Voltage across a Capacitor More than the Source Voltage?

[goldensquid2000]

Hi everyone, I'm new to this forum so I apologise in advance if I may have posted in the wrong forum or made a similar error.

I recently conducted an experiment with some peers where we measured the voltage across a charging capacitor in an RC circuit over a period of time. The diagram is attached at the bottom of this post.

The problem is, with some of the latest time values, the voltage read by the voltmeter exceeded 12 volts, the input voltage in the entire circuit. At this stage, the only explanation I have for this phenomena is the idea of current overflow into the voltmeter.

From my understanding, a voltmeter measures voltage through the current flowing into the voltmeter which interacts with a magnetic field, turning a coil and displaying a voltage value. The torque of the coil is proportional to the amount of current flowing through said coil. My guess is that when the capacitor was fully charged, no current would run through it, thus meaning all of the circuit's current would be redirected through the voltmeter and treating it as if it were in series. This extra amount of current might have caused the increased "voltage" value displayed.

At this stage, I'm mostly hypothesising, any feedback and alternative suggestions would be great, thanks!

 

[lekh2003]

Sometimes, in capacitors, if you wiggle around the positions of the plates (if it's parallel), then the voltage can change. This is since charge is conserved and capacitance changes when it wiggles around.

That is not to say that this is what is happening here. Could be something else.

 

[Nugatory]

You are expecting 12v and the meter is reading more. By how much? And what are you using as a voltage source?

 

[Dale]

the voltage read by the voltmeter exceeded 12 volts, the input voltage in the entire circuit.

Did you measure the voltage across the battery and find it was exactly 12 volts, or are you just reporting the nominal voltage.

 

[goldensquid2000]

@lekh2003 Hmm, not sure. The capacitor we had was one of those ones with non-transparent filament around it, so anything could have happened inside with the plates. Although, if this were the case I wouldn't expect that the voltage would consistently stay above the source voltage (unless, perhaps the source voltage from the power supply prevented it from discharging from the capacitor?)

@Nugatory Depending on the value of time, the meter read up to 14 volts across the capacitor, which exceeds the source voltage by 2 volts. The data curve seems to suggest that the voltage would eventually converge to 16 volts.

@Dale We used an analogue adjustable power supply and set the value to 12 volts. After we saw the strange value, we measured the voltage of the power supply when it was set to 12 volts, and got a value of about 12.07 volts (not a very substantial distance)

 

[Dale]

After we saw the strange value, we measured the voltage of the power supply when it was set to 12 volts, and got a value of about 12.07 volts (not a very substantial distance)

Interesting. I have no clue then.

 

[TurtleMeister]

DC power supplies can have a certain amount of ripple, spikes, and noise in their outputs. If you have only a capacitor without any load on the output, then the capacitor will eventually charge up the the maximum, or peak value, of the ripple, spikes, and noise. Two volts is a lot but not out of the question, depending on the quality and condition of the power supply. If you have an oscilloscope you can check this out.

 

[Tom.G]

If you do not have an oscilloscope (or even if you do), try putting a resistor in parallel with the capacitor -- a value between 560 and 1200 Ohms should do the trick. As @TurtleMeister pointed out, there may be noise or ripple on the power supply output. If there is some occassional noise the resistor will supply a path to drain the extra charge. Additionally, the output voltage of some power supplies may drift up when there is no load on them. As the capacitor charges with DC, it starts looking like an open circuit. (doesn't draw any current from the supply).

Cheers, and let us know what you find!
Tom

 

[goldensquid2000]

@TurtleMeister Unfortunately, I do not have access to an oscilloscope, but that did sound like a good idea.

@Tom.G Hi, Unfortunately I will not have access to the equipment again until a while after my report is due, but that is mostly inconsequential because I will not be assessed about this phenomena and was mostly just curious. I'll try to see if I can get access to the equipment again, but if not, then I'm pretty much stuck with hypothesizing. Thanks for the suggestion though

 

[sophiecentaur]

@TurtleMeister Unfortunately, I do not have access to an oscilloscope, but that did sound like a good idea.

@Tom.G Hi, Unfortunately I will not have access to the equipment again until a while after my report is due, but that is mostly inconsequential because I will not be assessed about this phenomena and was mostly just curious. I'll try to see if I can get access to the equipment again, but if not, then I'm pretty much stuck with hypothesizing. Thanks for the suggestion though

I could suggest measuring the output of the supply with the DMM set to AC and see how it changes when the capacitor is connected across it. This would be the next best thing to an Oscilloscope.
I remember measuring the DC volts on my battery charger and being disappointed that it was only 12V but, with a battery connected, it went up to >14V. The AC 'ripple' was enormous when open circuit.