What is a Filter Circuit?

What is 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.

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.

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

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.

Comparison of filters

Comparison between Active Filters and Passive Filters

List few advantages and disadvantages of PI filter

Advantages

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

Disadvantage

• 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.

FAQ on Filter Circuit

What are the four types of filter circuits?

The four types of filter circuits are:

1. Low-pass filter (LPF): Allows signals with a frequency lower than a certain cutoff frequency to pass and attenuates higher frequencies.
2. High-pass filter (HPF): Allows signals with a frequency higher than a certain cutoff frequency to pass and attenuates lower frequencies.
3. Band-pass filter (BPF): Allows signals within a certain frequency range (band) to pass and attenuates frequencies outside this range.
4. Band-stop filter (BSF) or Notch filter: Attenuates signals within a certain frequency range and allows frequencies outside this range to pass.

What is the function of a filter?

The function of a filter is to selectively pass signals of certain frequencies while attenuating or blocking others. Filters are used in various applications, including signal processing, communications, and power supplies, to remove unwanted noise or harmonics.

What is the meaning of a circuit filter?

A circuit filter refers to an electronic circuit designed to allow certain frequencies of a signal to pass through while blocking or attenuating others. It is used to refine signals and reduce unwanted noise or interference.

What is the principle of a filter circuit?

The principle of a filter circuit is based on impedance, where components like resistors, capacitors, and inductors create a frequency-dependent response. This allows the circuit to either pass or block specific frequencies, depending on the type of filter.

Where are filter circuits used?

Filter circuits are widely used in:

• Power supplies: To smooth out rectified AC signals.
• Audio systems: To separate different frequency bands.
• Communication systems: To isolate desired signals from noise or interference.
• Signal processing: To refine signals and remove unwanted frequencies.

What is the advantage of a filter circuit?

The main advantages of filter circuits include:

• Noise reduction: Filters eliminate unwanted noise from signals.
• Signal refinement: They allow for the precise selection of desired frequency ranges.
• Improved signal quality: Particularly in power supplies, they help in smoothing out voltage outputs by removing ripples.

What are the uses of a filter?

Filters are used in various applications such as:

• Audio engineering: To balance sound frequencies.
• Radio communications: To select and tune to specific frequencies.
• Power supplies: To smoothen the output voltage.
• Signal processing: To extract or emphasize particular components of a signal.

What is the role of a filter in a rectifier?

In a rectifier circuit, the role of a filter is to smooth the pulsating DC output of the rectifier. The filter reduces the ripple voltage, resulting in a more stable and continuous DC output, which is essential for sensitive electronic devices.

Why Do We Need Filters?

Filters are needed to remove unwanted noise or interference from electronic signals, allowing only the desired frequencies to pass through. They refine signals for better quality, improve the performance of communication systems, smooth out voltage in power supplies, and ensure that electronic devices operate efficiently and accurately. Filters are essential in applications such as audio processing, radio communications, and signal conditioning to maintain clarity and precision