Voltage balancing resistors for caps in series-paralell

[Forman313]

Once again im in over my head, and just to make sure it doesn´t get boring its a bit more voltage than im used to. Its still less than what I get from time to time when I cant be bothered to turn various equipment off before I start fiddeling or measuring.

The problem .. Im having a hard time wrapping my head around the calculations needed for a voltage balancing resistor network.

I have some 10.000uF 80v capacitors (eight of them) I want to use so I can do some testing. The rail voltage is +-92vdc... so maybe all I need is to balance 2caps .. times 4. When I have two caps in series with the required resistors, I can just paralell them.

I have been trough a few articles, but it seems caps in series are quite rare. The understanding of and know how, even more so.
Suffice to say, during my search it seems there are more dispute than consensus regarding the matter.

Some (a well known designer) even claim its impossible. Caps in series does not allow higher voltage. WTF?


I know what day it is, so you all probably want some fireworks. But please dont give me a "happy new near" schematic. I just had a couple of 100uf 100v caps blow up in my face. I get the chills just looking at a power button

[dtproff]

Usually I use a 2 transistor circuit for balancing caps. See figure 1B http://www.pes.ee.ethz.ch/uploads/tx...T-PE_2008_.pdf

Tony

[barrymagrec]

It`s not all that rare, most switched mode PSUs use two 200 volt caps in series for the rectified 240 volt input (or votage doubled 110 volts) They are usually balanced with resistors of 100k -220k. For your application the values would be lower, you just need enough voltage divider current to swamp out slight differences in electrolytic leakage current.

[AndrewT]

Originally Posted by Forman313

I just had a couple of 100uf 100v caps blow up in my face. I get the chills just looking at a power button

Use a bulb tester.

[Enzo]

Most SMPS have series caps in the main supply, and certainly there are TONs of guitar tube amps with series caps on the B+. (High voltage supply)

Series caps may be rare in SS amplifiers like that, not because it won;t work, but because there is no need for it. You are a hobbyist wanting to use some caps up that don;t meet the basic spec for the circuit, we've all been there. But if MArantz or someone wants to design an amp, there is no need to double up caps when they can just as easily order the parts that meet the spec in the first place.

[eem2am]

in theory , if leakage currents arent exactly the same, then in theory, one of the capacitors will slowly charge up more than the other............but in order to breach the 200V rating, well, to be honest, the leakage difference is not likely to be that great.......then theres the capacitance tolerance, so the one with the highest capacitance will have the lower voltage, so you just need to use V=Q/C , in the capacitve voltage divider......to be honest, you can snap it together in LTspice, and adjust the leakage "resistor" there so you can model it inn ltspice.

[Mooly]

Originally Posted by Forman313

I have some 10.000uF 80v capacitors (eight of them) I want to use so I can do some testing. The rail voltage is +-92vdc... so maybe all I need is to balance 2caps .. times 4. When I have two caps in series with the required resistors, I can just paralell them.

Resistors are fine as long as the current is magnitudes more than the leakage current of the caps. Something like 10K or 22K 1watt carbons would be OK.

You can parallel as many pairs of "balanced" caps as you wish.

[zigzagflux]

If you apply the ballast resistors in double duty, to perform discharge service in one minute when the supply is turned off, you will address the leakage current issue without having to know what the leakage is.

Simple calculation to discharge to one minute in 4 or 5 time constants. Calculate your wattage required (with margin of 3X or 4X to keep heat down) and make sure the resistor can handle the continuous voltage.