How does capacitor block DC and allow AC

 How does capacitor block DC and allow AC

Table of contents

1)what is capacitor
2)difference between ac and dc
3)why does capacitor block dc
4)Thumb rule
5)application of capacitor in DC
6)application of capacitor in AC

what is capacitor?

capacitor (also known as a condenser) is a two metal plates device separated by an insulating medium such as laminated paper air etc.

it store the energy in the form of electrostatic field and release to the circuit when needed in case of AC.

it storage ability is measures in farad, microfarad and nanofarad etc.

keep in mind that capacitor acts as an open circuit in DC.

Difference between AC and DC

DC is a constant value i.e. it doesn't change the polarity (direction) and magnitude while AC changes it's direction and amplitude continuously related to it's frequency.

why does a capacitor block DC?

keep in mind that capacitor acts as an short circuit at initial stage and a fully charge capacitor behave as an open circuit.

capacitor resist a change in voltage while inductor resist a change in current and acts as a short circuit in DC.

At initial stage when we connect the capacitor to the dc supply there will a small current of flow will occur until a plate becomes saturated.

In other word the positive terminal of the dc supply source will suck the electron from one terminal and push the electrons to the second terminal until the first plate becomes positively charge and second one is the negatively charge.

At this stage the applied voltage equal to the voltage across capacitor and capacitor plates are saturated and there is no more flow of current.

At this stage capacitor behave like as an open circuit and if we increase the value of applied dc voltage the capacitor may damage and explode.

let's see with a solved example of a dc connected capacitor

we know that there is no frequency 

if we put frequency =0 in the inductive reactance

                  Xc =1/2piFC

                 Xc=1/2pi×0×C

                 Xc=1/0

                 Xc=infinity

 

it means theoretically a capacitor will provide infinite resistance to the flow of current according to it's rating.

hence no current flow will occur as current in capacitive circuits are

I=V/Xc

if we put Xc is infinity the value of current would be zero

                   I=0amp

That is the exact reason why a capacitor block DC

As a rule of thumb.

AC marked capacitor can be used on DC

DC marked capacitor can not be used on AC

because the AC voltage shows the RMS VALUE where the peak value of AC is 1.414 times greater than DC.

 

Applications of capacitors in DC

1)rectifier

2)filter

3)conditioning

4)coupling capacitor and decoupling capacitor etc.

Application of capacitor in AC

1)power factor improvement and correction

2)split phase induction motor etc.