Filter capacitor
Encyclopedia
Filter capacitors are any capacitor
s used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:
Filter capacitors are not the same as reservoir capacitor
s, the tasks the two perform are different, albeit related.
s are usually used due to high capacity at low cost and low size. Smaller non-electrolytics may be paralleled with these to compensate for electrolytics' poor performance at high frequencies
are usually favoured due to extremely low inductance and low cost. Where precision is needed, silver mica capacitor
s offer superior precision and stability. Where manual tunability is required, plastic film trimmers are sometimes used, though it has long been more popular to adjust the inductor
to achieve tuning
.
. Often a ceramic disc sits in a notch cut in the PCB, with the tracks soldered directly to the disc for best performance.
frequencies, the (FR4) Printed circuit board
(PCB) material acts as a capacitor, and the PCB track has inherent inductance. The net result depends on the ratio of capacitance and inductance present.
Increasing PCB track width causes additional capacitance, and thus a net capacitance. Narrowing track width reduces capacitance, produces a net inductance.
s use large numbers of filter capacitors, making size an important factor. High currents and low voltages also make low equivalent series resistance
(ESR) important. Tantalum capacitor
s meet all these requirements and are thus commonly used. Tantalums have an additional issue which must be addressed during the design stage, namely that if their current rating is exceeded they are liable to explode. Tantalums are a type of electrolytic capacitor, but when the word 'electrolytic' is used it normally refers to aluminium electrolytics.
-µF) and electrolytic (for 0.47 µF and up) are normally the preferred types where their performance is sufficient, since these are the lowest cost types of capacitors. Hence they are very popular in filters of many types.
are incorporated into a single component envelope for convenience and robustness. This reduces switch
arcing and RFI
. The most common combination of values is 0.1 µF + 100 Ω.
s for their low inductance
and low cost.
Speaker
Nonpolar aluminium electrolytics combine sufficient capacitance with low cost. In high end systems they may be paralleled with small non-electrolytic capacitors to improve their performance.
Switched mode power supply
Low ESR (effective series resistance) electrolytics are often required to handle the high ripple current.
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:
- Glitch removal on Direct currentDirect currentDirect current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) power rails - Radio frequency interference (RFI) removal for signal or power lines entering or leaving equipment
- Capacitors used after the reservoir capacitorReservoir capacitorA reservoir capacitor is a capacitor that is used to smooth the pulsating DC from an AC rectifier.-Performance with low impedance source:...
to further smoothSmoothSmooth means having a texture that lacks friction. Not rough.Smooth may also refer to:-In mathematics:* Smooth function, a function that is infinitely differentiable; used in calculus and topology...
dc power suppliesPower supplyA power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy... - Capacitors used in audio, intermediate frequencyIntermediate frequencyIn communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(IF) or radio frequencyRadio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) frequency filtersElectronic filterElectronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
(e.g. low pass, high pass, notch, etc.) - Arc suppressionArc suppressionArc suppression is a method of attempting to reduce to near elimination the luminous discharge of electrical current , between two electrodes, through a gas...
, such as across the contact breakerContact breakerA contact breaker is a type of electrical switch, and the term typically refers to the switching device found in the distributor of the ignition systems of spark-ignition internal combustion engines.-Purpose:...
or 'points' in a spark-ignition engine
Filter capacitors are not the same as reservoir capacitor
Reservoir capacitor
A reservoir capacitor is a capacitor that is used to smooth the pulsating DC from an AC rectifier.-Performance with low impedance source:...
s, the tasks the two perform are different, albeit related.
Types
Many filtering tasks have specific types of capacitor that are required or typically used for the task.Power rail filtering
Electrolytic capacitorElectrolytic capacitor
An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume than other types. They are often referred to in electronics usage simply as "electrolytics"...
s are usually used due to high capacity at low cost and low size. Smaller non-electrolytics may be paralleled with these to compensate for electrolytics' poor performance at high frequencies
Radio frequency work
Ceramic plate capacitorsCeramic capacitor
In electronics, a ceramic capacitor is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric. The temperature coefficient depends on whether the dielectric is Class 1 or Class 2...
are usually favoured due to extremely low inductance and low cost. Where precision is needed, silver mica capacitor
Silver mica capacitor
Silver mica capacitors are high precision, stability and reliability capacitors. They are available in small values, and are mostly used at high frequencies.-Clamped Mica capacitors:...
s offer superior precision and stability. Where manual tunability is required, plastic film trimmers are sometimes used, though it has long been more popular to adjust the inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
to achieve tuning
Tuned filter
In signal processing, a tuned filter is a stage in the processing channel which accepts or rejects signals which are tuned for a specific type.-History:...
.
Ultra High Frequency-UHF
Disc and plate ceramic capacitors are used due to particularly low inductanceInductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
. Often a ceramic disc sits in a notch cut in the PCB, with the tracks soldered directly to the disc for best performance.
Microwave
At microwaveMicrowave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
frequencies, the (FR4) Printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
(PCB) material acts as a capacitor, and the PCB track has inherent inductance. The net result depends on the ratio of capacitance and inductance present.
Increasing PCB track width causes additional capacitance, and thus a net capacitance. Narrowing track width reduces capacitance, produces a net inductance.
Computer power rail filtering
ComputerComputer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s use large numbers of filter capacitors, making size an important factor. High currents and low voltages also make low equivalent series resistance
Equivalent series resistance
Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated to a very good approximation as ideal capacitors and inductors in series with a resistance; this resistance is defined to be the equivalent...
(ESR) important. Tantalum capacitor
Tantalum capacitor
The tantalum capacitor is a highly reliable type of electrolytic capacitor, available in both solid-bodied and separately encased forms. The encased "wet" variant is not used often in modern designs...
s meet all these requirements and are thus commonly used. Tantalums have an additional issue which must be addressed during the design stage, namely that if their current rating is exceeded they are liable to explode. Tantalums are a type of electrolytic capacitor, but when the word 'electrolytic' is used it normally refers to aluminium electrolytics.
Lowest cost
Ceramic (for values <0.47 micro-faradFarad
The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
-µF) and electrolytic (for 0.47 µF and up) are normally the preferred types where their performance is sufficient, since these are the lowest cost types of capacitors. Hence they are very popular in filters of many types.
Mains filtering
Mains filter capacitors are usually encapsulated wound plastic film types, since these deliver high voltage rating at low cost, and may be made self healing and fusible. The additional safety requirements for mains filtering are:- Live to neutral capacitors are flame retardant, and in Europe are required to use use class X dielectrics
- Live or neutral to earth: The requirements for L to N use apply, plus the dielectric must be self healing and fusible. In Europe these are class Y capacitors.
Contact suppression
A wound plastic film mains rated capacitor plus series resistorResistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
are incorporated into a single component envelope for convenience and robustness. This reduces switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
arcing and RFI
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
. The most common combination of values is 0.1 µF + 100 Ω.
DC motor suppression
Ceramic disc capacitors are usually used on low voltage motorBrushed DC electric motor
A brushed DC motor is an internally commutated electric motor designed to be run from a direct current power source.-Simple two-pole DC motor:The following graphics illustrate a simple, two-pole, brushed, DC motor.DC Motor Rotation...
s for their low inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
and low cost.