# Best way to test large capacitors?

#### Hearcostello

Joined May 14, 2016
7
Large capacitors!
First off, yes I'm aware of the safety concerns here... Large capacitors can have deadly amounts of voltage stored, and should be handled carefully and knowledgeably!
My question: is there a reliable process for testing a large capacitor? I have a 6800uf 63v DC cap off an old audio mixing board, and I'm trying to determine if it's gone bad or not. Despite learning a few test procedures I've seen posted online, I can't understand what my digital multi-meter (DMM) is telling me when I use those same procedures. Is my DMM not capable of testing such a large cap? If I handed you this cap, how would you test it?

#### Bushdog

Joined May 4, 2017
3
I have no idea why your DMM isn't just spitting out the capacitance, but you could recharge through a known resistance and then use your DMM to measure the voltage and how long it took.

#### AlbertHall

Joined Jun 4, 2014
11,610
I have no idea why your DMM isn't just spitting out the capacitance, but you could recharge through a known resistance and then use your DMM to measure the voltage and how long it took.
And you can do that with a reasonably low voltage (<10V) so there are no safety concerns.

#### #12

Joined Nov 30, 2010
18,223
Make a 63 volt DC power supply. Use a 5.6K resistor to limit the current to 11 ma. Connect the parts to charge the capacitor. Watch the voltage to see if the capacitor charges to its full rated voltage. When it has stabilized, no matter what voltage it stabilizes at, measure or calculate the leakage current.

Build a power supply filtering stage with an AC source, a rectifier, and the 6800 uf capacitor. Connect a load resistor and calculate the capacitance using the formula 1.414 x C x Er(p-p) x F = I

#### DickCappels

Joined Aug 21, 2008
8,000
To check capacitance:
With a capacitor this large you can test it for capacitance by shorting the leads then charging the capacitor through a 1k resistor while watching the voltage across the capacitor with a voltmeter. The time it takes the capacitor to reach 6.3 volts is the time constant of the circuit in seconds. The time constant divided by 1,000 gives you a good approximation of the value of the capacitance. (Take care that you don't get anything in your eyes when shorting the capacitor.

To check leakage:
Charge the capacitance to 10.0 volts and leave the capacitor to sit for a while. Come back later and see how much the voltage dropped. From this you can calculate the self-discharge time constant. It should be very large.

To check for high ESR (Equivalent Series Resistance -you will need more than a DVM to do this) :
Put a low value resistor in series the capacitor and drive the resistor with a sine wave at a frequency at which the reactance of the capacitor should be very small compared to the resistor. Measure the voltage across the resistor and the capacitor. The AC voltage across the resistor tells you the current through the ESR of the capacitor. The AC voltage across the divided by this current is the ESR of the capacitor. (In realityi it is the impedance taking ESR and capacitive reactance into account but with a high enough frequency the reactance can be ignored.)

#### BobaMosfet

Joined Jul 1, 2009
1,900
@Hearcostello

6800uF @ 63V.

It is not the size of the capacitor, but the rate of discharge that tells you whether it's good or bad. This is called 'RC Time Constant'.

In short-- Charge a capacitor to a known value-- like 1V. Let it discharge through a 1K-Ohm resistor, and measure the length of time it takes for it to discharge, and divide by 5. In the first 1/5th of the entire discharge period, it will lose 63% of it's charge across the resistor. That's what you're looking for. To see that voltage is about 37% of the initial 1V (or 370mV) at the end of about 900us.