Why a Break in the Circuit Can Raise the Voltage
Current flowing through an inductor—such as a coil of wire—creates a magnetic field around it. That field stores energy. If a switch suddenly opens, the current loses its path, but the magnetic field cannot vanish instantly. As it collapses, it pushes to keep current flowing in the same direction. The resulting voltage rises until the current finds another route, such as an electrical arc across the opening switch.
The Smaller the Interruption, the Sharper the Spike
An inductor’s voltage depends on how quickly its current changes: the faster the current is forced to fall, the larger the voltage needed to drive that change. A switch that snaps open can therefore produce a brief spike far above the circuit’s normal supply voltage. The spike is not extra energy appearing from nowhere; it is the inductor’s stored magnetic energy being released over a short time and through whatever path is available.
That path may be a spark, or it may be a sensitive electronic component. Repeated spikes can wear switch contacts and damage circuitry. In relay coils and motors, the effect is familiar as a small spark when power is cut.
Giving the Stored Energy a Safer Route
Designers often place a protective component across an inductive load. In a direct-current relay circuit, a diode can provide a loop for current after the switch opens, letting the magnetic field fade more gradually. Other circuits use components designed to limit voltage. These protections trade a rapid, potentially destructive discharge for a slower or more controlled one.
The practical lesson is that switching off an inductor is not simply removing power: the collapsing field keeps pushing until its energy has somewhere to go.
