Author Topic: Typical Electrolytic capacitor current rating  (Read 1178 times)

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Offline ZeroResistanceTopic starter

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Typical Electrolytic capacitor current rating
« on: September 09, 2018, 02:34:28 pm »
If I take a typical electrolytic capacitor say 100uF , 160V. I charge it to 100V DC. And then discharge it through a 3.3 ohm resistor. Would the peak current in the resistor be 33.33A ?
Is it practically possible for the electrolytic capacitor to source this amount of current for small period of time?
 

Offline helius

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Re: Typical Electrolytic capacitor current rating
« Reply #1 on: September 09, 2018, 03:24:24 pm »
If I take a typical electrolytic capacitor say 100uF , 160V. I charge it to 100V DC. And then discharge it through a 3.3 ohm resistor. Would the peak current in the resistor be 33.33A ?
There appears to be a problem with your arithmetic. 100V divided by 3.3Ω is 30.30A, not 33.33. So the answer would be no.
Additionally, this is only the case for idealized components. In any real circuit, the capacitor has ESR (series resistance), which limits its current capacity, and wires have inductance, which limits the derivative of the current (you cannot go from 0 A directly to 30).

Quote
Is it practically possible for the electrolytic capacitor to source this amount of current for small period of time?
This is a different question. If discharged through a (approximately) 0 ohm wire, the peak current through an elco can be higher than that. It may cause damage to the capacitor's plates if it is too high or repeated too fast. You may need to conduct tests to see how hard you can abuse a capacitor before it dies; the data sheets are intended for more typical applications, where the capacitor is charged and discharged at 120 Hz, and specify the maximum current capacity at this frequency (the rated "ripple current"). A typical ripple rating for a 100 uF cap would be 1 to 2 A RMS.

The data sheets may give you the ESR, or express it in terms of dissipation factor or tan ø. You can calculate the ESR from the DF as follows: \$ ESR = {DF \over 2 \pi f C} \$, where f is the frequency in hertz and C is the capacitance in farads.
« Last Edit: September 09, 2018, 03:38:04 pm by helius »
 
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Offline ZeroResistanceTopic starter

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Re: Typical Electrolytic capacitor current rating
« Reply #2 on: September 09, 2018, 05:31:19 pm »
If I take a typical electrolytic capacitor say 100uF , 160V. I charge it to 100V DC. And then discharge it through a 3.3 ohm resistor. Would the peak current in the resistor be 33.33A ?
There appears to be a problem with your arithmetic. 100V divided by 3.3Ω is 30.30A, not 33.33. So the answer would be no.
Additionally, this is only the case for idealized components. In any real circuit, the capacitor has ESR (series resistance), which limits its current capacity, and wires have inductance, which limits the derivative of the current (you cannot go from 0 A directly to 30).

Quote
Is it practically possible for the electrolytic capacitor to source this amount of current for small period of time?
This is a different question. If discharged through a (approximately) 0 ohm wire, the peak current through an elco can be higher than that. It may cause damage to the capacitor's plates if it is too high or repeated too fast. You may need to conduct tests to see how hard you can abuse a capacitor before it dies; the data sheets are intended for more typical applications, where the capacitor is charged and discharged at 120 Hz, and specify the maximum current capacity at this frequency (the rated "ripple current"). A typical ripple rating for a 100 uF cap would be 1 to 2 A RMS.

The data sheets may give you the ESR, or express it in terms of dissipation factor or tan ø. You can calculate the ESR from the DF as follows: \$ ESR = {DF \over 2 \pi f C} \$, where f is the frequency in hertz and C is the capacitance in farads.

Many thanks for the answer.
So the effective resistance with limit the max current and inductance will limit the rate of rise of current right?.
So if the esr of the Cap was 0.7 ohms. That would make the total resistance 4ohm. So that would make it 25A peak right.
But you also have said that rms current of a electrolytic cap is around 2A rms.
So would that mean that eletrolytic caps are not meant to be used in this kind of application and only strobe or flash capacitors should be used.
 

Offline helius

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Re: Typical Electrolytic capacitor current rating
« Reply #3 on: September 09, 2018, 06:00:55 pm »
The capacitor may be rated for 2A continuous ripple, but be capable of higher currents if they are not continuous. This is not guaranteed by the datasheet and needs to be tested.
 


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