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I have an RC circuit where I measure the voltage across the capacitor over time with a voltmeter. For the supply voltage I am using a 5V phone charger.

I know what is supposed to happen, the voltage on the capacitor is going to increase rapidly in the beginning and will slow down as it charges until it reaches the supply voltage.

What I am measuring however is very different. The first couple of seconds the voltage increases, like it should, until 3.9V and then starts to drop while the capacitor is still supposed to be charging.

I'm very curious as to how this can be explained.

Edit:

The capacitor's value is 150uF, the resistor is 20kOhm, the supply voltage is 5.22V measured with the voltmeter.

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    \$\begingroup\$ And after some time is settle down to a constant value? Or maybe your volt meter is measuring AC voltage? \$\endgroup\$ – Andy aka Jan 15 '17 at 16:04
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    \$\begingroup\$ Give us the details of the RC and the voltmeter you are using. \$\endgroup\$ – Peter Smith Jan 15 '17 at 16:05
  • \$\begingroup\$ @Andyaka I measured it for a minute and it takes 7 seconds to reach 3.9V after that it drops first relatively rapidly and then more and more slowly. After a minute it had reached 2.6V again and was still slowly decreasing. \$\endgroup\$ – Mathieu David Jan 15 '17 at 16:10
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    \$\begingroup\$ Maybe it's an old leaky electrolytic capacitor? \$\endgroup\$ – Andy aka Jan 15 '17 at 16:15
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    \$\begingroup\$ That may very well be the problem, I just switched out the capacitor for another one and it seems to behave correctly this time! I should have tried that earlier.. Thanks a lot for the help! \$\endgroup\$ – Mathieu David Jan 15 '17 at 16:19
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A case of comments graduating to become an answer: -

Maybe it's an old leaky electrolytic capacitor?

That may very well be the problem, I just switched out the capacitor for another one and it seems to behave correctly this time! I should have tried that earlier.. Thanks a lot for the help!

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