Hi ,

I have two zener diodes whose measured reverse zener voltages are 3.18 V and 3.2V respectively when used "alone" in the circuit below with the 900 ohm resistor and 5V supply. However, when i connect them in series, reverse biased, I measure voltage across D1 as 2.48 V and across D2 as 2.40 V respectively.
I would have expected 3.18V at D1 and (5V-3.18V=1.82V) across D2. But not. I am figuring why?

 

I would have expected 3.18V at D1 and (5V-3.18V=1.82V) across D2. But not. I am figuring why?

Why would you expect that?
Just because D1 is closer to the 5V supply doesn't affect its voltage.
The order of series connected devices devices has no effect on their relative voltage drop.
You can put them in any order and the drops will be the same.

Since they are essentially identical Zeners, the expectation is that the voltage drop will be near equal across both Zeners, and that is what you are seeing.

 

@BC547 There are some problems with this, in part because of the supply voltage and the zeners in series.... everything is interacting together- you can't take a single component out of context, because it's interaction with all the others. If you want to isolate a component, you have to do it in parallel, carefully, instead of serial.

 

I would be nice to think that zeners started to conduct at their rated voltage, and didn't conduct at all below that voltage.
Unfortunately, it's not like that, and, especially low voltage zeners start to conduct below that voltage.
So at below 3.3V, a 3.3V zener might just be starting to conduct a little bit, so looks like a high value resistor.
You have two high value resistors in series across the 5V supply, so you will see 2.5V across each zener.