# Capacitors power rating

Discussion in 'The Projects Forum' started by yassser, Dec 4, 2012.

1. ### yassser Thread Starter Member

Jul 25, 2011
91
0
Hello ,

I want to ask about the power rating of ac capacitors like this one :
http://www.elecsound-components.com/Film-Capacitors/cl21.pdf

what is mentioned is only its voltage rating , why is there no mention of the power rating , how could I know that it would withstand the current that will pass through it .

thanks

2. ### crutschow Expert

Mar 14, 2008
13,508
3,385
You need to know the ESR of the capacitor. The power loss in that resistance due to the AC current determines how much the capacitor heats up. Typically I would say you would want that power loss to be less than a watt.

3. ### yassser Thread Starter Member

Jul 25, 2011
91
0
thanks , I just want to make sure I understand it correctly

1.calculate the cap. impedance Z = (1/wc)
2.calculate current passing through cap. I = V/Z
3.Voltage drop across ESR : Vd= I*ESR
3.Dissipated power in cap. = I*Vd

is that correct?

and I can't find any datasheet that mentions the cap ESR as well , is there someway to measure it?

4. ### crutschow Expert

Mar 14, 2008
13,508
3,385
There are meters that can measure the ESR. But film capacitors, such as the one you referenced, generally have such a low ESR that it's not of concern unless you are putting very high frequency, high currents through them. What is your application?

5. ### yassser Thread Starter Member

Jul 25, 2011
91
0
it's an h-bridge circuit , i need to stabilize the glitches that happen because of the stray inductance and affect the electronics , V=160v , f=10KHz

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
For your question, please post a schematic of the circuit, voltage it is operating at, load current/load type, power supply information, and the exact problem you are having with it (scope pictures, overheating, etc). That will make it easier to suggest solutions.

Read these articles and see the advantages/disadvantages of various dielectrics (capacitor types), some go into great detail with math, one is a charts, the rest are a combination.

ESR/ESL (Disspation Factor/DF when combined) is the 3rd aspect of capacitors, second to voltage and value. Some applications aren't "picky" about ESR/DF, while others, such as filter capacitors, must have low ESR capacitors.

Mathematical and in-depth paper on capacitors and ESR/ESL

Quick Reference Sheet showing pros, cons, and uses of various types of capacitors by dielectric material

Analog Devices Engineers paper, easy to read, with table at end for capacitors

AVX capacitor comparison chart

Visual Guide to capacitor types

Polyester Film aren't the best choice for bypassing noise, but there are worse options around. If high current is expected to be pass through, look at type types with low inductance and low ESR or low DF

Feb 19, 2009
6,357
718