inductor

An inductor is a coil of wire, usually wound around an iron or ferrite core, that resists changes in electrical current. When current flowing through the coil tries to increase or decrease, the inductor generates a magnetic field that opposes that change. This property, called inductance, is measured in henries (H). A small inductor might measure millihenries (mH) or microhenries (uH); large power inductors can reach multiple henries.

The core material matters greatly. Air-core inductors have low inductance and are used where space permits and core losses must be minimal, such as radio frequency circuits. Iron-core inductors pack much more inductance into small physical size and are the workhorses of power supplies and filters. Ferrite cores offer a middle ground, resisting core losses at higher frequencies. The wire gauge, number of turns, and coil diameter all affect the final inductance value.

Where inductors actually work

In switching power supplies, inductors smooth current ripple and store energy between switching cycles. A typical buck converter uses an inductor in series with the output to reduce current spikes that would otherwise damage load circuits. In electromagnetic interference (EMI) filters, inductors block high-frequency noise while allowing low-frequency signals to pass. Input filters on industrial machinery often combine inductors with capacitors to eliminate conducted emissions.

Inductors fail when the wire insulation breaks down from heat or mechanical stress, when the core saturates at unexpectedly high current levels and loses inductance, or when the core develops eddy current losses that turn the component into a heater rather than a filter. Designers must account for the inductor's resistance (the wire is not pure conductor) and the core's frequency-dependent behavior. An inductor rated for 60 Hz mains may overheat if used in a 20 kHz switching circuit without derating.

The term comes from the verb induce, reflecting the device's function: it induces a voltage opposing current change according to Faraday's law. Unlike resistors and capacitors, inductors are bulkier and heavier at equivalent ratings, which is why they appear mainly in power circuits rather than signal circuits. Their cost per henry rises sharply at lower inductance values, making a 100 uH inductor proportionally more expensive than a 10 mH inductor.