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Re: Bleed resistors for MMC caps

Original poster: "Scott Hanson" <huil888-at-surfside-dot-net> 

Has everyone forgotten the inexpensive VR37 resistors previously offered by
Phillips and now distributed by Vishay (BC Components)? These are advertised
as "High Ohmic - High Voltage Resistors", and the 1/2 Watt series is rated
for 3500 VDC, 2500 VRMS. These resistors consist of a metal oxide film
deposited on a ceramic core, covered by a hard baked lacquer finish.
Although they cost more than the dirt-cheap garden-variety carbon film or
carbon-composition resistors, the last time I bought the VR37's they were
only 12 or 15 cents each. Given the cost and effort required to assemble a
clean looking and conservatively-rated MMC, why use anything less?

The Vishay datasheet for the VR37 resistors can be seen at:

Note that the peak pulse voltage rating (shown on page 3 of the spec) for a
10 meg resistor appears to be 10 KV! This implies a fair safety factor for
flash-over or arcing.

Regardless of the resistor's actual flash-over voltage, I have seen more
MMC's fail due to flashover to the adjacent capacitor than across a
resistor. If you are mounting your MMC capacitors in parallel rows, I would
leave no less than 1/2" to 3/4" gap between any resistor/capacitor pair and
next resistor/capacitor in the string.

Scott Hanson

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, June 07, 2004 6:29 AM
Subject: Re: Bleed resistors for MMC caps

 > Original poster: "Antonio Carlos M. de Queiroz" <acmdq-at-uol-dot-com.br>
 > Tesla list wrote:
 >  >
 >  > Original poster: "Peter Terren" <pterren1-at-iinet-dot-net.au>
 >  >
 >  > I use 3 bleed resistors per cap.  Others use one standard one and get
 >  > with it. Others use none at all. I am not sure what the right approach
 >  > I don't know anyone who is using the HV ones in view of the cost.
 > I use four in series, or the smallest that I can find, arranged in a
 > zig-zag pattern across each capacitor (or array of capacitors in
 > parallel). So far, no problem at all. I check the resistors
 > periodically.
 > Antonio Carlos M. de Queiroz