# To measure some V AC imprinted on a capacitor,

Joined Aug 30, 2017
84
If a capacitor is imprinted such:
450V AC 8 uF
50 - 60 Hz
How to measure it?
The step is on Capacitor, but it clearly seems not working while the digital multimeter just did satisfying measurement on a non AC one

#### crutschow

Joined Mar 14, 2008
34,078
Are you trying to measure the capacitance?
What do you mean " The step is on Capacitor"?

#### ronsimpson

Joined Oct 7, 2019
2,904
Most meters can not measure capacitance. This cap should measure about 8uF with a capacitance meter. Should not matter if the capacitor said "AC" or not.

#### DickCappels

Joined Aug 21, 2008
10,128
So...momentarily short the capacitor's leads together to assure that it is discharged then connect it to a capacitance meter. Do you have such an instrument?

#### MrChips

Joined Oct 2, 2009
30,520
If all you have is a DMM you can conduct a simple RC time-constant measurement.

#### rsjsouza

Joined Apr 21, 2014
383
One additional aspect: you cannot measure the "450V AC" of the capacitor - it is just a statement of its voltage limit which, if surpassed, can cause internal rupture on its dielectric and render it unusable.

Joined Jul 18, 2013
28,533
Rough test.
Presumably a Electrolytic, place a 12v cell across it, observing polarity, and use your VOM to measure the voltage decay over time via the meter.

#### MrChips

Joined Oct 2, 2009
30,520
Correct.

Look up the specs of your meter and find the resistance of the meter while in voltage measuring range.
Most DMM are 10MΩ resistance.

Charge the capacitor to some voltage. Measure the voltage across the capacitor and observe the voltage as it drops.
An 8μF capacitor will take 80 seconds to reach 37% of the initial voltage while being measured with a DMM (with 10MΩ internal resistance).

Do the math:
time constant = R x C = 10MΩ x 8μF = 80 seconds

#### ronsimpson

Joined Oct 7, 2019
2,904
you cannot measure the "450V AC" of the capacitor
Yes you can.
Not easy. Not with just a volt meter.
I have high voltage power supplies. I would put 450V dc on the cap and measure leakage current. Not something most people can do.
--------
450AC x 1.414= peak voltage. So the cap should hold 630V dc.

#### ronsimpson

Joined Oct 7, 2019
2,904
There is simpler ways
Please show us how to measure "450V" of the capacitor.
The 8uF part is easy. Use a capacitor meter and it will read 7.83uF or 8.09uF or .......
I do not know of a way to put a meter on the capacitor and have it read "450Vac". or read 630Vdc. On most electronics reading the breakdown voltage will kill the part.

#### rsjsouza

Joined Apr 21, 2014
383
Please show us how to measure "450V" of the capacitor.
Simply use a capacitor tester and run the leakage test, it applies full rated voltage.
Well, that is the point of my post. You are measuring leakage at a given voltage, not the maximum applicable voltage itself.

#### rsjsouza

Joined Apr 21, 2014
383
Well, I am not sure if I quite understand your assertive. Are you agreeing you can't measure the maximum voltage anyways but only have a rough estimate based on another parameter?

#### tautech

Joined Oct 8, 2019
378
Those values in the OP make it sound like it's a motor start cap and pretty simple to measure/check with a DMM in capacitance mode. If it doesn't measure somewhere close it's porked and should be replaced.