A choke is a coil made of thick insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current (ac) in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which is proportional to frequency. Chokes are typically used as the inductive components in electronic filters. Its inductance is quite large, though its resistance is very small. Thus it consumes extremely small electrical power. This enables it to be used in A.C. circuits to limit the current with extremely small wastage of energy as compared to a resistor.
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 
Chokes used in radio circuits are divided into two classes – those designed to be used with power and audio frequencies, and the others to be used with radio frequencies. To reduce the current in a circuit we use resistance.
Audio frequency coils, usually called A.F. chokes, usually have ferromagnetic iron cores to increase their inductance. 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 cores (see Ferrite bead). They are often wound in complex patterns (basket winding) rather than regularly to reduce self-capacitance. Chokes for even higher frequencies have non-magnetic cores and low inductance.
Common-mode choke 
Common-mode chokes, where two coils are wound on a single core, are useful for prevention of electromagnetic interference (EMI) and radio frequency interference (RFI) from power supply lines and for prevention of malfunctioning of electronic equipment. They pass differential currents (equal but opposite), while blocking common-mode currents.
Magnetic fields produced by differential-mode currents in the windings tend to cancel each other out; thus the choke presents little inductance or impedance to differential-mode currents. This also means the core will not saturate even for large differential-mode currents, and the maximum current rating is instead determined by the heating effect of the winding resistance. Common-mode currents, however, see a high impedance due to the combined inductance of the windings.
- "Understanding Common Mode Noise". Pulse. Retrieved 17 April 2012.