a single 10Meg, 1/2W resistor 
without problems.  Some aruge that's pushing one's luck and would recommend 
wiring several in series or using a single HV resistor.  The latter is 
significantly more expensive, though.

The value is not too important so long as the value isn't so small that it 
wastes significant energy nor too big so that you must wait several minutes 
before things are safe to touch (arguably about 20V).  It happens that 
10Meg, 1/2W is a good compromise for 2kV.  I would not recommend using one 
resistor on a 3kV cap due to thermal problems.

Mark Broker
Chief Engineer, The Geek Group

On Fri, 04 Jun 2004 18:29:50 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:

>Original poster: Chris Cudahy <ccudahy-at-yahoo-dot-com>
>   I'm collecting parts for an MMC cap. I've going with
>942C20P15K's for the caps but I'm not sure what to use
>for bypass resistors. The 1/2 watt 10M ohm seems to be
>reasonable but I can't find a resistor rated for 2000
>volts - 350normal/700max is the most I can find.
>   Which leaves me with 2 options:
>a) Wire up 3 3.3M Ohm resistors in series across each
>cap. Hit just under the max overload voltage of 2100V
>b) Wire one 10M Ohm resistor,  (someone has mentioned
>10MH-ND) and wait for smoke.
>   I'd rather not have to wire 3 resistors to each cap,
>has anyone had any failures with option b?
>   (BTW, what the heck is the point of making a 0.5 watt
>resistor in 10M ohm if it's limited to 0.12 watts by
>it's working voltage?)