RE: Bleeders for a cap bank

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

Original poster: "Loudner, Godfrey by way of Terry Fritz <twftesla-at-qwest-dot-net>" <gloudner-at-SINTE.EDU>

Hi Jason

A cap does not uniformly discharge. The discharge formula is Q = Qo e^(
-t/RC) where

   t = time (second),

  Q = cap charge at time (coulomb),

  Qo = initial cap charge (coulomb),

   e = the natural base = 2.718281828,

   R = resistance of bleeder (ohms), and

   C = capacitance of cap (farad).

You will need a hand calculator to use this equation. There is a more
practical manner of usage. A time constant is defined as the time required
for the charge to decay to 1/e of its initial value. Putting t = RC in the
discharge formula, we get Q = Qo / e. Hence one time constant is RC
(second). People can think in terms of 1, 2, 3, 4 or 5 time constants. Five
time constants is considered good for most caps, except perhaps the monster
ones. So putting t = 5RC in the discharge equation, we get Q = Qo e^(-5) =
0.006737947Qo =(0.6737947%)Qo . So the amount of charge left in the cap is
0.674% of its initial value. Maybe this is considered safe, but I would
still connect a wire across the cap before I would handle it. I work alone,
so I have to play real safe. The calculation for your situation is 5RC =
5x(0.000000068 farad)x(10,000,000 ohm) = 3.4 second which is a pretty good
time interval.

Helpful advise only---you need to go back and get total control over basic
electrical units. Example your 1 farad = 96,000 coulomb is not true. But I
do see what you were trying to get at. And I need to get control over my
spelling and grammar :-)

Godfrey Loudner




  



  

> -----Original Message-----
> From:	Tesla list [SMTP:tesla-at-pupman-dot-com]
> Sent:	Saturday, July 07, 2001 11:28 AM
> To:	tesla-at-pupman-dot-com
> Subject:	Bleeders for a cap bank
> 
> Original poster: "Jason Petrou by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <jasonp-at-btinternet-dot-com>
> 
> Hi all
> 
> I have a pretty standard MMC setup with 10meg bleeders across each cap -
> what kind of time do the resistros take to bleed the caps? This is my
> logic...
> 
> 1 farad=96000 coulombs
> 0.068uF = 0.000068F
> 0.000068*96000 = 6.528C
> 
> at 714.3V per cap, and I=V/R, 714.3/10,000,000 = 0.00007143A thru each
> resistor.
> 
> so, since Q=IT and therefore T=Q/I then T=6.528/0.00007143 = 80 thousand
> seconds.
> 
> which is obviously wrong.... so how long do the resistors take to bleed
> the
> caps?
> 
> Thanks,
> Jason
> 
> Geek # 1139 Rank G-1
> www.thegeekgroup-dot-org
> 
> 
>