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Caps in series or....

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captainkirksdog

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Hi,
I know that two capacitors in series are their product divided by their sum, and I know that caps in parallel simply add together. But...here's where I get stuck. Which way will increase the DC working voltage? Let's say I have two caps, each 47uF @ 50WVDC. In series, they would be 23uF, but at what working voltage? Also, in parallel, they would be 94uF, but again, what voltage? I suspect a series configuration would increase the WVDC, but would be unchanged in parallel. Am I correct or have I had a brain meltdown?

working voltage

Which way will increase the DC working voltage?
Series.

When placing electrolytic caps in series, they should be the same value and it is a good idea to connect a high value resistor across each one (same value) to be sure that the voltage divides equally. Capacitors in parallel don't need any special consideration. I might mention, that a reason for connecting caps in parallel is to reduce the equivalent series resistance . That is an important consideration in switching power supplies.

Hi russlk,
Should I use the resistor(s) on replacement caps? I have an old SW radio that uses three 30uF caps @600V and one 50uF @600V. Will the resistor change any aspects I should be aware of? What value would be appropriate? Thanks.~~ckd

captainkirksdog said:
Hi russlk,
Should I use the resistor(s) on replacement caps? I have an old SW radio that uses three 30uF caps @600V and one 50uF @600V. Will the resistor change any aspects I should be aware of? What value would be appropriate? Thanks.~~ckd

Yes, you should use balancing resistors on any capacitors in series - the usual reason for using electrolytics in series is to increase their voltage rating. As capacitors have wide tolerance ratings, and their values will change over time, one capacitor will always have more voltage across it than the other one - this capacitor will have a greater tendency to fail.

A common use was in PC and monitor PSU's - two capacitors are commonly used, switched in parallel for 110V and in series for 230V. I've seen monitors where the balancing resistors were not fitted (the PCB had spaces for them) - they didn't last a year!.

As for values, you can work out the dissipation and plan the values and resistor wattage accordingly - I would advise at least 1W resistors, around 100K should be fine. This also gives the added bonus of discharging the capacitors when the power is removed - a much more friendly way of building power supplies :lol:

Nigel wrote:

A common use was in PC and monitor PSU's - two capacitors are commonly used, switched in parallel for 110V and in series for 230V.

in PC and monitor supply the capacitors connected always serial, each cap. charged up to 160V, because in 110V mode the switch make a voltage doubler from bridge rectifier...

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