Is it OK to connect multiple Zener diodes in series?

Yes, it is possible to connect multiple Zener diodes in series in order to obtain a desired Zener voltage, but in this case, care should be exercised as to Zener current, Iz. For series-connected Zener diodes, the maximum allowable Zener current becomes equal to the lowest one among their specifications. Ensure that this value is not exceeded.

Furthermore, if two Zener diodes with different Zener current specifications are connected in series, one of them will not exhibit the specified Zener current and thus the specified Zener voltage.