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Capacitor help

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Cypher

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Hi all,

I'm trying to learn about Electronics and theres something that i just can't get my head around :)

I've been told to use a Capacitor between positive and ground to filter out AC current. How exactly does this work? If i have the following:

+5v ------------------------------- Circuitry>>
|
R
|
C
|​
0v ------------------------------- Circuitry>>

Where R is a resistor, and C is a capacitor, how does the AC current only flow through the Capacitor and leave the DC current to flow through the rest of the circuitry?

Apologies if it's a simple answer! I just can't get my head around it :)

Many thanks.
 
You can probably understand why the dc doesn't flow through the cap if you know how it is constructed. The difficult concept to grasp is how the ac gets through.
Threre are lots of sources of explanation - mostly physics or basic electronics text books but I'm not sure how many of us really grasp the explanation. Sometimes its better to just accept it and move on.
 
What I don't get is how come the AC current doesn't go through the rest of the circuitry? I thought the capacitor would have to be in series with the circuitry to be able to filter out the AC flowing to it, instead of parallel?
 
Cypher said:
Hi all,

I've been told to use a Capacitor between positive and ground to filter out AC current. How exactly does this work?

Many thanks.

I think you are referring to what is commonly called a by-pass capacitor. Contrary to your diagram, there is no resistor in series with a bypass, it is simply connected between positive and negative (ground) rails.

The reason, as I have been told, is that capacitors hate a change in voltage. Therefore, if there is a sudden surge or spike on the mains, the cap will tend to smooth out this surge, eliminating electrical noise and usually preventing the gliches that commonly occur, especially in digital work. It is the same principle that allows a capacitor to smooth the residual ripple following the bridge rectifier in a power supply.

There is a rule of thumb that, the slower the frequency of operation, the larger cap you need for bypass.
 
First, you don't need the resistor if you are just talking about decoupling the supply lines.
This means sticking a cap directly accross the lines and is meant to filter out spikes and noise so it doesn't get to the rest of the circuit.

The ac (ie spikes and noise and any unwanted RF or audio) sees the capacitor almost as a short circuit whereas the rest of the circuit looks like a higher resistance, so nearly all the ac current goes down through the cap to ground and returns to the supply. That's why these caps are sometimes called bypass capacitors.

Got to go now so cannot post until tomorrow.
 
The essential charastic of a capacitor is that it stores energy and opposes a change in voltage. In a circuit it charges up to the new voltage level that is applied to it and if the voltage tries to decrease it will discharge to try to maintain the voltage.
In AC it has a charastic known as capacitive reactance. This can be calculate by the formula Xc= 1/(6.28 x frequency x capacitance in farads).
So when the capacitor is used as a bypass the reactance should be low at the operating frequency.
 
Cypher said:
AC current only flow through the Capacitor and leave the DC current to flow through the rest of the circuitry?

Nothing flows through a capacitor! don't get confused... :)
inside a cap there are two metal plates with an insulator between them and the only thing it happens is the plates that get charged... the amount of charge it keeps is the capacitance.

When the voltage drops to a lower level than the charged voltage of the capacitor... power is released from the plates! this way the
voltage is constant through the power lines. (theoretically...because there is no ideal capacitor) imagine that extra power like if you have connected a battery instantly after you have realized a voltage drop to your circuitry.

the way you have connected your capacitor in the above circuitry give the following names: filtering capacitor/smoothing capacitor/bypass capacitor/decoupling capacitor, all are the same thing. The resistor is not necessary because it slows the capacitor charging and discharging.

they are some other ways and uses to connect a capacitor...i wont tell you unless you want it! ;)
 
A capacitor is merely an electrostatic storgage device. We normally talk about capacitance the capacitance between two plates; this is really mutual capactance. It's possible for an object consisting of a single conductor to have a capacitance, for example a van de graff generator sphere might have a capacitance of 100pf.
 
Ahh I understand now, I was mistaken and thought that a current always flows through a capacitor whereas a current only flows when it is charging or discharging.

Thanks for all your help guys! :)
 
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