Re: Bleeders for a cap bank

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Ed,

On 7 Jul 01, at 13:28, Tesla list wrote:

> Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<evp-at-pacbell-dot-net>
> 
> Tesla list wrote:
> > 
> > Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> > 
> > Hi Jason,
> > 
> > Caps will bleed down to 0.674% of their original voltage in 5xRxC seconds.
> > So 0.068uF and 10Meg gives 3.4 seconds.  Which is just right.  Fast enough
> > to discharge the caps before you should be touching them but slow enough
> > not to bother the coil's operation.
> > 
> > The equation is:
> > 
> > Vt = V0 x e^(-t/RxC)
> > 
> > Where:
> > Vt = Voltage at time "t"
> > V0 = Initial voltage on cap
> > e = Natural log function (2.71828...)
> > t = time in seconds
> > R = Resistance in ohms
> > C = Capacitance in Farads
> > 
> > Cheers,
> > 
> >         Terry
> 
> 	I still worry about deterioration of those resistors due to operating
> at much beyond their rated voltage.  Has anyone ever measured any after
> a few hours of operation?  In my experience they usually fail open, an
> unsafe thing if you're expecting them to discharge the bank.
> 
> Ed

I'm using Philips glazed metal film resistors rated for 3.5kVDC 
across my caps (one per cap). I have seen no signs of deterioration 
bearing in mind that they are being used at less than half their DC 
rating. I can't speak for other types but I do know from extensive 
repair experience that carbon comp types are prone to aging and going 
a bit high. Carbon film types are terrible if used beyond their 
ratings and sometimes not so good when used close to their ratings. 
Furthermore, some carbon film types from certain manufacturers appear 
to be more failure-prone than those from other manufacturers.

Regards,
Malcolm