Dry electro capacitors heat dependant?

Question

Had a device that was failing when it got warm (continually resetting etc). The power PSU supplies 30V,5V,3.3V and 1.8V. Didn't see anything unusually but I replaced all the electro's on the PSU and now it works fine even after it gets quite toasty. Is it likly the caps were dry and can they loose capacity or ripple current reduces when they warm up? All the caps look as good as they did brand-new (they are about 6 years old) and were 105 rated, im measuring about 60-70 degrees that they get to.

Is there anyway I can test the caps I removed to find out which one (s) were responsible (i have a CRO and power supply, but no capacitor or esr meters etc).

Davo · Accepted Answer

Apply a voltage with a high value resistor connected in series with the supply. Use the time constant formula you learnt years ago in your electronics training to find the value of capacitance. You could also check for electrical leakage through the capacitor by using the ohms range.

Phil Allison · Answer

** Almost certainly the caps had dried out and developed high ESR ( ie high internal resistance).

With some SMPS topologies, such faulty caps cause diodes and FETs to run very hot as well as causing high level of switching noise on the DC rails.

** If you have a signal generator that can be set to between 10 and 100kHz, just connect each cap across the output terminals while monitoring the residual signal voltage on your CRO.

Compare results with a known good cap.

..... Phil

Phil Allison · Answer

"Davo"** That will not find a dried electro cap with high ESR.Cos loss of electrolyte makes the ESR rise first and only later with much more loss will the capacitance value drop.This is WHY service techs use ESR meters to find bad electros -  and  NOT capacitance testers......   Phil

Franc Zabkar · Answer

On Wed, 7 Jan 2009 17:23:00 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:

Does your CRO have a 1kHz (?) calibration signal output? If so, then you may be able to test your capacitor with this. At 1kHz, 10uF and

100uF caps will have relatively high impedances of 17 ohms and 1.7 ohms respectively, so it may be difficult to detect a small increase in the ESR. However, it may be interesting to watch the signal amplitude as you heat and freeze the caps, especially when you compare their behaviour against known good ones.

Another possibility is to drive your caps with a sinusoidal test signal from the line output of your sound card. The output impedance is typically in the range of 20 ohms to 500 ohms, and the level can be as high as 2V. At 10kHz, the abovementioned impedances become 1.7 ohms and 0.17 ohms.

- Franc Zabkar

Dry electro capacitors heat dependant?

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