Voltage drop across resistor in series with zener diode

[rich77777]

how do i find this? i thought i would take the 22v source and just subtract the 0.7v that the diode takes.

but i am wrong, please help me!!

[vtech]

The .7v drop across a standard silicon junction is correct, however a zener diode behaves as an open circuit until reaching it's rating & acts like a voltage regulator/crowbar during it's avalanch mode. In other words the voltage drop across a Zener diode will equal to whatever it's rated when "zenering" ---NOT .7v as in a regular diode.

Purpose of the resistor is to limit the current through the zener during it's breakdown voltage region.

Here's a detailed example;
http://www.allaboutcircuits.com/vol_3/chpt_3/9.html