Coupling capacitors!

[akki]

Hello everybody!
Can anybody of u fine people tell me bout the operation of the coupling capacitors. or how these capacitors r capable of blocking DC signals & pass AC signals.

 

[Kevin Weddle]

A capacitor draws current from both sides when there is a rate of change to the applied voltage. Since the current that is being drawn through a resistor creates voltage, you have what appears to be a passing of the voltage.

 

[prateeksikka]

hi all!
i guess capacitor only has a fictitious displacement current through it and no actual current is drawn by itself.isn't it the case?
prateek

 

[audioguru2]

No Prateek,
A capacitor does draw a DC current when it is charging, and supplies a DC current when it is discharging.
Don't your room lights dim when you turn on your computer or high-power stereo? That is caused by its filter capacitor drawing current when it is charging.

 

[Kevin Weddle]

Hi prateek. What I think you are talking about is how there is no current through the capacitor. The capacitor draws current from both sides and equally as a matter of fact.

Hey audiguru, about the triangle applied to an RC circuit. It appears the current of the capacitor is constant yet there is a change in voltage across the the resistor which means a change in current.

10 5
resistor capacitor
5 2.5

change in current through resistor. Does this have to do with phase difference? Yes it does.The actual voltage is :


10 8.75
resistor capacitor
5 6.25

This means the current is constant and the phase between the capacitor voltage and the source voltage is 45 degrees. Interesting isn't it.

 

[prateeksikka]

hi kevin and audioguru!
although no current flows through the capacitor plates ,but i clearly remember a term called displacement current associated witha capacitor.wat i that,who is getting displaced there!!!?????
plz reply
prateek

 

[audioguru2]

Hi Prateek,
Sure, displacement current occurs when a capacitor is charged or discharged. One plate is stripped of electrons while the other plate has plenty when charged. Discharging developes a current that equalizes the charge of the plates.

 

[prateeksikka]

hi aludioguru!
i must add here that some books also say that there is no displacement
current as such .it is just like a fictitious current just an analogy with
the psuedo force (backward force when a car starts suddenly)
this concept is introduced in security of maxwells equations of flux.any replies are welcome
prateek

 

[audioguru2]

Hi Prateek,
You can call the current in a charging and discharging capacitor circuit "displacement" or any other kind of current name. There is actually real current flowing, it is not psuedo current.
The ripple current flowing into and out of a filter cap heats the cap due to its Equivalent Series Resistance and if the current is high enough can cause the cap to blow-up!

 

[Shahriar]

Hi

I Think this is why a Cap do not pass DC current...
The current Formula in a Cap is as Follow:

i=C dV/dt + v0

This formula indicates that we have current in a cap If the Cap Voltage Changes (The meaning of dV/dt). So If you just imagine DC voltage across a Cap, dV/dt = 0 ----> i=0

This formula also indicates why we have spark, when we short a charged Cap. when you do this, dt becomes very small so dV/dt increses so current will increase.

This Formula aslo indicates why small Caps are uses to remove small and fast changes in regulator voltage (i.e. before and after a 78xx regulator) but how?

Delta(v) = 1/C Integral ( i dt )

if dt become very small, Delta (v) becomes zero, it means the voltage across a cap do not accept fast voltage changes.

If you ask where did this formula came from, you can find it in Electronic Books.

HTH - Shahriar

 

[audioguru2]

Hi Shahriar,
A small cap like a ceramic or tantalum one removes fast voltage changes because its internal resistance and inductance is very low. An electrolytic cap is poor for filtering high frequencies because it is wound like an inductor. You frequently see an electrolytic cap to filter lower frequencies in parallel with a small ceramic cap to filter higher frequencies.

 

[prateeksikka]

HI AUDIOGURU!
THE FORMULA POSTED BY SHAHRIAR CLEARLY TELLS US THAT DISPLACEMENT CURRENT IS A FICTITIOUS CURRENT ASSUMED TO SATISFY THE EQUATION!!!!
(MAXWELL'S EQUATION)
SOME BOOKS ALSO SAY THAT NOTHING IS GETTING DISPLACED,THE NAME IS JUST A JAM DUMMY AND PURELY AN ASSUMPTION,YOU COULD CALL IT AS YOU LIKE BUT IT IS JUST A CONVENTION TO CALL IT A DISPLACEMENT CURRENT(FUNDAMENTALS OF PHYSICS BY RESNICK AND HALLIDAY SAYS THIS)
PRATEEK