Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Capacitor in DC and AC

Status
Not open for further replies.

skylines

New Member
Hey,
Can anyone explain me how Capacitor blocks DC and allows AC?

I can understand this from mathematical expression of capacitive reactance (Xc).
where Xc = 1/(2*3.14*f*c)

In DC f is zero that means Xc is infinife. since reactance is infinite , it does not allow current to flow through it and acts as open circuit.

But i am not satisfied with the above one(correct me if i am wrong).Can anyone let me know scientifically the reason for capacitor to block DC and allow AC?
 
The simple answer is that with AC electrons pile up on one plate and are sucked off the other. Then the polarity reverses and the electrons are sucked off the full plate and move through the external circuit to fill the empty plate. You can’t see it unless you consider the whole circuit rather than just the single device.
 
You may want to read this thread. I believe you will find it pretty well covered in the first few pages.

Ron
 
skylines,

Can anyone explain me how Capacitor blocks DC and allows AC?

Yes, I can.

In DC f is zero that means Xc is infinife. since reactance is infinite , it does not allow current to flow through it and acts as open circuit.

But i am not satisfied with the above one(correct me if i am wrong).Can anyone let me know scientifically the reason for capacitor to block DC and allow AC?

Whether the voltage is AC or DC, a perfect capacitor does not allow any charge to flow through it. A capacitor does not store or release a net charge, it stores and releases energy. It does this by separating and combining the opposite charges between the plates, and that requires the storage and release of energy. An ammeter placed in series with a capacitor appears to show a charge flow through the capacitor, but it is really showing the rate of charge accumulation on one plate, and the rate of charge depletion on the other. To iterate, no current exists through the capacitor.

Ratch
 
Hello Ratch,

Are you sure you want to bring that up again? :)
 
A capacitor consists of two conducting plates separated by a dielectric. Thus it is impossible for a DC current to flow since there is no conductive path.
 
Hey,
Can anyone explain me how Capacitor blocks DC and allows AC?

I can understand this from mathematical expression of capacitive reactance (Xc).
where Xc = 1/(2*3.14*f*c)

In DC f is zero that means Xc is infinife. since reactance is infinite , it does not allow current to flow through it and acts as open circuit.

But i am not satisfied with the above one(correct me if i am wrong).Can anyone let me know scientifically the reason for capacitor to block DC and allow AC?

When a DC voltage source is connected to a capacitor, current flows from the source into one plate, causing charge to build up on that plate. Current also flows out of the opposite plate, back to the source, which causes charge to deplete on that plate. As charge builds up on one plate and an equal charge is depleated from the other, the voltage of the capacitor rises in step with the accumulated/depleated charge across the capacitor. Once the voltage rises to the same level as the voltage source, then no more current can flow into the capacitor, because the voltages are equal.

If however, the DC voltage changes for any reason (battery runs down for example) then current will flow into or out of the capacitor, until a new voltage equalibrium is established between the source and the capacitor.
 
Last edited:
I am not able to get the exact answer from discussion.

First how can we tell DC is blocking current? Then we can go to AC.

If we apply a DC voltage to capacitor and connect a bulb in series with capacitor ,does the bulb glow?
 
...If we apply a DC voltage to capacitor and connect a bulb in series with capacitor ,does the bulb glow?

Assuming the capacitor was initially discharged, then for a little while...:D
 
what happens if AC is applied to a capacitor?

I am able to understand that ,For 1st half of the cycle,i.e for +ve cycle, during the voltage rise from v0 to vpeak, the capacitor charges, and during the voltage drop from vpeak to v1 the capacitor discharges.Let me know if this is true.

I assume
v0 as the intial voltage at an angle 0
vpeak as the highet voltage at an angle 90
v1 as the voltage at an angle 180 i.e voltage at the end of the +ve half cycle.

And also let me know what happens during -ve half cycle?
 
Look at these two simulations: They should help you visualize what is going on.

In the first one, a 2V step is applied to the coupling capacitor. Current flows until the capacitor is charged to 2V. The current decays from 20mA to near zero in the classic exponential way.

In the second one, a 1Vp-p 100Hz sine wave rides on top of a 2V dc level. The sim begins after the capacitor is already charged to 2V, which means that V(out) is just the peak to peak AC component (the DC component has been blocked by the capacitor).
 

Attachments

  • DF166D.png
    DF166D.png
    24 KB · Views: 285
  • DF166A.png
    DF166A.png
    27.7 KB · Views: 240
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top