Capacitors

[Jeanguypataterubberboot]

Hi there, I'm new to electronics and I am trying to understand the behavior of capacitors. I read that when they are connected to ac, as the frequency goes up, the more current they let through. When in hook one up to a function generator and an ammeter the amperage goes up to about 1 KHz then drops at about 3 MHz. Am I testing it wrong?
Thanks for your help.

Pierre

 

[davenn]

hi there
happy NY

you haven't told us what type or value of capacitor the voltage out of the function generator
A given value and type of capacitor will have a self resonant frequency
Also your function gene, depending on its quality, may not have a stable voltage output across its freq range

show us a drawing or preferably a sharp and well lit photo of your setup

Dave

 

[KrisBlueNZ]

When in hook one up to a function generator and an ammeter the amperage goes up to about 1 KHz then drops at about 3 MHz.

How are you measuring the current? With a multimeter set to an AC current range? Have a look at the specifications for the multimeter. Multimeters' AC response falls off pretty quickly above around 1 kHz.

You should use an oscilloscope if you want a flat response. If you're already using an oscilloscope, show us a drawing of how you've connected everything and include manufacturers and model numbers of all the test equipment you're using.

 

[Jeanguypataterubberboot]

 

[Jeanguypataterubberboot]

The gray lead is the function generator and the black lead is the oscilloscope.
Sorry about the sideways photos. In the top one is the setup. The second is at 1 MHz and the 3rd is at 15 MHz. The voltage is less at a higher frequency. I expected it to be higher. This is what I thought I would see from an inductor.

Thanks for the help. I know I'm doing something wrong.

Pierre.

 

[KrisBlueNZ]

It looks like it's an electrolytic. You could be seeing its internal inductance. Can you draw a connection diagram of how you're testing it, and show the value of the current shunt resistor.

 

[Jeanguypataterubberboot]

Ok I hope this makes sense.

 

[Laplace]

If you have a 0.1 μF ceramic decoupling capacitor, place it in parallel with the 100 μF electrolytic and run the measurement again (especially out past 1 Mhz). It may provide a good demonstration why ceramic capacitors are distributed along the power bus on a PC board.

 

[Jeanguypataterubberboot]

I don't have one of those but I will get one and try it. I assume Radio SHack has those.

Thanks

 

[Ratch]

Hi there, I'm new to electronics and I am trying to understand the behavior of capacitors. I read that when they are connected to ac, as the frequency goes up, the more current they let through. When in hook one up to a function generator and an ammeter the amperage goes up to about 1 KHz then drops at about 3 MHz. Am I testing it wrong?
Thanks for your help.

Pierre

Please understand that a capacitor does not let current exist through it. If it did, it would be considered "leaky" and defective. A perfect dielectric has a infinite resistance. So how does an AC ammeter register alternating current when an alternating voltage is applied to a capacitor? The answer is that when a voltage is applied between the two plates of a capacitor, a charge builds up on one plate and depletes equally on the opposite plate. While the charge is building up, it is registering as a current on the meter. When the voltage reverses, so does the current. So although no current exists through the capacitor, current does exists in the circuit branch containing the capacitor.

Make sure you don't apply a reverse voltage to an electrolytic capacitor. They don't like it.

Ratch

 

[Laplace]

So although no current exists through the capacitor, current does exists in the circuit branch containing the capacitor.

So we can see electrons going into one wire of the capacitor, and we see electrons coming out of the other wire of the capacitor, but no current exists through the capacitor? Is that because it is not the same electrons going in as are coming out? But the same situation exists with just a wire, electrons go in and electrons come out and they are not the same electrons (because the drift velocity is low), yet we say that current flows through the wire.

If we have a branch circuit from point:A to point:B and a wire carries current from 'A' to one terminal of a capacitor while another wire carries current from the other capacitor terminal to 'B', then current flows from 'A' to 'B'. How does it not also flow through the capacitor? Is not current the movement of charge? Does charge flow into one side of the capacitor, and an equal amount of charge flow out the other side of the capacitor? How is that not current flowing through the capacitor?

 

[Ratch]

Laplace,

"So we can see electrons going into one wire of the capacitor, and we see electrons coming out of the other wire of the capacitor, but no current exists through the capacitor?"

Yes, that is correct.

"Is that because it is not the same electrons going in as are coming out?"

No, while the above statement is true, that has nothing to do with no current existing through a capacitor.

" But the same situation exists with just a wire, electrons go in and electrons come out and they are not the same electrons (because the drift velocity is low), yet we say that current flows through the wire."

Yes, current does exists through a wire or resistor, but the situation is different in a capacitor.

