Magnetism, inductors & transformers
Current, coils & magnetic fields
A coil organizes the field created by electric current.
Electric current produces a magnetic field. Winding a conductor into a coil concentrates that field, and a magnetic core can increase flux for a given current. Field direction follows the current direction and winding orientation. Core materials are not unlimited field multipliers: saturation, hysteresis and heating matter. Magnets and magnetic fields also form the basis of many position and current sensors.
Induction & transformer ratios
Changing magnetic flux can transfer energy between windings.
Faraday’s law relates induced voltage to changing magnetic flux. Lenz’s law gives the opposing direction. Two windings linked by that changing flux form a transformer. For an ideal transformer, the voltage ratio follows the turns ratio and current changes inversely. A steady DC source does not provide normal continuous transformer action and can overheat a winding.
Inductor energy & switching kickback
An interrupted current still needs somewhere to go.
An inductor resists a change in current by developing voltage. When a transistor switches a coil off, stored magnetic energy must go somewhere. A suitable diode, clamp or snubber provides a controlled path. A flyback diode across a DC coil is normally reverse-biased during energization; after switch-off it permits recirculating current. Clamp choice affects voltage stress and how quickly the coil releases.