I try to build a low voltage drop regulator to bring the voltage down from 12 V to 5 V (ignore everything right of the capacitor C3). I have a 12 V voltage supply and I want to feed my OpAmp with a voltage below 5 V. To do this I put 3 bzx84c3v3 zener diodes in series (U3, U4, U5), everyone of them has a breakdown voltage of 3.3 V. So theoretically, the voltage should drop from 12 V to 2.1 V, but it drops to only 5.56 V. Putting more zener diodes doesn't really help. Does any one have an idea why the voltage doesn't drop accordingly to the breakdown voltage?
If you observe the datasheet, you would find the appropriate working conditions where zener diode provides 3.3V drop across it on reverse breakdown. Just observe the below image from the datasheet. Bottom curve shows for 3.3V.
Closely looking at the given values, maximum voltage drop it would observe is 2.9V at 1mA current flow.
And the op-amp that you used, generally have input currents in the range of µA. That would mean the voltage across zener diode goes even low. From the result you posted. 5.56/3 = 1.85V drop is there.
For proper 3.3V. appropriate amount of current should flow through the diode.
Try looking at datasheet for in-depth reference.
Solution: Try putting pull-down resistor at opamp input(Proper value has to be selected).