Voltage across zener diode

[MechTron]

Hi,

I have tried to simulate this circuit, which I have previously build on PCB. I just wanted to confirm that data from real example will match data in simulation, and in that way to gain more confidence in using this simulation software. In the simulation I got correct output voltage (5.5 V), but when I measure voltage across zener diode, voltmeter doesn't measure 6.2 V as it measures on real board (also voltage across diode D1 is not 0.7V). On real board output is connected to a microcontroller, here I connected 555 timer just to have some load. What I am missing here ?

[Dave@RPS]

For D1 you may not be putting enough current across it to generate the full forward voltage drop.  I was stung by this in a circuit I did a while back, zener diode might be the same thing.

You might have to put a current limiting resistor in series with these two diodes and then measure the voltage drop while that current is flowing.  

The spec sheet for D1 is here  https://www.vishay.com/docs/81857/1n4148.pdf

The two left charts on sheet 2 of the data sheet illustrates that forward voltage is a function of current.  At room temp you need 10mA to get .7 volts.

Hope that helps.

Dave