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. I've been doing some reverse engineering work and 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.)
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 diode's position within the circuit shouldn't affect the reading.