Ian Field wrote:



"klem kedidelhopper" wrote in message
...

I just pulled a suspect diode out of a dead board. It bears the
numbers SR506. This seems to be a Schottky barrier diode. I haven't
had much experience working with these at all. Normally when checking
conventional diodes I would use my 260 on RX100 ohms. It's very
simple. I read about 500 ohms one way and infinity the other. With
this diode though just after pulling it I read in one direction about
5K ohms with the reading gradually going up.almost like a capacitor
charging. In the other direction I read about 160 ohms. Now after it
has sat on the bench for a few minutes it reads 160 ohms in the one
direction and now reads between 1 and 3 meg depending on which scale
the meter is on.

***SB diodes have measurable leakage - I mistakenly binned a few in my
learning years before I knew this. The forward drop is lower than a
silicon diode and is quite variable depending on the power rating of the
diode - very small SB diodes can have Vf as high as 0.4V, only just less
than very fast silicon types, large SB rectifiers can have 0.2Vf, or
even as low as 0.1V for a hefty one.

***A DMM with a diode check function that gives you a direct reading of
Vf is pretty much indispensible, a little experience interpreting the
readings is also quite useful.

***If you replace a SB rectifier with a silicon type; the rectifier will
overheat - likely so will the reservoir electrolytic.

***If you replace a silicon rectifier with an SB type; you may find it
breaks down on reverse voltage, SB types start around 20V with ratings
up to about 60V being fairly common - I've seen 90V SB rectifiers, but
they cost more so manufacturers only use them if they have to.

***Don't forget; in a flyback converter a forward rectified voltage rail
can have flyback pulses 5 - 8x the voltage on the other side of the diode!
Last time I checked, there were some 1200 volt Schottky types that came
out not to long ago. The problem is finding a supply house that has some.

Jamie