# Capacitor operating current rating?

Discussion in 'General Electronics Chat' started by Leorenzo, Sep 30, 2013.

1. ### Leorenzo Thread Starter New Member

Sep 21, 2013
27
6
Do capacitors have current rating? I believe I have seen a datasheet who shows a maximum current rating of the capacitor but I thought that the amount of current going in and going out of the capacitor just fasten its charging and discharging rate and the capacitor is alright with any value.

I have seen capacitors discharging, although at a very fast time around 200ms, at a current around 400A (used for CD spot welding).

Also, do capacitors necessarily have lower operating voltage? I thought they are just fine as long as you don't exceed its maximum operating voltage?

2. ### MaxHeadRoom Expert

Jul 18, 2013
13,167
3,633
They have a voltage rating, when AC is applied to a perfect capacitor the current leads the voltage by 90° so no heating effect takes place at the rated voltage.
Capacitors posses ESR (equivalent series resistance) which will affect the phase angle between voltage and current, the lower the ESR the higher the current, capacitors such as Electrolytics have very low ESR, one reason why for example start and run capacitors on split phase motors are oil filled paper for run (high ESR) and electrolytic on start in order to get a large value in a small package, but low ESR which causes heating effect if left in circuit too long.
To assess a capacitor, you need to look at the ESR value.
Max.

3. ### crutschow Expert

Mar 14, 2008
16,540
4,458
As Max stated, capacitors do have ESR. This dissipates power when charging and discharging the capacitor. This causes heating of the capacitor and it's the maximum capacitor operating temperature which limits how much current and the frequency of the current pulses that the cap can tolerate.

As long as you never exceed the capacitors maximum voltage rating you are OK. But good design practice is to derate the operating voltage 50-75% of the maximum value to improve reliability.

4. ### Leorenzo Thread Starter New Member

Sep 21, 2013
27
6
Oh, I see. So what is the purpose of the maximum current rating on datasheets? I'm currently choosing the right capacitor for my capacitive discharge spot welder and it seems like that with my discharge current at 400A, it easily exceeds most operating temperature.

However, since the 400A would just be around for about 200ms, I believe with that time it would not be enough for the capacitor to increase its temperature at very high value? If this do work, surely it has its limits for even greater current. What could be that current value that would damage the capacitor?

I'm really confused at these capacitor ratings so thank you for help.

5. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,309
Repetitive peak current ratings only tell you part of the story. I think you might need to test on a real cap, and measure the temperature rise.

A high-current low-ESR cap is a good starting place if the cap will be subjected to high repetitive discharge currents. Also, for larger caps (>470uF) the ESR tends to get lower on caps with higher rated voltage. So using higher voltage electros can give you greater voltage safety margin and will also give you less cap heat (due to lower ESR and higher cap mass and surface area).

6. ### Leorenzo Thread Starter New Member

Sep 21, 2013
27
6
Oh I get it. So, paralleling two of my desired rated capacitor (but twice the capacitance since they will be paralleled) will give me more safety margin as it decreases the ESR and the current that will be drawn from each will be halved?

7. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
508
They have a ripple current rating. Some caps have a peak surge current rating.

Oct 9, 2007
2,392
355