The Zero crossing technique

An alternating electrical signal oscillates between positive and negative values. The zero crossing point, just before the sign change, is called zero crossing.

RL621 is a third generation relay designed to seize the opportunity of voltage zero crossing.

With the same electrical contact used, zero crossing switching allows to control higher loads with considerably higher reliability and durability compared to asynchronous switching (that is, at random instants). The increase in performance and reliability derives from the fact that zero crossing switching ideally does not generate electrical stresses either on the electrical contact or on the controlled load.

To intuitively understand what happens we think of alternating current as the oscillation of a pendulum: if we try to stop a heavy pendulum when it is at maximum speed we can get hurt because it will hit us hard (Figure 1); if instead we grasp and hold the pendulum at the highest point of its stroke, when it is about to reverse the direction of motion, we will not suffer shocks. This also happens with an alternating current: a switch that tries to block a current at a random point will undergo stresses that will instead be absent if the switch is activated exactly at the precise moments in which the current is already stopped, i.e. at the instant of zero. crossing.