how do capacitors work as a filter ??

Answer 1

A capacitor passes high frequency signals with little apparent resistance (low reactance).

A capacitor looks like a high resistance load to a low frequency signal.

Put a capacitor is series with the load (such as a speaker) and the capacitor will allow only high frequency signals to reach the load (a tweeter speaker, in this example).

Put a capacitor in parallel to the load, and high frequency signals will by-pass (go around) the load -- since current follows the path of least resistance -- but low frequency signals will still reach the load (a woofer speaker).

Inductors tend to pass low frequency signals, but have more apparent resistance to high frequency signals (the opposite of capacitors). So connecting inductors and capacitors in different circuit configurations will give you different types of filters that block or pass different frequencies.

Since capacitors tend to store charge during periods of high electrical charge in a circuit, and discharge during low periods, they can also be used as a smoothing filter to cut off the peaks and smooth out the valleys in a waveform.

Answer 2

We aren’t going to discuss filter theory – we’re going to discuss why filters don’t work as intended. The major reason is failure to account for the parasitic circuit elements within the component itself and to the coupling between adjacent circuit elements.

We’ll start by looking at the basic filter elements, the capacitor and inductor. Figure 1 shows an ideal, or textbook, inductor and capacitor and its real world equivalent circuit. Basically, all capacitors have some series inductance, resulting in a series resonant circuit; and all inductors have some parallel capacitance, resulting in a parallel resonant circuit. Below resonance, the component performs pretty much as basic circuit theory predicts. Above resonance, the capacitor turns into an inductor and the inductor turns into a capacitor, a notable reversal of roles.

At and above resonance, the published component value ceases to have any meaning. But resonance is the interesting aspect of this article – funny things happen at resonance, and here is where EMI problems, both emissions and immunity, tend to surface.

To get an idea of where these resonances occur, let’s take a look at some typical circuit parameters. Ceramic capacitors are pretty good performers – most of the series inductance occurs due to the external lead length. In the case of surface mount capacitors, the inductance is primarily in the vias, which is about one nH per via, or two nH per capacitor mount. Calculating the resonance frequency from fr = 1/2*pi*sqrt(LC), we find that a 100nF capacitor self-resonates at about 11MHz and a 1nF capacitor at about 110MHz. Above the resonance frequency the capacitors become inductive. That is not to say the capacitor stops working above resonance, but the impedance is definitely on the rise, and the capacitor becomes increasingly ineffective.

For a wound inductor, we like to use the empirical relationship fr = 200/sqrt(L), where L is in uH and fr is in MHz. Thus 100uH inductor resonates at about 20MHz and a 10mH inductor resonates at about 2MHz. Again, above resonance, the inductor looks like a capacitor.

If this is your first real look at resonances, you may be surprised at how low a frequency the resonances occur.

Answer 3

Capacitor works as a filter for low frequencies.

The Reactance Xc=1/2*pie*f*c
Where f=frequency of input
pie=22/7
c=capacitance of the capacitor under consideration

So if u observe, the reactance which is a synonym for resistance is indirectly proportional to frequency.It means that if frequency is less then the reactance is more n Viceversa.

So for dc i.e, for zero frequency, the reactance will be infinity.It implies that it acts as a open circuit.

Its obvious that it is stopping or acting as a filter for DC.

As the frequency is increased the reactance goes on decreasing so that at high frequencies it acts as a short circuit.
Another way of explaining is as follows:

i=CdVc/dt
So the output current will be zero if (dVc/dt) =0
that means for no variation it will be zero which is nothing but dc voltage.Zero current means that it is open circuit which is same as in the previous case

Generally noise signals are low signals.Also the output current will be zero for a sudden increase.It also avoids surge and glitches in electric circuits

Same analogy can be applied to inductor which is just opposite.

Answer 4

A capacitor is a passive electronic component that stores energy in the form of an electrostatic field. In its simplest form, a capacitor consists of two conducting plates separated by an insulating material called the dielectric. In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy.

Answer 5

Capacitors hold a charge. This means that the incoming power can be stored for short periods in a capacitor. If a surge or droop is experienced, the outgoing power from the capacitor will not be affected until the droop or surge is long enough.

Answer 6

Another idea is to look at the reactance of a capacitor. It is lower at higher frequencies, so when a capacitor is connected from a line to "ground" it tends to "short" higher frequencies. So noise on a line is "filtered."

Ron.

Answer 7

They work very well, however they require an upstream resistor or inductor to work against, this requirement necessarily raise the output impedance of the filter thus limiting power available to the load.