# Why is Capacitor used in filtering Dc?

• May 20th, 2010, 01:02 PM
Ekechukwu
Why is Capacitor used in filtering Dc?
Capacitors allow ac to pass thruogh it but NOT dc.

Why?
• May 20th, 2010, 01:42 PM
Janus
When you apply DC to capacitor, electrons begin to fill up one plate and the capacitor charges. This repels the electrons o the other plate, pushing them out the other lead of the capacitor. Once the first plate "fills up" and the other plate "empties", no more current can flow.

With AC, the direction of the current changes back and forth. So before the capacitor can fully charge and stop current flow, the direction reverses and all the electrons piled up on one plates start leave an the other plate starts to fill up. As long as the time it takes for the capacitor to charge in one direction is a fair amount shorter than the time it takes for the current to change direction, you get continuous AC.
• May 20th, 2010, 02:34 PM
Harold14370
Quote:

Originally Posted by Janus
When you apply DC to capacitor, electrons begin to fill up one plate and the capacitor charges. This repels the electrons o the other plate, pushing them out the other lead of the capacitor. Once the first plate "fills up" and the other plate "empties", no more current can flow.

"Filled up" being defined as when the voltage on the capacitor is equal to the voltage of the power supply. (You could always fill it up a little more by raising the voltage.)
• May 20th, 2010, 05:08 PM
Janus
Quote:

Originally Posted by Harold14370
Quote:

Originally Posted by Janus
When you apply DC to capacitor, electrons begin to fill up one plate and the capacitor charges. This repels the electrons o the other plate, pushing them out the other lead of the capacitor. Once the first plate "fills up" and the other plate "empties", no more current can flow.

"Filled up" being defined as when the voltage on the capacitor is equal to the voltage of the power supply. (You could always fill it up a little more by raising the voltage.)

As long as you don't exceed the voltage rating of the capacitor. Then you'll just ruin the capacitor. :D