Re: Bleeder (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 20 Oct 2007 17:14:35 EDT
From: Mddeming@xxxxxxx
To: tesla@xxxxxxxxxx
Subject: Re: Bleeder (fwd)

 
In a message dated 10/20/07 4:28:30 P.M. Eastern Daylight Time,  
tesla@xxxxxxxxxx writes:

---------- Forwarded message ----------
Date: Sat, 20 Oct 2007  12:04:22 -0700
From: Anthony R. Mollner  <penny831@xxxxxxxxxxxxx>
To: Tesla list  <tesla@xxxxxxxxxx>
Subject: Bleeder

I'd like to ask a question  and find out what the common consensus is
regarding bleeder resistors. What  does the group recommend for bleeder
resistors on an MMC bank? 1/2 watt, 5  watt, 1 meg, 10 meg etc.??
Tony




Hi Tony,
 
    :
The Main considerations:
1. Excess heat in the cap bank is your worst enemy.
2. Too long an RC time constant means the MMC can bite you.
 
Here's how to do a quick calculation, to avoid this:
Let's say you have a 15 kV NST with 15 caps and thus 15 resistors in  each 
string. Then the drop across each one is 1 kV. Power dissipated is E^2/R so  a 1 
Megohm resistor would dissipate 1 Watt, and each string would dissipate  15 
Watts as heat! If they were 10 Megohm each, then each would dissipate 0.1  Watt 
and thus 1.5 Watts for each string (much cooler). Power-wise, 1/4 watt  
resistors would be adequate, but their small physical length would lead to  dangers 
of flash-over, thus a 1/2 watt would be safer. You could, of course, go  with 
higher on resistance, but that slows down the safe discharge time at  
power-down, which is proportional to R x C. Using 5% tolerance resistors will  give 
you more uniform results than 10% ones. Higher precision would be costly,  and 
IMO overkill.
 
Hope this helps,
 
Matt D.
 



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