# Capacitor

Discussion in 'General Electronics Chat' started by Manasakrishna, Dec 7, 2010.

1. ### Manasakrishna Thread Starter New Member

Sep 7, 2010
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According to the equation Xc=1/2(pi)fC capacitor should behave as a closed circuit for DC current.But if it were so it could never be charged using DC current,because current doesn't pass through it if resistane is infinity.
Also I have not understood how a capacitor can separate DC component from AC component when it is used in amplifier circuits.

Thank you.

2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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The impedance equation shows what the resistance would be at a given frequency.

The plates of the capacitor do not touch, so when DC is applied, a short burst of current does flow through the capacitor, as the edge of a square wave is essentially a very high frequency component, high frequencies do pass through capacitors easier.

Caps block DC, they are used in input stages of an amplifier to remove any DC bias from an incoming AC signal so the signal can be "re-biased" to match the amplifier. On the output of the amp, there is a "decoupling cap", which removes any DC bias the amplifier may have added to the signal.

Last edited: Dec 7, 2010
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3. ### mik3 Senior Member

Feb 4, 2008
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$Xc=\fra{1}{2pifC}$

This equation is valid only for steady state conditions. During transients use the following equation:

$i=C\fra{dV}{dt}$

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4. ### steinar96 Active Member

Apr 18, 2009
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Manasakrishna, what you need to keep in mind is that many assumptions are simply to avoid unneeded mathematics. These are approximations which are quite good when they can be applied.

For example, when you turn on your DC circuit then for the first mili or microseconds your circuit is not composed of DC current but AC currents while the circuit is charging towards "steady state". This brief moment is referred to as transient response when the circuit is "setting" itself up to a point where it's working as it was designed to.

During this time capacitors see "changing voltages" where they charge themselfs up.

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5. ### Manasakrishna Thread Starter New Member

Sep 7, 2010
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I thank you for answering my question.But I am still uncertain about the thing that current does flow in simple circuit of capacitor connected to a DC source which is contradicted by theoretical value of impedance being infinity.It is true that current doesn't flow across the capacitor plates but it does flows in external circuit.So how can impedance be infinite?