# Decoupling and Filtering Capacitor Value?

yy.toh
Guys, dc decoupling, ac decoupling and ac filtering capacitor are frequently used. How do we determine(select) the value of its capacitance?

Fish4Fun
yy.toh,

Decoupling capacitors typically refer to capacitors placed physically near the Vcc pin(s) of ICs to prevent voltage transients from affecting IC operation. ( http://en.wikipedia.org/wiki/Decoupling_capacitor ). A common value for digital logic ICs is .1uF, though some particularly sensitive chips will call for parallel capacitors (eg: 10uF Tantalum, .1uF Ceramic and .001uF Ceramic.)

If by "ac decoupling" you mean a capacitor to block a DC bias voltage, then the value is a function of the AC frequency response and magnitude of the voltage. http://en.wikipedia.org/wiki/Capacitive_coupling

If by "ac filtering" you mean power supply filter capacitors placed after the rectifiers, then the value is selected based on the load requirements and the transients on the unfiltered supply. In some cases as little as 10s of uF, in other cases thousands of uF.

Fish

yy.toh
I am confused.

1. What is the name of the capacitor to block the DC?How to select the capacitance?
1. What is the name of the capacitor to block the AC?How to select the capacitance?
3. What is the name of the capacitor conneted between the terminals of a dc motor, what is its function and how to select its capacitance?

Fish4Fun
1. All capacitors block DC in a series circuit.
2. Capacitors do not block AC, the relationship of capacitive impedance to frequency is Rc = 1/(2 * Pi * f * C)
3. On a brushed DC motor, a capacitor is frequently used to reduce brush noise and to some degree help limit voltage spikes. The value is typically small, 1uF or less with a high voltage rating (10 times supply voltage or more). In some cases two capacitors of different values are used, typically one in the low uF range and the other in the pF range. For some AC motors a capacitor is used for starting and/or running, you can read about them here: http://en.wikipedia.org/wiki/Motor_capacitor

Fish

yy.toh
1. All capacitors block DC in a series circuit.
2. Capacitors do not block AC, the relationship of capacitive impedance to frequency is Rc = 1/(2 * Pi * f * C)
3. On a brushed DC motor, a capacitor is frequently used to reduce brush noise and to some degree help limit voltage spikes. The value is typically small, 1uF or less with a high voltage rating (10 times supply voltage or more). In some cases two capacitors of different values are used, typically one in the low uF range and the other in the pF range. For some AC motors a capacitor is used for starting and/or running, you can read about them here: http://en.wikipedia.org/wiki/Motor_capacitor

Fish

1.When capacitor block dc, we call it coupling, right?How to determine the capacitance to block the dc?
2. When capacitor connected in parallel to ground, it short-circuited the ac to ground, we call it decoupling, right? How to determine the capacitance to short-circuit the ac?

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1.When capacitor block dc, we call it coupling, right?How to determine the capacitance to block the dc?

Used in an AC or varying DC operating circuit ... say between stages of an amplifier
the value of the capacitance will vary depending on the frequencies that you want to pass between stages in the circuit

eg an audio cct say between a microphone amplifier stage and the next amp stage, the value may be ~ 4.7uF - 10 uF

in a RF circuit the value between stages could vary from 100's of pF @ 30MHz to only a 2 to 4pF at 5GHz

2. When capacitor connected in parallel to ground, it short-circuited the ac to ground, we call it decoupling, right? How to determine the capacitance to short-circuit the ac?

Yes it is called decoupling....
a common place will be in a power supply after the rectifier where there will be 1 or more big smoothing caps generally "rule of thumb" is 1000uF for every amp of current
So a PSU supplying 10Amps you would want at least 10,000uF for good smoothing.
You will also use some small value caps like 0.1uF or 0.001uF for decoupling any higher freq's that may be present on the DC line.

Dave