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

I suspect that resistor manufacturers rate the max voltage to accommodate a lot of worst-case scenarios that we're unlikely to encounter.  For instance, if you take a resistor that's been operating in a TV chassis is a household environment for 15 years, coated with 15 years of dust and grime, it's going to have a much lower flashover voltage than a brand-new resistor, and the manufacturer needs to anticipate that.  So if we're going to run these resistors so far beyond their ratings, we're going to have to treat them well.  Keep them clean and dry.  

As far as each resistor seeing less voltage than the previous one - no, the voltage across each one, terminal-to-terminal, is identical (one-tenth of the total voltage applied to the string), assuming that each capacitor is the same value.

Regards, Gary Lau

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of BunnyKiller
> Sent: Friday, May 15, 2009 9:25 AM
> To: Tesla Coil Mailing List
> Subject: 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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