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Choke (electronics)
Encyclopedia
Magnetic core
A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, inductors and magnetic assemblies. It is made of ferromagnetic metal such...
, used as a passive inductor which blocks higher-frequency alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(ac) in an electrical circuit while passing signals of much lower frequency and direct current
Direct current
Direct 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...
by having an impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
largely determined by reactance, which is proportional to frequency (see 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...
and 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...
). Chokes are typically used as the inductive components in electronic filter
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
s.
The name comes from blocking—"choking"—high frequencies while passing low frequencies. It is a functional name; the same inductor is often called a "choke" if used to block higher frequencies, but a "coil" or "inductor" if, say, part of a tuned circuit.
Types and construction
Audio frequency
Audio frequency
An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...
coil
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
s, usually called A.F. chokes, usually have ferromagnetic iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
cores to increase their 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...
. Chokes were used as filters, in conjunction with large electrolytic capacitors, in power supplies; working at low power-line frequencies they were large, heavy, and expensive, but more effective and power-efficient than resistor-capacitor hum filters. Modern components and circuits, with very large and cheap electrolytic capacitors and electronic circuits which suppress hum, have long made chokes obsolete in mains-frequency power supplies, although small and inexpensive inductors are used in high-frequency switch-mode power supplies.
Chokes for higher frequencies often have iron powder or ferrite
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
cores (see Ferrite bead
Ferrite bead
A ferrite bead is a passive electric component used to suppress high frequency noise in electronic circuits. It is a specific type of electronic choke. Ferrite beads employ the mechanism of high dissipation of high frequency currents in a ferrite to build high frequency noise suppression devices...
). They are often wound in complex patterns (basket winding
Basket winding
Basket winding is a pattern of winding electrical wire in a coil, used in making certain types of electronic components, such as inductors and transformers. The purpose of this type of winding is to minimize having adjacent turns of wire running parallel...
) rather than regularly to reduce self-capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...
. Chokes for even higher frequencies have non-magnetic cores and low inductance.
Common-mode choke
Common-mode choke coils are useful in a wide range of prevention of electromagnetic interferenceElectromagnetic 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...
(EMI) and radio frequency interference (RFI) from power supply
Power supply
A 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...
lines and for prevention of malfunctioning of electronic equipment. They pass differential currents (equal but opposite), while blocking common-mode currents.