Inductors & Coils:
Magnetism in Motion.

Current through a conductor produces a magnetic field. A coil concentrates magnetic flux, and its changing field induces a voltage that opposes the change in current (Lenz’s law). Inductance, L, is measured in henries (H); 1 mH = 0.001 H. A coil may also have wire resistance, but resistance and inductance are different quantities.

A current-carrying ideal inductor stores energy in its magnetic field: E = ½ L I². If inductance doubles while current stays the same, stored energy doubles. If current doubles, stored energy quadruples. This is not energy generated by the coil; the source supplied it.

For an ideal series RL circuit connected to a DC supply: I(t) = (V/R)(1 − e^(−tR/L)). Initially, an unenergized inductor opposes a sudden rise in current. Over time the current approaches V/R. At long-term steady DC, ideal inductive voltage goes to zero while the winding still has resistance.

The characteristic time constant is τ = L/R. After 1τ, current reaches about 63.2% of its final value; after 5τ, it is above 99%. A larger L slows the rise; a larger R shortens τ but also reduces final current. In real parts, resistance and inductance can vary with heat and operating conditions.

Coil current also resists an abrupt decrease. When its circuit is opened, the magnetic field releases stored energy and the inductor produces whatever voltage is necessary within the physical circuit to try to maintain current. An unprotected opening can cause a large voltage spike, arcing, or damage; its actual peak depends on the complete circuit and parasitics.

A flyback diode placed across a DC coil is reverse-biased during normal energization. When the driver opens, the diode provides a recirculating path, limiting the voltage across the coil to around a diode drop in a simple model. That protection often slows relay or solenoid release. TVS devices, active clamps, and OEM drivers use different voltage–speed tradeoffs. Do not add suppression to a vehicle circuit without service guidance.

Air-core inductors generally maintain inductance more consistently with current but need more turns for the same inductance. Ferromagnetic cores can increase inductance; their permeability is nonlinear. At high magnetic flux, saturation causes effective inductance to fall, potentially allowing a faster current rise. This ideal workbench does not model saturation.

An inductor is defined by its inductance; a solenoid or relay coil uses the magnetic field to move an armature. A transformer has magnetically coupled windings and transfers energy through changing flux. Ignition coils are specialized coupled-winding devices, not a simple 12 V RL load. Transformers are studied more deeply in Mini Course 05.

Examples include relay coils, injector and solenoid windings, alternator field windings, motor windings, noise-filter chokes, and power-converter inductors. Actuators have motion, driver electronics, back-EMF, and protection strategies; their real waveforms differ from an ideal isolated RL circuit.

An ohmmeter measures DC winding resistance, not inductance under operating conditions. An LCR meter can estimate inductance of a properly isolated component at its test frequency. For in-vehicle faults, first verify supply, ground/driver command, wiring, and the OEM test specification. A good resistance reading does not prove correct magnetic or mechanical operation.