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Home > Motor Control > Understanding Filter Capacitor

Understanding Filter Capacitor

tempo di aggiornamento: 2023-03-09 17:20:00


What is Filter Capacitor?

Filter capacitors are a type of capacitors. A filter capacitor, also known as a smoothing capacitor, is used in electronic circuits to filter out unwanted signals or voltage fluctuations and provide DC put out smooth.

Filter Capacitor.png

Working Principle of Filter Capacitor

Filter capacitors work based on the principle of capacitive reactance. Capacitive reactance is a capacitor's opposition to the flow of alternating current (AC). In an AC circuit, the voltage and current continuously change direction, and the capacitor charges and discharges in response to these changes.

As the AC voltage increases, the capacitor charges up to the peak voltage, storing electrical energy. As the voltage decreases, the capacitor discharges, releasing the stored energy to the circuit. This charging and discharging action of the capacitor helps to smooth out the AC waveform and filter out any unwanted fluctuations or noise, thereby providing a stable DC voltage output.

In a filter capacitor circuit, the capacitor is placed in parallel with the load or the power supply, and the capacitor is chosen based on the frequency range of the noise or ripple to be filtered. The capacitor's capacitance value and the frequency of the AC waveform determine the capacitive reactance, which is used to filter out unwanted signals.

Filter Capacitor Formula

In power supply circuits, the capacitance of the filter capacitor can be calculated using the following formula:

C = (I × t)/(ΔV)


C = capacitance in farads (F)

I = maximum load current in amperes (A)

t = one-half of the AC cycle time in seconds (s)

ΔV = maximum allowable voltage ripple

The capacitance value calculated by this formula is the minimum required value for the filter capacitor. A larger capacitance value may be chosen to provide additional filtering or to account for other factors, such as tolerance and aging.

It's worth noting that this formula assumes a half-wave rectifier circuit. For a full-wave rectifier circuit, the value of t would be one-fourth of the AC cycle time.

Filter Capacitor Circuit

A filter capacitor circuit is typically used in power supply circuits to smooth out the voltage ripple and noise generated by the rectifier circuit. The filter capacitor is placed after the rectifier and before the load. It stores electrical energy during the peak of the AC voltage waveform and releases it during the troughs. This results in a smoother DC voltage output.

The simplest filter capacitor circuit consists of a single capacitor placed across the DC output voltage, also known as a shunt capacitor. However, this circuit is ineffective at reducing voltage ripple and noise, especially at higher frequencies.

shunt capacitors.png

A more effective filter capacitor circuit is the pi filter, which consists of a series inductor and two shunt capacitors. The inductor helps to smooth out the voltage ripple and noise by providing a low-impedance path for high-frequency signals, while the capacitors provide a high-impedance path for high-frequency signals.

Another common filter capacitor circuit is the RC filter, which consists of a resistor and capacitor in series. The resistor helps to limit the current flow, and the capacitor helps to smooth out the voltage ripple and noise. The cut-off frequency of the RC filter can be calculated using the formula:

f_c = 1 / (2 * pi * R * C)

Where f_c is the cut-off frequency, R is the resistance in ohms, and C is the capacitance in farads.

Features of Filter Capacitor

Filtering: The primary function of a filter capacitor is to filter out unwanted noise and ripple voltage in the power supply circuit, resulting in a more stable and smoother output voltage.

Energy storage: Filter capacitors can store energy, which helps to supply short-term bursts of current to the load when there is a sudden increase in power demand.

Low ESR: Filter capacitors have low equivalent series resistance (ESR), which ensures that they can handle high-frequency ripple currents effectively and maintain the stability of the output voltage.

High capacitance: Filter capacitors typically have high capacitance values, which allows them to store more charge and provide greater energy storage capacity.

Compact size: Filter capacitors are available in various sizes and form factors, including surface-mount devices (SMDs), making them highly versatile and suitable for use in a wide range of applications.

Filter Capacitor Applications

Power supplies: Reduce ripple voltage and noise.

Audio systems: Remove unwanted noise and improve sound quality.

Motor control circuits: Reduce noise and protect the circuit from voltage spikes.

Radiofrequency (RF) circuits: Reduce noise and interference and stabilize the circuit's frequency response.

Communication systems: Filter out unwanted signals and improve the overall signal quality.

Medical equipment: Reduce noise and interference and improve the overall performance and reliability of the equipment.

Precedente: CR1220 Battery Equivalent, Specification, Application

Prossimo: Snubber Capacitors: Applications, Selection and Types


  • How do you choose the right capacitor for a capacitor filter?
  • The right capacitor for a capacitor filter should have the appropriate capacitance value, voltage, and temperature rating for the specific application. Other factors to consider include the physical size and cost of the capacitor.
  • What is the maximum allowable ripple voltage in a capacitor filter?
  • A capacitor filter's maximum allowable ripple voltage depends on the application and circuit requirements. In general, the lower the ripple voltage, the better the circuit's performance.
  • What are the advantages of using a capacitor filter?
  • The advantages of using a capacitor filter include reduced AC ripple and noise, improved stability and regulation, and improved performance of electronic circuits.
  • What is the difference between a shunt capacitor filter and a series capacitor filter?
  • A shunt capacitor filter is placed in parallel with the load, while a series capacitor filter is placed in series with the load. Shunt capacitor filters are used for low-current applications, while series capacitor filters are used for high-current applications.
  • What are the types of capacitor filters?
  • The two main types of capacitor filters are the shunt capacitor filter and the series capacitor filter.
  • What is the purpose of a capacitor filter in a power supply?
  • The purpose of a capacitor filter in a power supply is to smooth out the DC voltage by removing any AC ripple or noise. This helps to provide a steady and stable DC voltage for electronic circuits.

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