I've designed a number of boards which utilise different kinds of diodes, and thought I had a good handle on how they work and their properties - however recently on doing some reverse engineering work i've found some diodes with properties that have somewhat confused me.
On probing boards with a DMM in diode mode, I would expect forward-based diodes to display a voltage drop ranging from 0.5ish to 0.8ish Volts - but on two boards this week i've found diodes measuring around 0.1V.
It could be that these diodes are faulty, however the boards are in as-new working condition so I suspect what's really going on here is that there is some scenario where if a diode is used in a certain way within a circuit, it gives low readings when measured in situ.
On the most recent of the two boards (above) there are 5 identical Zener diodes (ZPY24 - I believe), all of those circled blue have a 0.5V to 0.7V forward voltage measurement, the pink has a forward voltage of 0.1V (I know it's physically reversed - I measured with correct polarity).
My question is, how would you have to place a diode within a circuit for it to measure this way? Surely if my meter leads are passing a small voltage out of one probe, through the diode, and back into the other probe, the diodes position within the circuit shouldn't affect the reading?
Thanks in advance!