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Capacitor-input filter

Thecapacitor-input filter, also called the pi filter due

to its shape that looks like the Greek letter , is a type

ofelectronic filter. Filter circuits are used to remove un-

wanted or undesired frequencies from a signal.

A simple pi filter, containing a pair of capacitors, an inductor,

and a load.

A typical capacitor input filter consists of a filter or reser-

voir capacitorC1, connected across the rectifier output,

an inductorL, in series and another filter or smoothing

capacitor, C2, connected across the load, RL. A filter of

this sort is designed for use at a particular frequency, gen-

erally fixed by the AC line frequency and rectifier config-

uration. When used in this service, filter performance is

often characterized by itsregulationandripple.

1 Operation

The capacitor-input filter operates in three steps:

1. The capacitor C1 offers low reactance to the AC

component of the rectifier output while it offers in-

finite resistance to the DC component. As a result

the capacitor shuntsan appreciable amount of the

AC component while the DC component continues

its journey to the inductor L.

2. The inductor L offers high reactance to the AC com-

ponent but it offers almost zero resistance to the DC

component. As a result the DC component flows

through the inductor while the AC component is

blocked.

3. ThecapacitorC2 shunts the AC component which

the inductor had failed to block. As a result only the

DC component appears across the load RL.

The component value for the inductor can be estimated as

an inductance that resonates the smoothing capacitor(s) at

or below one tenth of the minimum AC frequency in thepower supplied to the filter (100 Hz from a full-wave rec-

tifier in a region where the power supply is 50Hz). Thus if

reservoir and smoothing capacitors of 2200 microfarads

are used, a suitable minimum value for the inductor would

be that which resonates 2200 microfarads (F) to 10 Hz,

i.e. 115 mH. A larger value is preferable provided the

inductor can carry the required supply current.

In general, the relationship between the resonant fre-

quency, which should be less than or equal to one tenth

of the minimum AC frequency, in this case 100 Hz, the

capacitance, and the inductance can be characterized by

the followingresonance equation: f0 =1

2LC

.

2 Comparison with other filters

Capacitor input filters can provide extremely pure DC

supplies, but have fallen out of favour because inductors

tend to be unavoidably heavy, which has led to the often-

preferred choice ofvoltage regulatorsinstead.

Advantages of pi filters:

More output voltage

Ripple-free output

Disadvantages of pi filters:

Large size

Heavy

High cost

3 References

4 See also

Electronic filter topology - contains a general definition of

a pi section filter topology, of which this is an example.

1

https://en.wikipedia.org/wiki/Electronic_filter_topologyhttps://en.wikipedia.org/wiki/Voltage_regulatorhttps://en.wikipedia.org/wiki/LC_circuit#Resonance_effecthttps://en.wikipedia.org/wiki/Capacitorhttps://en.wikipedia.org/wiki/Inductorhttps://en.wikipedia.org/wiki/Shunt_(electrical)https://en.wikipedia.org/wiki/Reactance_(electronics)https://en.wikipedia.org/wiki/Capacitorhttps://en.wikipedia.org/wiki/Ripple_(electrical)https://en.wikipedia.org/wiki/Voltage_regulationhttps://en.wikipedia.org/wiki/Inductorhttps://en.wikipedia.org/wiki/Capacitorhttps://en.wikipedia.org/wiki/Electronic_filterhttps://en.wikipedia.org/wiki/Pi_(letter)https://en.wikipedia.org/wiki/Greek_alphabet

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2 5 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

5 Text and image sources, contributors, and licenses

5.1 Text

Capacitor-input filter Source: https://en.wikipedia.org/wiki/Capacitor-input_filter?oldid=700574696Contributors: Charles Matthews,

Robbot, AJim, Icairns, Hooperbloob, DV8 2XL, Joriki, SeventyThree, Gaius Cornelius, Xapxivos, SmackBot, Rdarlington, Steve carl-

son, JL-Bot, Addbot, Redheylin, Matj Grabovsk, Gongshow, Gargan26, Materialscientist, Armstrong1113149, Erik9bot, EmausBot,

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5.2 Images

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