Can Zener diodes be placed in series to increase the zener voltage.
That is, will two 10 volt zeners in series function identically to a single 20 volt zener?
No problem. Zeners connected in series are good to go.
Add all the Zeners breakdown voltages together. There you have your end result and your final Zener Voltage.
Something to remember: Don't mix 400Mw and 1W Zeners in the same series circuit.....the 400Mw are going to go faulty way before the 1W Zeners.
Do the stufff/maths and check what is needed for your application.
Can Zener diodes be placed in series to increase the zener voltage.
That is, will two 10 volt zeners in series function identically to a single 20 volt zener?
Not true always, its depends on the BD voltage of the diode, ex: you can connect a 10V (100mA)(=1W) diode in series with 5V (100mA) (=0.5W) without any problem.
in another word, if you connect a 10V (1W) zener in series with 3V(500mW) zener, 1W one will fail 1st, not the 500mW one.
never connect zeners in parellel, in series no problem. some times you can series rectifier diodes too in forword position to add 0.7V etc. to a zener.
The main differences of using them in series versus one higher voltage unit will be a difference in the knee impedance and likely a difference in the temperature coefficient of breakdown voltage.