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balancing resistors


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4/21/2003 9:25 PM
Dr. Blues
balancing resistors
here's a question for the cognoscenti. i want to use 2 series 180uf @ 400Vcaps in series for the first caps in a power supply filter. the voltage off the rectifier is 600V. according to the cornell dubilier site, (see page 13, http://www.cornell-dubilier.com/appguide.pdf )  
this equation is used to calculate balancing resistors:  
For 2 capacitors in series: R = (2Vr - Vb) / (0.0015 C Vb)  
 
For n > 2 capacitors: R = (Vr - Vb/n) / (0.00075 C Vb)  
 
 
where  
R = resistance in Megohms  
Vr = max rated surge voltage  
Vb = max voltage across entire bank of caps  
n = number of caps in series  
C = capacitance in uF  
 
using this formula the resistors should be 1.2M. i don't recall seeing any circuits using such high balancing resistors. usually they're 220k. e.g., the aa270 super reverb uses two 70uf @ 350V and two 220k balancing resistors. with a standby voltage of 510V, by the formula above the balancing resistors for the aa270 should be 3.5M.  
 
is there another formula to calculate the balancing resistor value? should i use the calculated value or go with convention?  
 
ciao...dr. blues
 
4/21/2003 9:54 PM
R.G.
That formula is intended to tell you the *maximum* resistance you should use to let the resistors swamp the internal leakages well enough. It's perfectly permissible to go lower than that.  
 
I tend to approach it from the other way. Bleeder resistors are a good thing for a hacker to use. The smaller the resistor value (that is, the more current the resistors draw) the sooner the caps have run down to a safe voltage level when you turn the switch off.  
 
So I tend to say "how much static bleeder resistor power can I stand in there?" and put in something that dissipates about that much. Practical values are about a watt. So for a cap with 300V across it, the resistor would be (300**2)/1W = 90K. So if I put in a 100K 3W power resistor across each one, and I'm done.  
 
Doubling the resistance cuts the power in half, so a 200K resistor would be about half a watt, and you could use a 1W resistor, or two half watt resistors in series or parallel to get the same value.  
 
Anything between 100K or so up to the value the formula gives you will do a fine job of equalizing the voltage across the caps.  
 
R.G.
 
4/22/2003 4:45 PM
Dr. Blues

R.G.,  
 
thanx for the response. very helpful.  
 
ciao...dr. blues
 

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