# Capacitors in parallel question

#### TimCollins

Joined Sep 19, 2011
9
I am a bit stumped by this capacitor configuration It just doesnt look right, but maybe I am missing some purpose for doing this... I have two capacitors in parallel. C1=12000uF/25V (polyester film) C2=4.7uF/100V (Low ESR Electolytic). The power source is 12V DC (not sure if that matters or not in this case). Just looking at these two capacitors their uF and voltage numbers seem so far apart from each other I cant understand what the intent of putting them in parallel could be? I am wondering if there is some common reason that I am not aware of that these two seemingly diverse capacitors would be combined in parallel into a circuit?
Also,
Im not sure if I am using this formula correctly but here is what I calculated:
C(equivalent)=Q1/V + Q2/V
C(equivalent)=12000/25V + 4.7/100V
C(equivalent)=48000/100V + 4.7/100
C(equivalent)=48004.7/100V
However, is 100V even possible since one of the capacitors is only 25V.

Thanks in advance for any help...

#### bertus

Joined Apr 5, 2008
21,041
Hello,

If you put capacitors parallel, the capacitance values can be aded.
The voltage however is taken from the lowest voltage rating of the capacitors.
So the capacitance is 12000 + 4.7 = 12004.7 μF
And the voltage is 25 Volts.

Bertus

#### #12

Joined Nov 30, 2010
18,216
Here's the easy explanation: Big electrolytics don't work well at high frequencies. Little poly-films and ceramics conduct at a higher range of frequencies. One of each is an insurance policy.

#### MrChips

Joined Oct 2, 2009
22,521
Don't even bother to sum the two capacitance values. #12 has the correct answer.

#### TimCollins

Joined Sep 19, 2011
9
Thanks you for the explanations. #12 seems to make since...

#### GetDeviceInfo

Joined Jun 7, 2009
1,742
sometimes, when you are tuning the frequency response of a circuit, it is easy to mount the largest whole value component, then fine tune with what might be in this case parallel values.