I was going through some of the theory behind Zener diodes. I know what they're used to work at constant voltage, but I don't know how they work. Actually, I kind of get that too, I know that voltage is constant until breakdown occurs (avalanche breakdown), but I don't get why a minimum current is needed to maintain a constant voltage, which is what the textbooks say. Why is there a minimum? I thought any current below the breakdown voltage was constant. I don't see why the voltage would vary through the diodes if it's below a set amount of current, yet remain constant between voltages x-y?

Any help would be greatly appreciated, I can't wrap my head round exactly what's going on inside the little buggers.

P.S, is there more of an appropriate question for the electronics section? As I know electronics is a natural part of physics, and I heard this kinda stuff gets studied quite a lot in physics exams.


This diagram (cribbed from here) shows the voltage current curve for a typical zener diode:

Zener diode

Below the breakdown voltage the diode does not behave as a perfect insulator, but has a small leakage current. This means the voltage across the diode is strongly current dependant. The minimum current, $I_{Z(mini)}$, is simply the current at which the breakdown is established and the voltage becomes (roughly) constant. If you're using the zener diode as a voltage regulator you need to maintain at least this minimum current to keep the voltage drop across the diode constant.

  • $\begingroup$ Ahhhh I see, I see. So Right, I always got why it had to be above a certain value, but the graph clearly shows it's basically forward voltage in reverse! Thanks man, you offered much clarity, and the text books really should show this graph and just say it's the same god damn thing but backwards (albeit with a very minor difference). Thanks again :) James $\endgroup$ – user108262 Nov 12 '13 at 20:14
  • $\begingroup$ @JohnRennie Although the graph clearly shows that after the breakdown voltage the voltage remains constant can you please explain what happens at the microscopic level which result in this constant voltage? $\endgroup$ – Rajath Krishna R Jan 18 '14 at 13:55
  • $\begingroup$ @RajathKrishnaR: that's better addressed as a new question. $\endgroup$ – John Rennie Jan 18 '14 at 15:58
  • $\begingroup$ OK.sir I will put a new question then. $\endgroup$ – Rajath Krishna R Jan 18 '14 at 16:01

A Zener is not like a normal diode. A normal diode lets current flow in only one direction and needs to be installed in the correct direction. A Zener diode is placed in the opposite direction, against the flow of current. A Zener diode will prevent current from flowing until it reaches a certain voltage, depending on the diode rating. Once this critical value is reached, current is allowed to flow. The diode will also try to keep the outgoing voltage at its rated level, say 5 volts. If you feed 10 volts through the diode, the voltage might climb to 5.5-6 volts, but it will try to stay at the same voltage. If you feed too high a voltage through the diode, it will break down and fail.


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