# capacitors in series

Discussion in 'Electronic Basics' started by Ken O, Jul 2, 2006.

1. ### Ken OGuest

HI,

I have two capacitors in series. I want to put 35 volt acrosse ( cause I
dont have 50v rated) so I want to use 25 v rated. I know its going to make
50v (and 1/ 1/c1 +1/c2) but they are rated 25v, do I need 50v rating
capacitors or my two 25v will be fine ? I think it should be ok to use two
rated 25v but I wanted to make sure. There are a lot of parallel and series

ken

2. ### Ken OGuest

it is going to work, sorry for the post.
One of the capacitor is rated 1000uF and the other 2200uF, I am just
thinking that one might take more voltage then the other. v=Q/C
proportionally, so 1000uF will have twice more voltage.

K

3. ### John O'FlahertyGuest

If the capacitors are equal, the applied voltage will split equally
across them, and you can use two 50V caps in a 35V circuit. If the caps
aren't equal, the voltage will be higher across the smaller cap. (The
charge is the same on both caps because they're in series, and voltage
= charge / capacitance V=Q/C). So if you had two capacitors with a 25V
rating, and the ratio between their values was greater than 25/10, or
5/2, the rating of the smaller cap would be exceeded. For example, if
you used two 25V caps in a 35V circuit, and one was 5uF while the other
was 2uF, you would end up with more than 25V across the 2uF cap.

4. ### Ken OGuest

I did the circuit like said, I get 35 volt at the bridge, but when I add the
capacitors, I get 50v (49.8v), I was sure I would be getting 35 volts across
both...

ken

5. ### John O'FlahertyGuest

I see you figured it out for yourself. I'd just like to inject a note
of caution about the electrolytics. It's very common for large caps to
be 20 or 30% off nominal value, often higher. If the 2200 is that much
higher and the 1000 isn't, the latter might have end up with more than
25V on it. It might be alright or not.

6. ### John O'FlahertyGuest

For electrolytics as DC filters, the voltage split may depend more on
the relative leakage current of the two caps than on their capacitance
values. It's probably safer to use a single capacitor with a voltage
rating higher than 50V for your power supply.

7. ### G. SchindlerGuest

Something to consider:
The tolerance of electrolytic caps is typically very large, maybe even
+100%, -20% depending on the units in question.

Second, leakage and ESR, and probably a lot of other things will also
contribute to voltage drop.

So, the actual capacitance ratio only controls the AC voltage division
across the capacitors. If the capacitors are filtering rectified AC and
thus dropping a significant DC Voltage component then the Capacitance
ratio cannot be used to predict the (DC) voltage drop across the
individual capacitors. For this the other effects become dominant. For
this reason, series connections of electrolytics in power filters is

In situations where it is done by paralleling the caps with resistors
controls the (DC) voltage division.

Sorry, but I never went down this road so I don't know the criteria for
selecting the resistors other than the basic voltage division. I would
think that the resistor current would have to be selected as a ratio of
the ripple current but that is only a guess.

Hope this helps ....

8. ### EeyoreGuest

You mean you got 35V without the caps ? That's because it was just rectified ac.

The caps will charge up to the peak of the recified ac's votage which will inded
be ~ 50V in this cae.

Graham

9. ### ehsjrGuest

Have you considered the ripple voltage? Assuming you
are using 60 Hz AC, it will be about I*.0083/C where
I is the current you draw from the supply in amperes,
For example, assume you draw 1/2 amp from your ~50
volt supply, and have a 2200 uF 63 volt cap. Your
ripple voltage will be .5*.0083/.0022 or about 1.89
volts. And rather than using the nominal 2200 uF
figure, you should look at the the +/- spec on the
cap and use - spec to compute the lowest value the
cap might be.

There's a whole lot more to consider than the above
when designng a power supply. This site may help:
http://www.electronics-tutorials.com/basics/power-supply.htm

Ed

10. ### jasenGuest

are they the same capacitance?

Bye.
Jasen

11. ### David L. JonesGuest

Here is one way to calculate parallel "ballast" or "bleeder" resistors
for a DC application of 2 series caps:

First you need to know the approximate worst case leakage of the
capacitors in question.

Then take the following worst case conditions:
- The maximum voltage rating of one of the caps
- Assuming one cap has maximum leakage and the other has zero leakage

i.e. The capacitor with the leakage with drag the mid rail voltage away
from its nominal half rail, creating a greater voltage across the cap
with no leakage. You don't want to have more than the maximum capacitor
voltage across the non-leaky cap.

Once you assume these worse case conditions then the circuit is easy to
analyse.

Each resistor is:
R=(CapLeakRes * (MaxCapVolt-(Vrail/2)) / (Vrail-MaxCapVolt)) * 2

The leakage will be voltage dependant, but this simple formula is a
good starting point.

Dave

12. ### Alan BGuest

Assuming you are using a DC voltmeter: you are measuring average voltage.
assuming you are measuring the output of a rectifier: an unfiltered
rectifier will have a lower average voltage than a filtered rectifier.

Now, about ratings. When designing circuits, it is always a good idea to
use a process called "derating." Derating is the use of components with a
rating comfortably in excess of the actual running conditions of the
circuit. In this case, with a filtered DC output of 50V, good derating is
the use of a capacitor rated at 75-100VDC.

13. ### Ken OGuest

yes you are right, I just calculated that I needed at least 2700uF and
approximately 75v, but I'll go to 100v to make sure. That means that the
capacitors I used are know blown up. ..

Ken

14. ### Guest

Yes, two 25V caps in series will give 50V operational overall but cap
total cap will be 1/2.
I'll also add a 470K resistor across each one to ensure that each one
share the voltage equally.
Steve Balstone stephen balstone