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

[bartb <bartb@xxxxxxxxxxxxxxxx>]

Hi Greg,

The voltage rating is a "working voltage" rating and is not a voltage 
standoff rating from end to end. Similar to a single wire conductor, the 
working voltage of a resistor is determined by the insulation, or in 
this case the composition, however it is standard practice to rate the 
working voltage at 1/3 breakdown.

However, 1000V across a 1/2W carbon resistor is too close for comfort in 
my little book horrors. You probably should double up or seek out HV 
resistors. An ounce of prevention....

Take care,
Bart



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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