Assume we have two zeners connected in parallel, one with a zener voltage of x volts, and one with a zener voltage of y volts. What will be the equivalent zener voltage of the circuit?
The one that has the smaller voltage. It will hog the current until it's voltage compliance brings the voltage into the range of the higher zener, which then will start to flow some current. But even if the higher zener is flowing some current the majority will still flow through the smaller voltage unit.
Heating of the diodes also plays a role, especially if the voltage shifts with increased temperature. While the term Zener is generically applied to these diodes, the Zener breakdown (negative temperature coeff.) and avalanche break down (positive temperature coeff.) are two distinct mechanisms.
The zeners will establish an intermediate bias point, depending on bias current at a point where their V/I characteristics cross ... which will generally mean:
- the zener with the lower voltage will hog the current, and the higher voltage zener won't get into it's avalanche conduction region. The terminal voltage will be determined by the lower zener voltage.
- since there is some intrinsic resistive slope to the avalanche conduction, the higher voltage zener may begin to conduct, establishing an intermediate terminal voltage, and eventually limiting the voltage at the higher zener voltage.
- At some point the current will become high enough to burn out the lower voltage device, then of course we step to the higher zener voltage.