Re: Bleeders for a cap bank

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Jason,

There are, as you guessed, a few problems with your calculations:
  - Your capacitor is 0.000000068 Farad and not 0.000068 Farad
  
  - One Farad does NOT equal 96000 Coulombs. A 1 Farad capacitor will store
1 coulomb of charge for for every volt. 
 
  - The discharge current is not a constant as the capacitor discharges
through the resistor
     
As a result, the rate that the voltage declines is not linear, but is
instead "exponential" in shape.
You need to get a better understanding of RC circuit operation and circuit
fundamentals - as a start, check out the "RC Discharge Curve" section of
the following site. You may also want to look at some of the other sections
as well, since they are fairly well written and easy to understand.

http://www.sweethaven-dot-com/acee/forms/frm1002.htm

To estimate the amount of time it takes for the capacitor to discharge to
less than 1% of its original value, you'd calculate the "Time Constant" of
the RC circuit and multiply it by 5. Note that it's completely independent
of the initial voltage across the capacitor. 

So, for your MMC:

    R = 10 Meg Ohms = 1e+7 Ohms
    C = 0.068 uF = 0.000000068 Farad = 6.8e-8 F
   RC = 6.8e-1 sec = 0.68 seconds
 5*RC = 3.4 seconds

Hope this helped,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com
 
Tesla list wrote:
> 
> 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