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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.

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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!

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  • \$\begingroup\$ In circuit testing is always fraught with difficulties due to parallel circuits. \$\endgroup\$ – Andy aka Aug 5 at 21:09
  • \$\begingroup\$ So far Schottky diodes have the lowest forward Vdrop at 400 mV to 500 mV. For valid test unsolder 1 leg and lift it up, then take a reading. \$\endgroup\$ – user105652 Aug 5 at 21:29
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If you are measuring the forward voltage of the diodes with the diodes in the circuit, then you are probably seeing effects from the rest of the circuit.

Take the diode out of the circuit and try it again. You'll probably measure the expected 0.7V

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The diode drop test works by pushing a small current through the diode and measuring the voltage. The diode you pointed out most likely has a resistor (possibly 120 ohms?) in parallel, which is passing the test current with a 0.1V drop.

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Diode tests vary with instruments but 0.5mA current source is about what I expect so 0.100 implies 200 Ohms. A diode would be closer to 0.6V. Reversing the polarity tells you if the difference is nonlinear from diode current as opposed to linear , (equal value) for resistance.

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