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Author Topic: Capacitor question  (Read 1296 times)

Samuel Rees

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Capacitor question
« on: October 07, 2012, 01:00:03 pm »

Hoping someone can double check my understanding on a simple capacitor function in a simple DC circuit question. I'm no electronics guy, just trying to bone up a little bit on small circuit design. If it helps, I'm just trying to understand a really simple cap charge and discharge circuit I made with a switch, an led, a 9v, and a cap.

Thanks!

1- Once a cap is charged, it functions as an 'open' circuit, correct? Any direct current flowing through a cap is 'leakage' and is not intentional by design, correct?

2- In a "perfect" capacitor, electrons never actually cross from plate to plate (or pin to pin), just a charge from field effect, correct?
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John Roberts {JR}

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Re: Capacitor question
« Reply #1 on: October 07, 2012, 02:39:23 pm »

Hoping someone can double check my understanding on a simple capacitor function in a simple DC circuit question. I'm no electronics guy, just trying to bone up a little bit on small circuit design. If it helps, I'm just trying to understand a really simple cap charge and discharge circuit I made with a switch, an led, a 9v, and a cap.

Thanks!

1- Once a cap is charged, it functions as an 'open' circuit, correct? Any direct current flowing through a cap is 'leakage' and is not intentional by design, correct?
In terms of a DC driving voltage it will look like a high impedance when charged because it is no longer drawing current, but it's reactance will always look like an impedance or resistance to an AC driving voltage. (technically not a resistance). 
Quote
2- In a "perfect" capacitor, electrons never actually cross from plate to plate (or pin to pin), just a charge from field effect, correct?

Yes, technically they just accumulate on one plate of the capacitor or the other. (or maybe it's the holes that appear to move).

Leakage current occurs between the plates because the insulation resistance is not infinite.

JR

PS: I vaguely recall in one night school semiconductor physics course while I briefly tried to matriculate (again) after getting out of the army (this second time didn't stick either). ;D. The night school instructor (a working engineer for his day job) drew an equation on the black board for electron motion wrt a capacitor plate, but he made a mistake and there was a + constant term in the equation so for every cycle of AC motion the electron would end up an incremental distance in one direction or the other away from the plate. I pointed that out to him after the class, and he said something like never mind, you'll never need that.  ;D  I suspect a more complete equation  cancelled out that constant term.

PPS: Your simple charging circuit could benefit from a small resistor in series, since there could be large instantaneous current when you first connect a fully discharged cap to a voltage source. This current surge will be somewhat limited by the ESR (series resistance) of the cap, but in extreme cases it could damage the cap, or LED from  momentary high current. (The battery will have an effective internal resistance too... 9V batteries are only around 250 mA-hours so peak current output into a dead short could be a couple amps, not exactly a car battery but not nothing.
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Samuel Rees

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Capacitor question
« Reply #2 on: October 08, 2012, 05:12:22 pm »

Thanks a lot, that really helped!
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Samuel Rees

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Capacitor question
« Reply #3 on: October 08, 2012, 05:25:52 pm »

double post
« Last Edit: October 08, 2012, 09:31:57 pm by Samuel Rees »
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John Roberts {JR}

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Re: Capacitor question
« Reply #4 on: October 08, 2012, 05:43:05 pm »

Your welcome.

JR

PS: Your welcome
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