LC Pi Filter:Pi Filter ,Working,Critical FACTS

This article will discuss about LC PI filter in detail.

What is LC filter circuit

The unique characteristics of the inductor and the capacitor are used to remove noise in a different configuration.

LC filter circuit is an LC circuit that is used as a filter, which consists of an inductor and capacitor; LC filter can cut or pass signal from a certain frequency range. An LC filter can be a high pass filter (HPF), low pass filter (LPF) or a bandpass filter (BPF).

LC filters can be classified by different designs or configurations:

  • L type filter.
  • Pi-type filter.
  • T- type filter.
300px Lattice filter%2C unbalanced T.svg
Image Credit: T type filter, SpinningSpark real life identity: SHA-1 commitment ba62ca25da3fee2f8f36c101994f571c151abee7, Lattice filter, unbalanced TCC BY-SA 3.0

What is Pi type filter

The Pi filter is majorly used as a low pass filter, which consists of three components. It can also be used as a high pass filter configuration.

The pi filter is a type of LC passive filter, which is constructed using an inductor and capacitor forming the shape of a greek letter (pie); that’s why it’s known as a pi filter.

This filter is also known as a capacitor input filter because the output of the rectifier is directly fed into the capacitor, which is connected in parallel with the rectifier. LC pi filter can also be known as CLC filter due to its design or components in use.

Image: High Pass LC Pi filter.

What does LC filter do

Application or use of LC filters:

Why LC filters are used at high frequency

Nowadays, more sensitive circuits, high-speed logic and more electrical noise at the high frequency require highly efficient filtering of noises for proper operations.

LC filter is used at high frequency because it can eliminate AC ripples with the highest efficiency than other filters and can provide a smooth DC signal as output.

LC pi filter formula

For low pass LC pi filter formula:

Image: Low Pass LC Pi filter circuit.

Cut off frequency(fc) = 1/ᴫ(LC)1/2
Value of the Capacitance is (C) = 1/Z0ᴫfc
Value of the inductance (L1) = Z0/ᴫfc
Where, the Z0 is the impedance characteristic in ohms and fc is the cut off frequency.
SparkX Pi-Filter
Image Credit: “SparkX Pi-Filter” by SparkFunElectronics is licensed under CC BY 2.0

Low pass LC pi filter

The pi filter is used where the high dc output voltage is required on small current drains because of its high voltage gain.

Low pass LC pi filter has an inductor in series with the load, and two capacitors, one capacitor in parallel with the source and another capacitor in parallel with the load.

lc pi filter
Image: Low Pass LC Pi filter.

The capacitor C1, which is in parallel with the source, has low reactance towards AC components of the input signal while having high resistance towards the DC component of the input signal, resulting in most of the Ac component of the signal passing through capacitor C1 and the DC component moving towards the inductor of the circuit. The AC component gets blocked by the inductor, now the capacitor C2, which is in parallel with the load filter out the AC component which inductor failed to block. The potential drop across the inductor and capacitor C2 is minimal. In this way, the load only gets the DC component of the input signal with the maximum AC component filtered out.

LC pi filter cutoff frequency

The cutoff frequency for a low pass LC pi filter is the frequency above which the filter starts filtering the frequency signal.

Image: Low Pass LC Pi filter circuit.

The cutoff frequency for a high pass LC pi filter is the frequency below which the filter starts filtering the frequency signal.

For low pass LC pi filter circuits, we know for cutoff frequency:

L     =     Zo / (2pi x Fc) Henries

 C     =     1 / (Zo x 2pi x Fc) Farads

 Fc     =    1 / (2pi x square root ( L x C) Hz

Which filter L or PI has better regulation

The pi filter has better load regulation than that of an L filter because the Ripple factor of the Pi filter is much lower than that of an L filter.

The pi filter is used for relatively lighter loads than an L filter.

What is the advantage of pi filter

Advantages and benefits of LC Pi filter:

  • High output voltage makes it capable of use in power-related applications, where high voltage direct current (HVDC) filters are required.
  • A high-frequency LC pi filter is easy to design, which is also immune to harsh environments and surges.
  • Highly efficient to filter out unwanted AC ripples.
  • If the source impedance is much higher than load resistance, then the pi filter is best for application.
  • It can be used with both a half-wave rectifier (HWR) and a full-wave rectifier (FWR).
  • For the smaller ripple factor, for identical values of inductor and capacitor, the ripple factor is much smaller than multi LC filters.
  • Smooth DC output.
  • This filter is used where low input current and high output Dc voltage is required.

Why Pi filters are not suitable for varying loads?

The pi filter is an LC passive filter which can be a low pass or high pass filter depending on the design or configuration of the filter.

The pi filter is not suitable because of the poor voltage regulation of the filter, and this is because the output voltage rapidly drops along with the increase in current through with the load.

Is LC a low pass filter

An LC low pass filter is a filter that allows passing signal frequency lower than the cutoff frequency. 

LC filter can be constructed as a low pass filter.

What is the ripple factor in LC filter

Ripple factor of LC filter is the ratio of the rectified output RMS value of AC component to the dc value of rectified output.

Ripple factor = √(Vrms/Vdc)2-1=√(Vm/2/Vm/Π)2-1 = 1.21. Effieiency = (Vdc/Vrms)2 = (Vm/Π/ Vm/2)2=0.405X100= 40.5%.

LC filter
Image: LC filter circuit.

Theoretical calculation (without filter) :
Ripple factor = √(Vrms/Vdc)
-1 = 1.21.
Effieiency = (Vdc/Vrms)
2 = (Vm/Π/ Vm/2)
=0.405X100= 40.5%.
Experimental calculation (without filter):
Ripple factor = √(Vrms/Vdc)
-1 = √(14/11.2)
-1 =0.75
Efficiency = (Vdc/Vrms)
2 = (11.2/14)2
= 0.64X100 = 64%
Theoretical calculation (with filter) :
Ripple factor = 1/ (2√3 f RLC) =1/2√3X 50X 103
= 0.577
Experimental calculation (with filter) :
Ripple factor = Vrms/Vdc =6.12/17.4=0.351

What is the difference between RC and LC filters

The comparison between RC and LC filters is as follows:

Parameter RC filters LC filters
Ripple factor High Low
Voltage regulation Poor Moderate
Cost Cheaper Relatively costly
Load current Small High
Power dissipation High Low

What is the use of LC circuit?

Use or application of LC circuit :

  • LC Pi filter is an efficient DC filter as it can filter out AC ripples easily, so LC pi filter is used designs such as AC to DC converter, frequency converter, etc.
  • LC pi filter can be used with a bridge rectifier.
  • They are used with communication devices to retrieve the original signal after modulation.
  • They are used for attenuating noise in signal and power lines.
  • It is used to select or generate a particular frequency signal.
  • They are used in designing radio receivers, amplifiers, tuners, filters, oscillators, television receivers, mixers, etc.
  • They are used for current or voltage magnification.
  • In induction heating, used as both parallel and series resonant circuits.
  • Used in constant voltage transformers.
  • In carrier line communication for high voltage transmission systems.

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