Capacitor Extraction: Charging & Isolating Electrons

[dansmith170]

Hi,

Is it possible to charge a capacitor, and then extract or isolate the part of the capacitor that has been charged with electrons?

Thanks.

 

[.Scott]

Yes.
It can be done mechanically (by disconnecting from all circuitry).
Or, you can gate one or both of its terminals with high-impedance devices.

In actual circuitry, this is often done for the purpose of holding a voltage level. Once the capacitor is charged to the target level, it is isolated from further charge or discharge currents. A voltage-following op amp can then be used to drive additional circuits with that voltage.

 

[sophiecentaur]

If you take two plates, charge one with +Q and the other with -Q then you bring them together, you will, in effect, have a capacitor that's charged with Q. Their charges will remain the same (they can't go anywhere) and the Potential difference V will be given by Q = CV where C is the so-called Capacitance. Capacitance is inversely related to the space between them.
So, taking a charged capacitor (which will be starting with V₁ across the terminals) and moving them apart will involve doing work (overcoming the attractive force). The Potential Difference will increase as the separation increases.

I seem to have repeated myself a bit there but never mind.

 

[.Scott]

When the capacitors are very large, the ability of of them to hold a charge even when the rest of the circuit is powered off can be a safety hazard. Such devices should not be opened up without appropriate expertise.

 

[DaveE]

Yes. If you want a more detailed answer, I think you'll need to ask a more detailed question.

 

[sophiecentaur]

When the capacitors are very large, the ability of of them to hold a charge even when the rest of the circuit is powered off can be a safety hazard. Such devices should not be opened up without appropriate expertise.

A very important safety point. ‘Big capacitors’ should be stored with shorting wire wrapped around the terminals. This risk also applies to old TV tubes that can hold their charge for a long time and surprise anyone who touches the electrodes.

 

[.Scott]

A very important safety point. ‘Big capacitors’ should be stored with shorting wire wrapped around the terminals. This risk also applies to old TV tubes that can hold their charge for a long time and surprise anyone who touches the electrodes.

Often "shorting wires" are not appropriate. Some electronic equipment is build over a capacitor bank. You cannot just short those out safely because there is the potential of explosively vaporizing a substantial chunk of your screw driver. You need a resistor - such as an incandescent lamp.

That's why I suggested "appropriate expertise".

 

[sophiecentaur]

Often "shorting wires" are not appropriate. Some electronic equipment is build over a capacitor bank. You cannot just short those out safely because there is the potential of explosively vaporizing a substantial chunk of your screw driver. You need a resistor - such as an incandescent lamp.

That's instructions how do deal with a capacitor that you take out of the drawer. A resistor is only needed if it's already (possibly) charged. Someone, before that, should have wrapped wire round it.

 

[dansmith170]

When the capacitors are very large, the ability of of them to hold a charge even when the rest of the circuit is powered off can be a safety hazard. Such devices should not be opened up without appropriate expertise.

Thanks Scott

 

[dansmith170]

A very important safety point. ‘Big capacitors’ should be stored with shorting wire wrapped around the terminals. This risk also applies to old TV tubes that can hold their charge for a long time and surprise anyone who touches the electrodes.

Thanks Sophie

 

[analogdesign]

Surprised no one mentioned this explicitly, but this exact procedure is the basis of Dynamic Random Access Memory (DRAM). The bit you are storing is represented by a presence or absence of charge on an isolated capacitor.

The technique also underpins most practical analog to digital conversion schemes. Instead of just presence or absence, the actual amount of charge is the variable of interest. This means if you continually charge and discharge caps by means of a clock you now have discrete-time analog signals. Very useful indeed! It that context it is called switched-capacitor signal processing.