I'm trying to regulate 12V to 4.7V with a zener diode. (I'm doing this to protect an AVR's gpio pin, so the current is very low.) I tried the simplest circuit with the zener, but it does not work as expected.
simulate this circuit – Schematic created using CircuitLab
I expect the multimeter to measure 4.7 Volts on the zener diode, but it only shows about 3.2-3.3 Volts. I replaced the diode with other ones with the same model, and the results are the same.
Are my diodes faulty, or am I doing something (very) wrong?
Best Answer
That is a fairly husky Zener diode. The test current shown in the datasheet is 45 mA. You are only allowing 2.5 mA, which is apparently not enough to get over the diode's "knee".