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Re: [TCML] Why Don't Bleeder Resistors Fail?

Hey Greg...
One good start is to think of the resistors in series as a 10 segment spark gap with each gap having a 3/8" space, thats alot of "gap" to jump as far as flashing over being a problem.... the 10Mohm value of each resistor is additive in a series situation...thus lending itself to be equal to 100Mohm of value. From what I remember from my EE classes, there is a voltage drop between each resistor and that voltage drop becomes additive also, thus leading to each resistor "seeing" alot less voltage at each succesive resistor as you get towards the "negative" end of the supply. In high voltage applications, there are resistors that are 2" long to keep flashover problems to a minimum ( commonly known as "tigerstripe" resistors). 


Scot D



G Hunter wrote:


All,

I'm building a new MMC to replace the one that was ruined by a garage flood in 2005.  The specs are seven series strings of ten, 68nf/1600v film & foil polypropylene caps, for a total of 47nF @ 16kv peak.  This isn't a very high voltage rating, but it's for a MOT-based SGTC that will charge it to only 10-12kvdc, so it will do.  The new MMC, like the old one, will employ 10M, 1/2W carbon film bleeder resistors across each cap.  I believe this is fairly common practice in the TC hobby community.  Since the resistors are only rated for 350v or so, why don't they fail?  The whole resistor string is only good for 3.5kv.  I would expect it to flash over within seconds.  Yet I can't recall a single bleeder resistor failure in my own experience, nor have I heard any complaints on the TCML.  No big deal really.  I'm just curious.  Any theories?  Too bad film caps are not so grossly underrated!  Would make MMCs much cheaper!

Regards,

Greg
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