Power Supply Caps in Series

[sunnydaze]

We often see the main supply caps wired in series to increase the voltage handling capacity. At the same time, a resistor is usually added across each cap to help equally apply the voltage to each cap.

Merlin's site notes the resistor should be 50/C or less and uses an example of two 100UF caps in series each needing a 500K resistor (use 470K). It sure seems like I see 220K - 270K in these spots even when the circuit might be two 220UF caps in series. 50/C of resistance would be in the 1 meg range.

Would appreciate enlightening comments.

Thanks
Mike

[martin manning]

The recommendation is for the balancing resistors to be no larger than 50/C is to insure that the current through the resistors is some multiple of the expected leakage current through the capacitors. Said another way, the impedance of the resistors must be lower than that of the capacitors so that the voltage at the mid-point will be dominated by the resistor current. Some capacitor manufacturer's app notes describe more precise calculation methods, but this rule of thumb is adequate.

For 100uF, 50/C gives 0.5 Meg, or 500k. Using 220k will work for balancing voltages, but wastes a couple of mA. For 220uF, 220/C gives 0.227 Meg, or 227k.

[sunnydaze]

Thanks Martin - for the both the explanation and the math lesson

C in Farads is too big.... Always get confused on the conversions. I guess uf has to be divided by one million to convert to Farads.

Thanks

Mike

[JMFahey]

I guess uf has to be divided by one million to convert to Farads.

Actually it's the other way round

[sunnydaze]

I guess uf has to be divided by one million to convert to Farads.

Actually it's the other way round

OK...

Now you guy's are messing with me.

You do have to divide uF (microfarads) by one million to get Farads:

1 uF = 0.000001 Farads, right?

Thanks
Mike

[JazzGuitarGimp]

Divide farads by 1,000,000 to get uF. Or multiply uF by 1,000,000 to get farads.

[sunnydaze]

Divide farads by 1,000,000 to get uF. Or multiply uF by 1,000,000 to get farads.

Somehow, I've fallen into a parallel universe.

https://en.wikipedia.org/wiki/Farad


1 Farad = 1,000,000 uF (microfarad) or stated differently - 1 uF = 1 millionth of a Farad. - I read that as it takes 1 million uF to equal 1 Farad.

You have to divide uF by one million (or multiply by 10^-6) to convert to Farads:

220uf = .000022 Farads. or 220/1,000,000 = .000022

thanks
Mike

[xtian]

1 Farad = 1,000,000 uF (microfarad) or stated differently - 1 uF = 1 millionth of a Farad. - I read that as it takes 1 million uF to equal 1 Farad.

Yes!

You have to divide uF by one million (or multiply by 10^-6) to convert to Farads:

Totally no!

[JazzGuitarGimp]

Divide farads by 1,000,000 to get uF. Or multiply uF by 1,000,000 to get farads.

Somehow, I've fallen into a parallel universe.

https://en.wikipedia.org/wiki/Farad


1 Farad = 1,000,000 uF (microfarad) or stated differently - 1 uF = 1 millionth of a Farad. - I read that as it takes 1 million uF to equal 1 Farad.

You have to divide uF by one million (or multiply by 10^-6) to convert to Farads:

220uf = .000022 Farads. or 220/1,000,000 = .000022

thanks
Mike

Yes, of course that is correct - I was on drugs or something..... probably wine....

[sunnydaze]

Wine is good for you, Lou! Wish I could have a glass now, but alas it is day time here in Taipei and I'm at work.

Thanks.
Mike