Depth of discharge of electrolytic capacitors.

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mrinalmani

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What should be a safe depth of discharge for aluminium electrolytic capacitors undergoing charge/discharge cycle at around 100Hz?
Ripple current is not the issue here, it is well within limits.

I need to place a capacitor across a 230V RMS bridge rectifier with 0.5A average load current. With each cycle the voltage across the capacitor will swing. Is 100V swing ok from capacitor's point of view?
 

Hi,

A schematic would be helpful.

The description is not clear to me.

Klaus
 

I am attaching the schematic with the oscilloscope output.
The question is how much ripple in voltage is acceptable.
Since electrolytic capacitors are not meant to be frequently charged and discharged, each ripple wave is equivalent to partial charge/discharge. To ensure long term reliability, what should be a rough accepted value of discharge percentage?
 

Hi,

It's much clearer now...

Since electrolytic capacitors are not meant to be frequently charged and discharged, each ripple wave is equivalent to partial charge/discharge. To ensure long term reliability...
Click to expand...

I think the capacitors are made for charge - discharge.
And the amount of voltage ripple is not the point to reduce lifetime.
It is more important to keep the current ripple (which causes power loss = heating) within it's specification.

How much voltage ripple is acceptable depends on your application. For a lamp or a heating application there is no need for a capacitor at all.

Klaus
 

Capacitors are indeed meant for regular charge/discharge, but not electrolytic ones. The electrolytic version have their life limited to generally 10K cycles.
 

Hi,

The electrolytic version have their life limited to generally 10K cycles.
Click to expand...
If you have a lifetime limitation, then you should find the information in the relating datasheets.

In the past I have read a lot of electrolytic capacitor datasheets, but can't remember such a specification.

In case your electrolytic capacitors have this limitation, then I recommend to choose capacitors without this limitation.

Klaus
 

As far as I can recapitulate, every note that I have read related to power storage devices say the same thing.
Electrolytic capacitors are meant to act as a voltage reservoir and just like batteries, they have a limited cycle life.
I am attaching a few clips that mention this.



 

Hi,

Really some interesting informations. Thanks for sharing.
You did your job right by reading datasheets and application notes directly from the manufacturer. There you get the most reliable information.

Now that there is the limitation, the only thing you can do is to use capacitors without this limitations.
The last picture of your post#8 shows a diagram. In the original PDF just below the chart there is the following paragraph :

Best is to rely on manufacturer information..

Klaus
 

This article gives a measurable relationship of lifespan of electrolytics and stress factors like temperature rise and voltage margin. T rise is due to circuit impedance relative to internal ESR.
 
The Epcos General technical informations for aluminium electrolytic capacitors give a different view on superimposed AC voltage:

There seems to be an unclarity about "charge/discharge operation". As far as I understand, the term refers to high curren discharge with low or zero series resistance, in other words doesn't apply to the present discussion.
 

The aging factor in capacitors is not the same as batteries which depends on product of full cycles * depth of discharge.
Capacitors life depends on levels of stress caused by margin to rated voltage and thermal rise from ripple current.

Choosing low ESR Caps , much lower than load will result in lower T rise and less aging.
ESR =1 to 5% Rload, depending on load would be reasonable.

Motor start Caps have to compare with ESR of motor coil on startup, and are rated for ripple current but are always higher ESR caps of similar material, size, voltage and general purpose low ESR use. But Motor start caps have lower duty factor use only at startup. They are also much cheaper than similar Polyester Vac rated caps with low ESR.

here is more stress with less ripple as less ripple also corresponds with a lower duty cycle for charging and thus a much higher ratio of peak power to load power.

In your thread #3, The discharge droop is about 20% of the peak voltage and the charge time is about 20% of the cycle, thus the charge current must be about 5 times discharge current and thus power dissipation 25x.during charge compared to discharge. As a check RC=>>10ms which is inverse to ripple factor.
 

I think Mr. FVM is right about the rapid nature of charge and discharge cycle which the datasheets are mentioning.
Last night after posting this thread I tested an 82uF 400V capacitor.
I applied 230V RMS through one diode, ie. a half-wave rectifier.
I adjusted the load such that the ripple voltage was 160V pk-pk
This leads to...
Vp = 325V
V_rip = 160V
I_RMS = 0.7A
Frequency = 50Hz

Although this is not a complete discharge, it still is a 50% discharge during each cycle.
I let the setup operate for two hours. But nothing happened.
Temperature rise was nearly 45 C
I also checked the vent area. There was no bulging and no convex appearance due to internal pressure.

Similar sized capacitors are averagely rated at around 0.6A to 1.5A. I think the temperature rise was justifiable.
I will test the circuit again with complete charge/discharge, but for that that a lower frequency source will be needed to ensure that the ripple current limit is not crossed.
 

Hi,

But nothing happened
Click to expand...
The datasheets say that there is a decrease in capacitance. Did you check capacitance before and after the test?

Your increase in temperature tells me it is a standard capacitor with about 2 Ohms ESR. Am I wrong here?
With a high quality high frequency, low ESR capacitor i'd expect less heating.

Klaus
 

I didn't notice before, but the first datasheet excerpt in post #8 is referring to polypropylene film capacitors. It's irrelevant for the present discussion. To anticipate possibly questions, the 20% ripple voltage rating isn't a general limitation of poylpropylene capacitors, it only exists for specific types.

Returning to the thread topic, besides the said high current charge/discharge and wellknown ripple current ratings, there are apparently no indications in datasheets about problems with higher ripple voltage.

So the test results are just expectable.
 

Oh yes! #8 image 1 is for film capacitors. It went unnoticed.

I don't know much about the capacitor rating. But after googling a bit, it appears to be rated below 500mA RMS.
And of course I checked only the physical appearance and the temperature rise and not the depreciation in capacitance.
 

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