• pr159260

MemberMar 10, 2013

## How the capacitor blocks dc and allows ac?

XC = 1/(2pi(f)C) capacitive reactance.

XC is inversely proportional to frequency f. Now for DC f=0. Hence XC is infinite(theoritically). Hence it blocks DC.

but any how when we connect capacitor to battery there will be dc current flow in circuit as long the capacitor is fully charged. Then what does actually mean that "capacitor blocks dc " and allow ac?

Anybody help me with this.
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Replies
• MemberMar 10, 2013

Then how the capacitor acts in filtering purpose while AC to DC conversion?
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• MemberMar 11, 2013

I can't understand the question.
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• MemberMar 11, 2013

Capacitor blocks dc current and allow ac ? what does it mean actually. When and where the capacitor blocks the dc current?
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• MemberMar 11, 2013

pr159260
Capacitor blocks dc current and allow ac ? what does it mean actually.
Okay.
Consider a path in which both AC & DC currents are flowing.But,we need only AC as output ;so before load we will mount a specific capacitor which won't allow dc to pass & thus we'll get required output as AC signal.

pr159260
When and where the capacitor blocks the dc current?
When & where ? umm weird question.
Well,it blocks whenever a capacitor is mounted.

Basically,Blocking of DC by Capacitor means Capacitor is charged upto its capacity when DC is applied(AC can't charge capacitors due constant change in frequency).If that capacitor's polarity is changed (after capacitor has blocked some DC) then Capacitor can act as Voltage source.
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• MemberMar 11, 2013

pr159260
Then how the capacitor acts in filtering purpose while AC to DC conversion?
nice question in this application there is a small twist Here you will find initially the capacitor is discharged. So no charge is there in the capacitor. During the positive cycle of the AC the capacitor charges till peak voltage

Once the peak is reached the capacitor starts to discharge But as the time constant of the capacitor discharging is large the voltage does not drop to 0 immediately. It takes a little time and this time is more than enough for AC signal to reach the next peak thus the discharge of the capacitor is more or less similar to DC signal
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• MemberMar 11, 2013

There are many theoretical way to understand how capacitor block D.C and pass A.C. To understand this firstly you have understand the concept of charging and discharging of capacitor. A capacitor is made up of two plates with a insulator between them, this means that these is no physical path between the two plate, this means DC does not flow. When you connect a changing supply it creates a changing magnetic field, this induces an emf into the other plate allow AC to 'flow'.
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• MemberMay 20, 2016

am confused......there is only phase and neutral does AC have polarity .
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• MemberMay 21, 2016

harini nair
does AC have polarity.
Instantaneously it does. One lead is positive relative to the other for half a cycle. Then voltage passes through zero (a "zero crossing") and the same lead is negative relative to the other for the remaining half of the cycle.
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• MemberJun 14, 2016

pr159260
XC = 1/(2pi(f)C) capacitive reactance.

XC is inversely proportional to frequency f. Now for DC f=0. Hence XC is infinite(theoritically). Hence it blocks DC.

but any how when we connect capacitor to battery there will be dc current flow in circuit as long the capacitor is fully charged. Then what does actually mean that "capacitor blocks dc " and allow ac?

Anybody help me with this.
During initial stage of charging the capacitor charges with high speed(max. current flows) and gradually decreases as it tends to get full charge so during this the flow of electric current get changes from maximum to minimum as a result capacitor allows current; but once it got fully charged then due to steady current cap. blocks flow of current in case of dc
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• MemberJun 22, 2016

capacitor allows DC current to flow through it when it is charging or not charging. for example the voltage across it changes. It’s the same case in AC. hence it allows AC current to flow through it. charge stored in it during AC.
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