# measure cap voltage

Discussion in 'General Electronics Chat' started by Yeti, Oct 4, 2005.

1. ### Yeti Thread Starter Active Member

Jul 26, 2005
35
0
If i charge a capacitor up can i connect my handheld Digital Multimeter to it to measure DC voltage.
There is no voltage marking on this capacitor. Not even any capacitance value like 470uF. It is a capacitor from a disposable camera if anyone knows about them.
Thanks
Yeti

2. ### Sebi Well-Known Member

Sep 24, 2005
59
0
First determine the capacity! You can measure it with cap-meter or compare it with know value caps: charge one with marking 220uF up to 24V with parallel 100k and measure the discharge time. Repeat this with unknown cap.
Most of case the flash caps are 400V 100...470uF.

3. ### Yeti Thread Starter Active Member

Jul 26, 2005
35
0
What i mean by measuring the voltage on it:
Charge the capacitor up, and read it on a voltmeter?

Will the voltmeter get damaged from completing the "discharge loop"?

4. ### n9xv Senior Member

Jan 18, 2005
329
1
As long as the meter range can handle the value of voltage the cap is charged up to, it will be ok. The cap will discharge VERY slowly depending on the internal resistance of the meter & the capacitance of the cap. You should be able to find the internal resistance of the meter by the "specs" listed on a manual or box the meter came in. The internal resistance X the capacitance is a "time constant".
t = R X C. 5 time constants is considered to be complete discharge. On discharge, the cap looses 37% of the "charged" voltage durring the first time constant. If the cap discharges slow enough, you can measure the time it takes for the cap to discharge 37% of the charged voltage. Solve for C, C = t/R. R being the internal resistance of the meter. If it discharges slowly enough, you can accuratly determine the capacitance of the cap.

EXAMPLE:

A 1000-uF cap is charged up to 100-VDC. The internal resistance of the meter is 10-Megohm. t = R X C, so t = 10,000-seconds. Therefore, in about 2.77-hours (2-hours & 46.2-minutes) the cap will have some 63-VDC accross it. I've done this countless times with a 100,000-uF computer supply cap. Its pretty cool!

Jan 22, 2004
1,437
1

oops!