What is a Filter Circuit?

Filter Circuit

         Rectifier converts in changing alternating current into direct current in the form of pulse, which only travels in one direction. The rectifier circuit are classified as half wave rectifier, Full wave rectifier & Bridge rectifier. All of these rectifier circuits’ outputs have some ripple factor.

Filter circuit

Full wave rectifier and its output response without filter

  • The rectifier circuitry takes the initial ac sine wave from the transformer or other source and converts it to pulsating dc.
  • A full-wave rectifier will produce the waveform shown to the right, while a half-wave rectifier will pass only every other half-cycle to its output.
  • We must find a solution to eliminate the pulsations and give the load circuit a much “cleaner” dc power source.
  • A filter is any circuit that eliminates some components of a signal or power source while enabling the other components to function normally.

Why are Filters Required?

  • The presence of an AC component is indicated by the ripple in the signal.
  • To attain pure dc output, this ac component must be entirely removed.
  • Therefore, a circuit is required to convert the rectified output into a pure dc signal.
  • A filter circuit is one that lets the dc component go to the load while removing the ac component from the rectified output. The simplest sort of filter used in rectifiers is the capacitor filter.
  • Inductor and capacitor are the two major parts used to build a filter circuit.
  • A capacitor permits ac and blocks dc.
  • An inductor permits dc and blocks ac.
  • Capacitor filter: The AC component of the rectified signal is filtered away, leaving only the DC component, using a capacitor in parallel with the load. A smoother DC output is produced as a result of the capacitor charging during the peak of the AC voltage and discharging during the off period.
  • Inductor filter: To smooth the output voltage, a choke filter (also known as an inductor filter) connects an inductor in series with the load. A more stable DC output voltage is produced by the inductor, which resists variations in current. As a result, energy is stored during the on period and released during the off time.

Compare Capacitor & inductor Filter.

  • Capacitor filters are less complicated and more affordable, but they have a propensity to produce more ripple voltage, which can be problematic in particular applications.
  • On the other hand, inductor filters are larger and more expensive, but they offer better filtering and are better suited for high-current applications.

Single Capacitor

  • If we place a capacitor at the output of the full-wave rectifier, the capacitor will charge to the peak voltage each half-cycle
  • The capacitor will discharge more slowly through the load while the rectified voltage drops back to zero before beginning the next half-cycle.
  • Thus, the capacitor helps to fill in the gaps between the peaks, as shown in red in the first figure to the right.
  • The decay is essentially the typical exponential decay of any capacitor discharging through a load resistor.
  • The capacitance of the capacitor and the amount of current drawn by the load essentially serve as the RC time constant for voltage decay, determining how much the capacitor voltage lowers.
  • Because of this, the actual voltage output from this design never reaches zero and instead assumes the form seen in the image to the right.
  • Whereas the red component of the waveform represents when the capacitor supplies current to the load, the blue portion of the waveform represents the portion of the input cycle during which the rectifier supplies current to the load.
  • As you can see, the output voltage has significantly less variance (also known as ripple) than the rectifier’s unfiltered output, while not being pure dc.
FWR - Filtered output response

RC Filters

  • The filter circuit needs to be somewhat extended in order to further reduce the ripple without significantly decreasing the dc output.
  • Although this circuit results in some dc loss in the resistor, if the needed load current is modest, this loss is acceptable.
  • The single capacitor filter is appropriate for a wide range of low-current, non-critical applications. When a load resistance is very low or ripple must be kept to a minimum, the capacitor value must be quite large. Electrolytic capacitors up to 10 mF are pricey. The RC filter, a more advanced filter with lower capacitor values, is more practical.
  • The use of the RC capacitor-input filter is restricted to low load current applications.
  • When voltage management is not required and the load current is constant, this sort of filter is employed in power supply
FWR with RC Filter

LC Filters

  • The RC filter in the example above reduces ripple voltage, but when the load current is large, it causes excessive resistive losses.
  • Replace the resistor with an inductor to further reduce the ripple without adding a lot of dc resistance.
  • The two capacitors in this circuit seek to maintain a steady output voltage in between input peaks from the rectifier while continuing to store energy as previously.
  • Additionally, the inductor attempts to maintain a steady current through itself by storing energy in its magnetic field and releasing it as needed.
  • This is yet another element that makes an effort to reduce ripple voltage.

PI Filter

  • To lessen the amount of ripple voltage on the output, power supply applications frequently utilize a pi filter type of filter circuit.
  • Two capacitors and an inductor are stacked in the shape of the Greek letter pi to make up the pi filter.
  • Capacitance blocks dc component but offers very low reactance to the ac component. Hence the majority of the ac component is bypassed by capacitor C1.
  • The choke L is then reached by the dc component. It has a very low reactance to dc and a very strong reactance to ac components. Hence, it prevents the passage of the dc component and blocks the ac component.
  • The capacitor C2 prevents the passage of any remaining ac component, allowing only nearly pure dc to travel to the load.
  • It is possible to employ several of these pi sections sequentially to produce practically pure dc to the load.
  • In power supply circuits, the rectifier and pi filter are frequently used in combination.
FWR with pi filter

Comparison of filters

Parameters FWR (60Hz)Types of Filter
No filterLCL SectionPi Section
Vdc at no load0.636Vm0.636VmVmVmVm
Vdc at load0.636Vm0.636VmVm– [4170Idc ]/ C0.636VmVm– [4170Idc ]/ C
Ripple Factor0.48RL/16000L2410/ [CRL]0.83 / [LC]3330/ [LC1C2RL]
Peak Inverse Voltage2Vm2Vm2Vm2Vm2Vm

List few advantages and disadvantages of PI filter


  • Provides high voltage gain
  • Low ripple factor
  • High output voltage & PIV


  • Pi has high output voltage but voltage regulation of pi filter is very poor, because the output voltage drops rapidly with the increase in current flowing through the load.

List few application of pi filter.

  • These are utilized in communication equipment to recover a specific signal following modulation.
  • The signal is modulated into high frequency multiples for transmission.
  • Filters are used to demodulate the specific frequency range on the receiver side.

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