How two capacitors in parallel are of different values?

in certain circuits i found two capacitors in parallel across the power supply...
i know they are noise rejection but i found that one is of lower value and other is of higher value why is it so...

capacitors

i think the larger the capacitor the better

Re: capacitors

Typically the large capacitor ( electrolytic, etc.) is used to reduce power supply ripple or low frequency components. The smaller capacitor is usually a ceramic or other high-frequency type to reduce digital and high frequency noise. The smaller capacitor is physically placed near sensitive components while the larger capacitor is generally placed near the power input.
Large electrolytic and similar capacitors generally have very poor high-frequency characteristics.

capacitors

There are two kinds of capacitors for power supplies, one for the input and another for the output.
The first one is to reject noise and the value depends on several conditions typical values are less than 1uF. On the other side the second case is important but no critical usually values are 10uF minimum, but recomendation is the bigger the better.

capacitors

hi
there is some capacitor because
for noise reduction we can place capacitor in the nearest position of the ICs.
I test it and in high frequency board it effects very good.
regards

capacitors

hi
a capacitor have their frquncy responce.because it consist of two parts, ideal capacitor and a leakage risistor. and also polarisation of dielectric have thei frquncy responce.
there is many material for dielectric , ceramic ,polyster,......

Re: capacitors

In practical applications, normally you would want a big enough capacitor (like 100uF) at the power pins. This acts as a bucket for any switching in the board. It's response is slow though. When the board needs current (charge), the capacitor will supply them before being re-charge back by the supply input.

To by-pass noise into the power line, we need to use values like 4.7uF, 0.1uF. These are fast, so that any high frequency noise can be by-pass to ground and doesn't stay on the power line.

The frequency response of the capacitors are important if you are working on a low noise board. Big capacitors usually have very low frequency response, their nominal values are only good at around DC. At higher frequency, they will drop.

So to cover a wide range of noise, you will need different capacitors with different values.

Re: capacitors

also, aside from the esr (effective series resistance) of capacitors. Different capacitor have different temperature stability/leakage/accuracy/range.

As a rule of thumb:
Electrolytic - sucks (but cheap), useful for ps filtering when u just need to plop as much capacitance as needed
Mica - good accuracy good at RF
Ceramic - cheap, small, inexpensive, popular
Tantalum - just like electrolytic except for the range (smaller) and size (better)
Teflon - expensive, highest quality

source: Art of Electronics (pg 22)

Re: capacitors

Cost of the component is vital.
A large capacitor is quite costly in comparison to two low value capacitors.
Also the ESR value of two parallel capacitors is low.

Re: capacitors

Hello !!!

I think that capacitors must be selected on conditions of frequency used and resistance of capacitors.

Xc = 1/(2*pi*F*C)

Re: capacitors

Shurik said:

Hello !!!

I think that capacitors must be selected on conditions of frequency used and resistance of capacitors.

Xc = 1/(2*pi*F*C)

Click to expand...

Hi
no,
best value is 100nF
because there is no polarity.
regards

Re: capacitors

Most of the large cap use Electrolytic which has a good low frequecy filtering capability and the Ceramic cap will be used to remove the high frequency.

Re: capacitors

if two capacitor are parallel.the total capacitor is C1+C2if total capacitor is large riplle is very low.

Re: capacitors

the large capacitor with larger inductance is for bypass low frequence noise.

the small capacitor with less inductance is for bypass high frequence noise.

best regards

A.Anand Srinivasan said:

in certain circuits i found two capacitors in parallel across the power supply...
i know they are noise rejection but i found that one is of lower value and other is of higher value why is it so...

Click to expand...

capacitors

hi
if capaciitor is very large,
it can damage your power supply or regulator.
because it acts as short circuit at start up.
regards

Re: capacitors

If we assume that the Large Capacitor is Ideal There is no need to use Small
Capacitor In Parallel with it.
But In Real the Lagre Capacitance is much Far than Ideal than Small Capacitors.

The Non Ideal Large Capacitor has resistance and inductance So if We use Only
the Large Capacitance and connect if to function Generator and increase the frequncey you will see after a specific frequency the capacitance Impedance
will Increase !!!
this is because of the inductance behavior that it shows in high frequencies.

So small Capacitance is much more ideal than larger ones so if we calculate
the impedance of the large Cap and then Parallel it to smaller one we see that
its inductive behavior decreases dramaticly.

this type of desigen is very common in HF RF Circuits we can see there most
of electrolit capacitance have a small capacitance in parallel.
i think the power supply that you told must be switching power Supply.

for example for 100uf Cap they parallel with 10nf cap NOT Because of making the
value of the capacitance EXACT! the 10nf has the Purpose of that i told