Don, what is a balancing resistor?

rictified · Post by **rictified** » Mon Jan 12, 2004 10:55 am

I recapped an SVT and had to series two caps to get the voltages needed, I've had no problems but someone here said that the resistors are a must to prevent over voltage in either cap. Are they just high impedance resistors in paralell with the caps or are they special resistors, and are they crucial?
I have two 220 uf. 250 volt caps in series for 100 uf. 500 volts, also in another section there are two 80 uf. 350 or 400 volt caps for 30 uf. and 600 volts. Do both of these need balancing resistors? thanks.

philco · Post by **philco** » Mon Jan 12, 2004 3:27 pm

They do if one cap develops more leakage than the other one, as the other cap might drop most of the voltage and then develop a catastrophic short due to overvoltage. Power dissipation in the resistors equals voltage squared across it divided by resistance. At 300 volts, that gives 0.9 watts for a 100K resistor, so a 2 watt resistor is the minimum power dissipation you use. The divider network will have 3mA flowing through it. With modern caps, you could probably back off to 1W dissipation at 200K as leakages are lower these days.

rictified · Post by **rictified** » Tue Jan 13, 2004 5:24 am

So can I assume then that as long as they are equal in leakage that there will be no problem and that this is more of a preventative measure than anything? But I also realize that there is probably a slight difference and should I assume that over time the weaker one will eventually fail as you mention with no balancing resistor? And what do you mean by divider network? I remember a divider network as a series of resistors with different voltage drops over the different resistors. Or do I just throw the 1W 200K resistor in parallel with the caps? It's been a long time since school. Thanks Phil.

rhampshire · Post by **rhampshire** » Tue Jan 13, 2004 5:57 am

For the 30uF/600V in my amp, I used two 100uF/450V in series (gives a total of 50uF/900V), with a 2W 220K resistor in parallel with each. The last thing a cap wants to see is overvoltage (KABOOM!), and the resistors prevent that.

rictified · Post by **rictified** » Tue Jan 13, 2004 6:19 am

Thanks both of you, I just found the formula online for the balancing resistors. Fliptops gave me two 80 uf. 450 V caps in series for the 30uf 600 V cap.
I see that I need one resistor for each cap in parallel for the voltage divider.

philco · Post by **philco** » Tue Jan 13, 2004 3:25 pm

It is assumed the two equal value resistors in parallel will have a much higher leakage current through them than the capacitors do, and thus force the voltage drops across the capacitors to remain fairly even. The cap with the lower voltage across it is the one with the most leakage, when leakages are unequal.

rictified · Post by **rictified** » Tue Jan 13, 2004 6:22 pm

Ok I got one for you, (I may change this) I have the approx. 650 volt plate supply going through a 100 uf. 500 volt section in a can, then I put two 220 uf. 250 V caps in series for the the other half of a series arrangement which was already there (was a 100 uf 500V). There is a voltage divider network with two 220K 2 watt resistors in it now. My question is this: Do I take out the 220 K resistor that is in parallel with the two series resistors that I put in, and put in two 110K 1 watt resistors for a voltage divider for this side of the voltage divider, or should I just try to find a 100 uf 500 volt cap somewhere? It is getting a little complicated for me, ahh!!!!!

philco · Post by **philco** » Tue Jan 13, 2004 6:48 pm

These stacked caps work best when they are the same capacitance and voltage.