Freewheeling Diode
A diode connected in reverse across a relay, contactor or valve coil that short-circuits and damps the reverse voltage peak appearing when the coil current is interrupted. In an unprotected circuit this peak can rise to tens of times the supply voltage and leave a permanent fault by puncturing the switching device.
While current flows through the coil, energy is stored in the magnetic field. When the switch opens, the current cannot drop to zero instantly; to keep the current flowing, the coil raises the voltage at its terminals in the reverse direction. When a diode is connected in reverse parallel with the coil, it offers this current a closed loop and the voltage peak is limited to the diode's forward voltage of about 0.7 V.
- Selection: the diode's reverse withstand voltage should be at least twice the supply voltage, and its continuous current greater than the coil current. In 12/24 V circuits a standard 1 A / 100 V class diode is enough for most relays.
- Orientation: the cathode (banded end) goes to the positive end of the coil. Fitted the wrong way round, the supply is short-circuited and the fuse blows.
- Side effect: a plain diode lengthens the coil's release time. If fast release is needed, a zener or resistor is added in series with the diode.
When connecting an external coil to the relay outputs of DC drivers and remote controls, if this diode is missing the fault shows up in the board's output stage. Check whether the coil's own socket already contains a diode before fitting.
Context in which this term is used: Installation, redundancy and field continuity