"If we have a branch circuit from point:A to point:B and a wire carries current from 'A' to one terminal of a capacitor while another wire carries current from the other capacitor terminal to 'B', then current flows from 'A' to 'B'. How does it not also flow through the capacitor?"

Because a capacitor accumulates and depletes a charge on its plates. It does that because its dielectric has a theoretical infinite resistance. If it did not have this infinite resistance, the capacitor action would not be possible because no charge would be able to accumulate efficiently. Because the resistance of the dielectric between the plates is infinite, no charge can pass through the capacitor and therefore the current through the capacitor is zero. The charge flow stops on one side of the capacitor where it accumulates. An equal amount of charge flow starts on the other plate on the opposite side of the capacitor. None of the charges go through the capacitor. Charges do not accumulate in a wire or resistor because the path is completely conductive.

Ratch

 

[davenn]

Because a capacitor accumulates and depletes a charge on its plates. It does that because its dielectric has a theoretical infinite resistance. If it did not have this infinite resistance, the capacitor action would not be possible because no charge would be able to accumulate efficiently. Because the resistance of the dielectric between the plates is infinite, no charge can pass through the capacitor and therefore the current through the capacitor is zero. The charge flow stops on one side of the capacitor where it accumulates. An equal amount of charge flow starts on the other plate on the opposite side of the capacitor. None of the charges go through the capacitor. Charges do not accumulate in a wire or resistor because the path is completely conductive.

yes, agreed and the bolded defines it

its just the movement of charge on and off the plates of the capacitor.
a build up of electrons on one plate repels and drives off electrons off the other plate

NOTE the overall charge of the capacitor remains zero/balanced
The number of negative charges on one plate will be balanced by an equal number of positive charges on the other plate
The energy is stored in the electric field that is formed between the plates

Dave

 

[Ratch]

davenn,

"NOTE the overall charge of the capacitor remains zero/balanced
The number of negative charges on one plate will be balanced by an equal number of positive charges on the other plate
The energy is stored in the electric field that is formed between the plates"

Correct, I have said many times elsewhere that it is a misnomer to call a capacitor "charged", because a cap has the same net charge when 100 volts is applied across it as it does when its terminals are shorted to zero volts. Specifically, its net charge is always zero. I think it is correct terminology to say a capacitor is "ENERGIZED" instead if there is a voltage across it.

Ratch

 

[davenn]

It took me many years to realise this fact

 

[Laplace]

While it is true that charge does not cross the dielectric, it is also true that charge goes in one terminal and the same amount of charge leaves the other terminal. There is a net movement of charge. How is that different from any other component where charge flows into one terminal and an equal charge flows out the other terminal? Isn't that the definition of current flow?

 

[Ratch]

While it is true that charge does not cross the dielectric, it is also true that charge goes in one terminal and the same amount of charge leaves the other terminal. There is a net movement of charge. How is that different from any other component where charge flows into one terminal and an equal charge flows out the other terminal? Isn't that the definition of current flow?

The difference is that in a resistor, the charge flows continuously at a constant voltage. In a capacitor, the charge flow will decrease until it stops when the capacitor is energized to the applied voltage. The definition of charge flow through a component needs to be more defined than just saying that the input of one side is equal to the output of the other side. To complete the definition, it must be stated that a path exists through the component for the charge flow. This path exists for a resistor, but not for a capacitor.

Ratch

 

[chopnhack]

Displacement current flows, but there is no direct current flow across the capacitor. In other words the electrons you put in on one plate of the capacitor will not be the same electrons that flow from the other plate. Dave over at EEVblog recently went on a tirade about flow across a capacitor.

 

[Ratch]

Displacement current flows, but there is no direct current flow across the capacitor. In other words the electrons you put in on one plate of the capacitor will not be the same electrons that flow from the other plate. Dave over at EEVblog recently went on a tirade about flow across a capacitor.

http://en.wikipedia.org/wiki/Displacement_current

A most confusing presentation. Displacement current is a misnomer. As the link above says, "However, it is not an electric current of moving charges, but a time-varying electric field." It has the same units as current, but it is not current. One only has to worry about it if one is studying Maxwell's equations.

Ratch

 

[chopnhack]

http://en.wikipedia.org/wiki/Displacement_current

A most confusing presentation. Displacement current is a misnomer. As the link above says, "However, it is not an electric current of moving charges, but a time-varying electric field." It has the same units as current, but it is not current. One only has to worry about it if one is studying Maxwell's equations.

Ratch

Yes, I didn't watch it all because I remember that a capacitor is two plates separated by an insulating dielectric material. Therefore direct current can not pass, but alternating current can "induce" a current on the far plate since it is time varying.