Inductors, also known as coils or chokes, are passive electrical components that use electromagnetic induction to store energy in a magnetic field. They are made by winding a conductive wire, typically copper, around a core material. The core increases the magnetic field and allows the inductor to store more energy. Inductors are used extensively in analog circuits and signal processing applications.
At its most basic, an inductor consists of a wire loop or coil. When electric current flows through the wire, a magnetic field is generated around the loop. This field represents stored energy. When the current changes, the stored energy is either increased or decreased, causing the inductor to resist changes in current flow. This property is called inductance and is measured in henries.
Ferrite cores are commonly used in inductor designs to increase the inductance. Ferrites are ceramic materials made from iron oxide combined with other metal oxides. They exhibit high permeability, meaning they concentrate magnetic flux lines. Ferrite cores allow inductors to be more compact while achieving higher inductance values.
There are several common types of inductors:
Air core inductors - These use a coil of wire without any core material. They are the simplest inductor design but have low inductance values. Air core inductors are mainly used at high frequencies where ferrite becomes ineffective.
Laminated core inductors - These use stacked sheets or laminations of magnetic material as the core. The laminations are insulated from each other to reduce eddy current losses. Laminated cores provide high inductance while handling high power levels.
Toroidal core inductors - The wire is wound around a ring-shaped toroidal core. This completely contains the magnetic field within the core, increasing inductance and reducing electromagnetic interference. Toroids are efficient and have high quality factors.
Ferrite core inductors - These use ferrite material in various shapes as the core, including rods, beads, pot cores, and E-cores. Ferrites allow compact, high inductance values but are limited to lower frequencies and power levels.
Powdered iron core inductors - These use iron particles compressed into a solid core shape. Powdered iron provides high inductance at low cost but has losses at high frequencies. The cores are also prone to breaking.
Chokes are a specialized type of inductor, optimized to oppose alternating current while allowing direct current to pass through. They are commonly used to block AC signals in power supplies and audio circuitry. Chokes typically have air or laminated cores to accommodate DC bias currents.
Variometers are variable inductors, used to smoothly adjust inductance rather than fixed increments provided by a multiple-tapped inductor. They consist of two coils with movable linkage. When configured for minimum mutual inductance, the inductance is limited to that of each coil. When the coils are aligned, the mutual inductance sums their inductances.
The inductance of a coil depends on the number of turns, core material and shape, type of winding, and presence of a magnetic object within the field. More turns and better core material (higher permeability) increase inductance. Layers spaced apart reduce stray losses. Inductance is proportional to the cross-sectional area of the core. A closed magnetic path structure like a toroid confines the flux for highest inductance density.
Inductors exhibit parasitic effects that must be considered in circuit designs:
Resistance - The wire in the windings has a small but significant resistance which dissipates power. Copper wire or litz wire helps minimize this loss.
Capacitance - Adjacent windings exhibit self-capacitance, allowing AC to bypass the coil. This parasitic capacitance limits high frequency response.
Skin effect - High frequencies current crowds to the surface of the wire, effectively increasing resistance. Litz wire mitigates this effect.
Proximity effect - Alternating current distributes itself within the wire to minimize inductance, increasing resistance. Litz wire or specially wound coils counteract this effect.
Self-resonance - The self-capacitance and inductance create a resonant tank circuit. Performance degrades above this frequency.
Saturation - In ferrite cores, too much DC bias current saturates the material, dramatically reducing inductance. An air gap in the core prevents saturation.
Losses - Eddy currents in core laminations and hysteresis losses in the core material cause heat dissipation, impacting power handling.
Stray fields - Magnetic flux can induce currents in nearby conductors. Shielding such as a foil wrap may be required.
Parasitic coupling - Unintended coupling between components degrades performance. Careful layout and orientation minimizes coupling.
Despite their drawbacks, inductors provide vital functions in electronic circuits:
Energy storage - Inductors store current as a magnetic field. This property is useful for pulse shaping and temporary energy storage.
Filtering - Inductors smooth out ripple currents by impeding high frequencies. They filter in tandem with capacitors.
Oscillation - An inductor and capacitor create an LC tank circuit to generate sine waves or tune radio signals.
Isolation - Inductively coupled coils transfer signals while isolating circuits. Transformers operate on this principle.
Impedance matching - Inductors match impedances between circuits to maximize power transfer.
Noise suppression - Common mode chokes block noise between transmission lines. Ferrite beads suppress high frequency noise.
Timing - Inductors shape pulse edges, delay signals, modulate duty cycle, and help generate precise clock signals.
Protection - Saturable reactor inductors sense and limit large current surges to protect circuits.
The performance of an inductor depends on selecting the right core material, winding technique, and shape/size for the intended operating frequency and current levels. Inductors range from discrete components to PCB traces acting as planar inductors. With their essential role in electronic systems, inductors will continue advancing hand-in-hand with circuit designs and technologies.