Zener diodes can be used in ac applications to limit voltage swings to desired levels. Below Figures shows three basic ways the limiting action of a zener diode can be used.
Part (a) shows a zener used to limit the positive peak of a signal voltage to the selected zener voltage. During the negative alternation, the zener acts as a forward-biased diode and limits the negative voltage to -0.7 V.
When the zener is turned around, as in part (b), the negative peak is limited by zener action and the positive voltage is limited to +0.7 V. Two back-to-back zeners limit both peaks to the zener voltage ;0.7 V, as shown in part (c).
During the positive alternation, D2 is functioning as the zener limiter and D1 is functioning as a forward-biased diode. During the negative alternation, the roles are reversed.