FAQ02088 of Solid-state Relays FAQ

A snubber circuit is a circuit consisting of a resistor (R) and a capacitor (C) that prevents faulty ignition from occurring in the Solid-state Relay triac by suppressing a sudden rise in the voltage applied to the triac.

Switching an inductive load, such as a motor, using an Solid-state Relay with triac or thyristor output causes the voltage to suddenly change (voltage transience) when the load is turned ON or OFF and results in incorrect operation (faulty ignition).

The characteristics for voltage transience with a triac or thyristor are expressed as dv/dt.

The maximum value at which voltage transience will cause switching from OFF to ON is called the critical rate of OFF-state voltage (static dv/dt). For a triac, the maximum value at which voltage transience will not cause switching from ON to OFF is called commutation dv/dt.

The snubber circuit works to suppress surges, but the output semiconductor elements may be damaged if a large voltage surge is applied. Therefore, measures against surge are required in addition to the snubber circuit if an Solid-state Relay that does not have a built-in surge-absorbing element (varistor) is used with an inductive load.