Why capacitor pass AC & block DC - Electronic Circuits Projects Diagrams Free

azaz742 · 23rd November 2004, 06:56 PM

Hello every body

My question is
Why capacitor allow to pass AC and not for DC?

Thank advance

Dean Huster · 23rd November 2004, 07:15 PM

Capacitive reactance -- the "resistance" of a capacitor to current flow -- is found by 1/(2*pi*f*C). For high frequencies, this "resistance" is low, allowing current flow. The lower the frequency, the higher this "resistance" is. DC or direct current is consider "zero" Hertz frequency and the capacitive reactance ("resistance") at this point is infinite, allowing no current flow at all. So, the higher the frequency, the easier it is for the signal to pass through the capacitor; the lower the frequency, the more difficult it is for current to pass and impossible for DC.

Dean

Styx · 23rd November 2004, 09:33 PM

well that was teh maths behind it

but physically, a capacitor is two plates with some form of insulator between them (that has di-electric proporties).

Hence a capacitor cannot pass DC since a capacitor is an open circuit.

For AC however, both plates get charged up with a certain polarity, when the voltage swaps over that charge then gets pulled off. So although capacitors "appear" to pass AC it doesn't really

no power is transfered, just signal infomation

Russlk · 24th November 2004, 01:18 AM

Styx was correct up to a point, but power can be transferred thru a capacitor. Power = I^2*R as always. The capacitor does not dissapate power.

Styx · 24th November 2004, 08:57 AM

What I mean is no "current" passes through a capacitor (if you ignore leakage)

checkmate · 24th November 2004, 09:10 AM

The threadstarter must be very new to electronics to ask such a question. I'd suggest he grab any electronic book and read through the introductory chapters. These kind of information is fundamental in all of electronics.

Russlk · 24th November 2004, 01:04 PM

A DC transient current does pass thru a capacitor, up to the point of the capacitor becoming fully charged. An AC current passes thru the capacitor continuously.

checkmate · 24th November 2004, 02:03 PM

Quote:

Originally Posted by Russlk

A DC transient current does pass thru a capacitor, up to the point of the capacitor becoming fully charged. An AC current passes thru the capacitor continuously.

The current doesnt really pass through in the physical sense, unless you are referring to displacement current. Anyway, venturing any deeper would be confusing the threadstarter even further.

Juglenaut · 24th November 2004, 04:18 PM

Current leads voltage.

Styx · 24th November 2004, 05:24 PM

Quote:

Originally Posted by Juglenaut

Current leads voltage.

WooHoo a good fraise I like for that is

CIVIL

For a capaictive load Current leads volts (C I V)

For an inductor load voltage leads current ( V I L)

Juglenaut · 25th November 2004, 11:23 AM

LoL :lol:

H /\ E K T P O N · 25th November 2004, 12:48 PM

Alternations (AC) between + and - cause the plates of the Capacitor to be equally but differently charged. They attract each other, and Electric field exists.

On the other hand, DC is constant and due to this, there are no opposite charges on plate 2 to attract the charges of plate 1. Consequently, DC is blocked.

azaz742 · 25th November 2004, 07:06 PM

Thank's for All
And really I guite understand.

Sorry my language is not very well

ops:

23rd November 2004, 06:56 PM	(permalink)
azaz742 New Member	Why capacitor pass AC & block DC Hello every body My question is Why capacitor allow to pass AC and not for DC? Thank advance

23rd November 2004, 07:15 PM	(permalink)
Dean Huster Experienced Member	Capacitive reactance -- the "resistance" of a capacitor to current flow -- is found by 1/(2pif*C). For high frequencies, this "resistance" is low, allowing current flow. The lower the frequency, the higher this "resistance" is. DC or direct current is consider "zero" Hertz frequency and the capacitive reactance ("resistance") at this point is infinite, allowing no current flow at all. So, the higher the frequency, the easier it is for the signal to pass through the capacitor; the lower the frequency, the more difficult it is for current to pass and impossible for DC. Dean __________________ Dean Huster, Electronics Curmudgeon Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines). R.I.P.

24th November 2004, 01:18 AM	(permalink)
Russlk Experienced Member	Styx was correct up to a point, but power can be transferred thru a capacitor. Power = I^2*R as always. The capacitor does not dissapate power. __________________ see my website: www.geocities.com/russlk

24th November 2004, 01:04 PM	(permalink)
Russlk Experienced Member	A DC transient current does pass thru a capacitor, up to the point of the capacitor becoming fully charged. An AC current passes thru the capacitor continuously. __________________ see my website: www.geocities.com/russlk

24th November 2004, 04:18 PM	(permalink)
Juglenaut Experienced Member	Current leads voltage. __________________ Your website has been blocked

23rd November 2004, 09:33 PM	(permalink)
Styx Experienced Member	well that was teh maths behind it but physically, a capacitor is two plates with some form of insulator between them (that has di-electric proporties). Hence a capacitor cannot pass DC since a capacitor is an open circuit. For AC however, both plates get charged up with a certain polarity, when the voltage swaps over that charge then gets pulled off. So although capacitors "appear" to pass AC it doesn't really no power is transfered, just signal infomation

24th November 2004, 08:57 AM	(permalink)
Styx Experienced Member	What I mean is no "current" passes through a capacitor (if you ignore leakage)

24th November 2004, 09:10 AM	(permalink)
checkmate Experienced Member	The threadstarter must be very new to electronics to ask such a question. I'd suggest he grab any electronic book and read through the introductory chapters. These kind of information is fundamental in all of electronics.

25th November 2004, 11:23 AM	(permalink)
Juglenaut Experienced Member	LoL :lol: __________________ Your website has been blocked

25th November 2004, 12:48 PM	(permalink)
H /\ E K T P O N New Member $H /\ E K T P O N is on a distinguished road$	Alternations (AC) between + and - cause the plates of the Capacitor to be equally but differently charged. They attract each other, and Electric field exists. On the other hand, DC is constant and due to this, there are no opposite charges on plate 2 to attract the charges of plate 1. Consequently, DC is blocked.
$H /\ E K T P O N is offline$

25th November 2004, 07:06 PM	(permalink)
azaz742 New Member	Thank's for All And really I guite understand. Sorry my language is not very well ops:

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