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Schottky diodes with 0.2V Vf?

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jogri:
I am currently trying to repair the switch mode power supply of an oscilloscope and i stumbled across three diodes on the secondary side with suspiciously low forward voltages:

-two Schottky rectifiers (B20H100G) that are supposed to have a Vf of .4V but barely manage .2V at room temperature
-one silicon diode (BYV29-400) that has a Vf of .4V instead of the .9V that the datasheet suggests.

They didn't show a reverse voltage when i tested them with my multimeter (the one schottky that i tested with my lab power supply had a Vref above 60V, so i guess they are fine in that regard). Am i correct in the assumption that those diodes are toast when they deviate that much from their datasheets?

oPossum:
What is your test current and what is the current specified in the data sheet? Diodes will have lower Vf at lower current.

jogri:
Thanks for the tip with the current, that solved it: The current vs voltage drop graphs on the datasheet goes down to 0.1A, so i just assumed that the reading on my multimeter should be at roughly the same voltage. I just tested them with 2A, they behave completely normal at that current (10A are specified in the datasheet, but i see no point in going that high if the Vf is within the specified range at 2A).

jackthomson41:
yup 2A battery source will work for that Schottky diode i.e. 1n5819 diode.

fourfathom:
For what it's worth, diodes usually fail shorted or open.  I don't think I've ever seen one where the failure mode was a lower than normal forward voltage drop -- I don't think semiconductor physics permit it.

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