Capacitor Value and Measurements

[DjKrish]

Hi all, ok im getting more and more knowledge from this forum and i have to say i highly appreciate about all the inputs and outputs out of this forum based on everyone's feedback about caps. Thank you very much for that. I just need to learn a bit more about caps.

Ok example. 220uf 450v cap. Lets take from the microfarad value. Original value of the cap is 220uf, but upon using various testers and meters, it shows

Cap - 240.8 UF
ESR - 0.16 Ohm

Ok so what i need to know is, the cap uf should be + or - 10% right? And if its above than that then its gone, correct? I'm not assuming, im still learning. Just need some guidance in understanding caps more deeper. So is the above reading for the cap is faulty or its in fine range?

[lexwalker]

Re: Capacitor Value and Measurements

Ok example. 220uf 450v cap. Lets take from the microfarad value. Original value of the cap is 220uf, but upon using various testers and meters, it shows

Cap - 240.8 UF
ESR - 0.16 Ohm

Ok so what i need to know is, the cap uf should be + or - 10% right? And if its above than that then its gone, correct? I'm not assuming, im still learning. Just need some guidance in understanding caps more deeper. So is the above reading for the cap is faulty or its in fine range?

Actually brand new capacitors very often will measure higher than its labeled value. Any electrolytic capacitor can have up to +/-20% tolerance. The capacitance value will decrease as it ages, subjected to too much heat or too much ripple current beyond its specifications. Usually if it measures way below its nominal 20% tolerance then its either aged a lot (getting too old) or already starting to fail (going bad).

[DjKrish]

Re: Capacitor Value and Measurements

20%, ok i got that So looks like the cap i stated is dying anyway.........

[lexwalker]

Re: Capacitor Value and Measurements

20%, ok i got that So looks like the cap i stated is dying anyway.........

Did you mean that 220uF being measured as 240.8uF? That one sounds fine, as higher is better. Only worry if the capacitor's reading measures way lower than 220uF. Here's an example of highly aged dying/failing capacitor (see attached image below) pulled out of a computer PSU. Interestingly that one does not show any external signs at all (no bulging on either top or bottom, and no leaking brown gooey).

[mariushm]

Re: Capacitor Value and Measurements

Capacitance is useless to determine the quality of the capacitor.

In lots of cases, the capacitance can remain the same while the internal resistance increases a lot, or the capacitance actually drops when there's higher voltage or higher frequency going through the capacitor.

A multimeter uses low voltage and low frequency, which is not the working conditions inside a power supply, monitor etc.

Also, two multimeters will report different capacitance values for the same capacitor simply due to the measurement technique, current and voltage used to do the measurement, and they'll both have a +/- 5-10% error margin by default.

You kinda need a bridge or a good LCR meter to test a capacitor completely. A minor variation in capacitance compared to what's on label doesn't mean anything.

[DjKrish]

Re: Capacitor Value and Measurements

Capacitance is useless to determine the quality of the capacitor.

In lots of cases, the capacitance can remain the same while the internal resistance increases a lot, or the capacitance actually drops when there's higher voltage or higher frequency going through the capacitor.

A multimeter uses low voltage and low frequency, which is not the working conditions inside a power supply, monitor etc.

Also, two multimeters will report different capacitance values for the same capacitor simply due to the measurement technique, current and voltage used to do the measurement, and they'll both have a +/- 5-10% error margin by default.

You kinda need a bridge or a good LCR meter to test a capacitor completely. A minor variation in capacitance compared to what's on label doesn't mean anything.

Which i used the ESR70 by atlas peak to measure the cap.

[mariushm]

Re: Capacitor Value and Measurements

They also use a low voltage to measure capacitance.

Also, the ESR70's capacitance measurement is decoupled from the ESR measurement, as far as I know. So the capacitance is not measured with the 100kHz frequency, but rather in a separate step.

Most cheap capacitance meters just use constant current pulses to charge the capacitor a bit, then let it discharge a bit and repeat and they use that time period and current amount to estimate the capacitance.

Things can change when the capacitor has 12v constantly at a 30-50-70kHz frequency.

[lexwalker]

Re: Capacitor Value and Measurements

They also use a low voltage to measure capacitance.

Also, the ESR70's capacitance measurement is decoupled from the ESR measurement, as far as I know. So the capacitance is not measured with the 100kHz frequency, but rather in a separate step.

Most cheap capacitance meters just use constant current pulses to charge the capacitor a bit, then let it discharge a bit and repeat and they use that time period and current amount to estimate the capacitance.

Things can change when the capacitor has 12v constantly at a 30-50-70kHz frequency.

Even if at low voltage (using the cheap capacitance meters) the capacitor measures way below its labeled capacitance, then its certainly a sign of bad capacitor. Also capacitance can change with temperature (at higher temperatures the capacitance goes up rather than down, and the ESR decreases), which is why some equipment with badcaps may behave oddly (e.g. would not function properly at cold start and needs "warming up").