To measure some V AC imprinted on a capacitor,

Thread Starter

abdulbadii

Joined Aug 30, 2017
38
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
 

ronsimpson

Joined Oct 7, 2019
1,659
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
7,985
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?
 

rsjsouza

Joined Apr 21, 2014
335
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.
 

MrChips

Joined Oct 2, 2009
24,627
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
1,659
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
1,659
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
335
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
217
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.
 

MaxHeadRoom

Joined Jul 18, 2013
23,938
If it is a motor start/run rated capacitor, at that value, usually it would be a oil filled paper or similar construction and generally do not lose capacitance.
The solid electrolyte start only versions are typically larger values and can in some cases, change value with age.
Also these later types are not polarity marked.
 
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