# Why is the voltage across a Zener diode so dependent on current?

I was going to use a 3.3V Zener diode to protect the analog inputs of an Arduino Due, but I found that the voltage drop across it was only 2V.

So I tested connecting it to my power source with a series resistor (see diagram) and I found that the voltage seems to vary somewhat widely:

R=15k   1.78V
1.5k    2.44V
150     3.3V


Incidentally, the datasheet for 1N4728A shows a test current of 76mA, which is close to the 58mA current that is obtained via the 150ohm resistor.

simulate this circuit – Schematic created using CircuitLab

I guess I shouldn't be too surprised by my findings, because it just means that the Zener diode has a positive effective resistance, but I was somewhat surprised that the variation was so high.

Also, it's surprising that even when the Zener diode is not "fully conducting" (or whatever the appropriate term is), it shorts out the lower resistor in my resistor network (see below). When I remove the diode, for an input voltage of 12V the voltage at the ADC is 2.89 V, but when I put the diode in, the voltage drops to 1.86V. Should I just choose smaller resistor values, or is the Zener diode useless for this purpose?

simulate this circuit

• Zener diodes have V-I curves in their datasheets. If you can work with their limitations, they are fine to use. Otherwise take a look at something like a TL431. Note: there is nothing wrong with using a Zener that has a higher nominal voltage if it works for your circuit. You can use a 3.9V (or higher) Zener, if you have a lot of resistance in series. May 31 '16 at 2:05
• Watch out for the temperature coefficient, too. If that is important to you, then you better test your circuit over temperature. May 31 '16 at 2:05