# Capacitors

Discussion in 'General Electronics Discussion' started by Muhasaresa, Jan 15, 2012.

1. ### Muhasaresa

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Jan 2, 2012
Hi everyone

I was wondering something about capacitors (I am a beginner). Can capacitors conduct when connected in series. When I was looking at the schematic for a low pass filter, there was a capacitor connected in series with a resistor, so does that mean that current goes through the capacitor?

Thanks for any help.

Muhasaresa

Last edited: Jan 15, 2012
2. ### davelectronic

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Dec 13, 2010
Hi there. If this is right and i am fairly sure it is, current travels through a component, voltage travels across it, thats the way its normally stated i think.

3. ### GonzoEngineer

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Dec 2, 2011
Simply put, capacitors conduct AC current, but block DC current.

4. ### NuLED

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Jan 7, 2012
do they not release the stored charge when accumulated charge exceeds whatever "threshold" they have? i took apart a toy that seemed to have a capacitor which provided a pulsed DC flow to an electromagnet. there did not seem to be any AC component (it was just a small solar panel feeding into the capacitor so i assume it must draw enough charge that once it releases it, it is a pulse of DC strong enough to power up the electromagnet for a short burst of time).

does this make sense?

5. ### duke37

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Jan 9, 2011
Capacitors do not have a threshold but like every other component they have a maximum rating.

In your application, the capacitor charges up from the solar panel, in other words the voltage has an AC component. When the capacitor is connected to the electromagnet, the capacitor discharges and the voltage drops. If you are a wizz kid, you can do a Fourier analysis!

A sine wave AC signal has a continously varying voltage so the capacitor will pass the AC current.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
When I think about capacitors, I like to think about water, and capacitors as tanks with flexible membrane in them.

If the water is flowing only one way, it can only flow into the capacitor until it is full. In the water analogy, until the membrane has been pushed as far across as the water pressure can push it. Note that in doing so, it would push water out of the other side.

If the water was flowing in both directions, the membrane would be first pushed one way (expelling water from the other side) then drawn back (allowing the other side to fill up again)

In the first case water seems to flow for a while then stop. In the latter case the water seems to flow back and forth with little impediment.

An astute observer who knows the inner structure will realise that the water which appears to flow through the capacitor is actually different water that was already stored on the other side of the capacitor. No water flows through, but the "push" from one side is transferred to the other. AN even more astute observer would note that it's not even the "push" that is transferred, but changes in the "push".

As AC (sine wave at least) is continually changing, a capacitor sees a continuing change in push and this is transferred across.

For DC, the capacitor sees a one time change (when it is turned on or off) and that change is transferred across, but it rapidly falls back to zero as the DC stops changing.

So you get a common use of a capacitor, you block a constant voltage, but allow through any changes -- these are typically used in amplifiers to allow the audio signal through, but not any DC component.

7. ### NuLED

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Jan 7, 2012
i thought solar PV panels are DC devices? so you reckon there must be some kind of inverter component in the circuit, that is changing it to AC, which is then fed to the capacitor for the oscillating electromagnet?

8. ### Muhasaresa

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Jan 2, 2012
Thanks for the replies, but there is one more thing I'm not sure about (still about capacitors). I was making a 555 circuit with a capacitor, and it worked. But what I do not understand is why does the capacitor need to be connected to ground, since it is 2 metal plates with insulation. Why should this made a difference when the capacitor charges and discharges on only the top plate as in this picture: http://www.williamson-labs.com/555-tutorial.htm

Thanks

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,412
2,780
Jan 21, 2010
Read my explanation about capacitors as tanks with a diaphragm in them. The same amount of water that is pumped in one side will come out the other. If that pope were blocked, no water could flow into the tank.

Just the same, as you charge a capacitor by pushing electrons in one side, the same number get pushed out of the other side. Connecting it to ground provides a path for these electrons to flow and allows the capacitor to work.

10. ### Muhasaresa

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Jan 2, 2012
Thanks for the help

MuhaSARESA

11. ### jackorocko

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Apr 4, 2010
Ground is your voltage reference, for there to be a voltage present there needs to be a voltage difference/potential between two points. If you never connected the capacitor to ground (or some other ref voltage) there would be no voltage difference and therefor electrons could not flow.

Last edited: Jan 16, 2012
12. ### BobK

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Jan 5, 2010
When a DC voltage is placed across an uncharged capacitor a current flows into the capacitor until the charge across the plates creates a voltage that matches the applied voltage, then no more current flows. So in the steady state, a capacitor "blocks" DC.

When an AC voltage is applied to a capacitor, first current will flow in in the positive half the cycle (as the voltage is increaing) then current will flow out during the negative half of the cycle. So current in contantly flowing in one direction or the other, which is, voila, an AC current. So a capacitor can "pass" AC currents. The amout of current it passes depends on the frequency and the AC voltage. The ratio is called impedance, which is analogous to resistance in a DC circuit.

Bob

13. ### Muhasaresa

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Jan 2, 2012
So I understand why this would happen in a circuit where there is a battery, a resistor and a capacitor, but in my example, what causes electrons to go into the ground end of the capacitor while the other end is discharging. Wouldn't the electrons just go straight into the battery 0V and not charge the capacitor that is connected in parallel? Also, while experimenting with a 1uF capacitor in the 555 example, I was told that I can only put it one way round, but for some strange reason, it also works when I flip it, but the output frequency is 2x slower.

Please can I just solve the last part of this capacitor mystery because I can not sleep at night.

Thanks for the generous help,

Muhasaresa

14. ### jackorocko

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Apr 4, 2010
Is it an electrolytic capacitor? These have a certain orientation, the negative side of the cap needs be connected to ground. I believe if you reverse it the capacitor has a higher internal resistance and this would explain why it drains slower.

what capacitor in parallel? The capacitor is acting like a power source when it is discharging. The stored electrons are leaving the capacitor.

Last edited: Jan 16, 2012
15. ### duke37

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Jan 9, 2011
I agree with Jackorocko about the capacitor likely to be an elecrolytic but do not agree with the mechanism.
Elecrolytic capacitors are complex beasts and have many parameters to specity them correctly. They need the positive terminal to always be more positive than the negative terminal to limit the DC current which can leak through. Reversing the capacitor increases the leakage and in your case this is enough to affect the circuit operation. Operating an electolytic capacitor in reverse is likely to damage it as the dielectric is destroyed.

If you wish to have a capacitor in a circuit with voltage guing positive and negative then you will need a non-polarised capacitor, these have negligible leakage in either direction but are larger.