Adding zeners in series

[Mark Hammer]

I need a 5.6V zener.  None of the local electronics places either have them in stock or are open when I can get to them.  Can I simply stick two zeners in series that add up to 5.6V (or close enough)? the same way adding regular silicon switching diodes are added to achieve different forward voltages?

[R.G.]

Yes. There are the usual string of caveats, but yeah, the voltages add.

At one time NPN transistors had a reverse base-emitter breakdown centered between 5 an 6 volts, but I notice that this has drifted upwards in recent years, so you may not get lucky and get one that you like.

The temperature drift of a zenered base-emitter and a forward biased base-collector tend to cancel, so if one needs a 6.x to 7.x volt low-tempco voltage, using an ordinary NPN with positive on emitter, negative on collector, base open, tended to produce that. I haven't tried this for a couple of decades, but it used to work.

Adding forward biased diodes to zeners works too, but the current/voltage knee gets sloppy compared to a nice, sharp avalanche device.