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the best procedure to check ultra fast diode by analogmeter

what is the best procedure?

how to know whith analog meter the diode is short or leckage

Measure the forward and reverse resistance. The forward resistance will be low, under 1K, depending on the meter type and sensitivity. The reverse resistance should be open circuit unles you have a very sensitive meter.

Use mega buck machinery to check the recovery speed, because this happens in the nano or pico second range.

replace it and see if the circuit acts differently

Robert A. Pease in his book Troubleshooting Analog Circuits, page 68 describes a low cost circuit for testing fast recovery rectifiers.
This circuit requires a high speed oscilloscope and a few discreet components.
Troubleshooting Analog Circuits is a good book and reasonably priced. I strongly recommend it.
Carl Pugh

Hello there,


A good test circuit is an actual switching
regulator circuit...Let me explain...


Someone gave me a zero recovery fast diode once and
i tested it with my o-scope, but my scope doesnt
go that high so i could only guess, but...

When i compared it to a regular Schottky type
i could definitely see some reverse current
going through the Schottky when the switcher
turned on (in a buck configuration) while with
the zero recovery diode i couldnt see anything.
Im sure there was a 'little' there somewhere, but
with my scope i couldnt see it. Even so, at least
i could know that it's got to be much less than
a regular Schottky diode.

On the down side, the zero recovery diode has
a much higher f voltage drop -- something like
1.4 volts compared to about 0.5 for the Schottky.

Oh yeah, the circuit i used was a simple buck
converter with NPN series pass switch transistor
and arranged for positive output, and loaded to
get the f diode current up high enough to test
recovery. When the NPN turns on it's in series
with the (reverse) diode, so if the diode was
previously conducting (and is was due to the
inductor current) the diode looks like a short
circuit for some nano seconds or so (Schottky)
but with the zero recovery diode it must be
much much faster. The reverse current peak
goes up quite a bit with the Schottky diode.

I ended up not likeing the z-rec diode as much
however, because the circuit i was doing was for
low voltage (15v out max) and the forward
drop was just too high compared to the Schottky
so the losses would be a bit higher. If it was
a higher output voltage (like say 100v or something)
it wouldnt have mattered as much so it would
have been a good choice and would have reduced
the recovery power quite a bit i bet.

In any case, if you intend to test the diode
to see if it's recovery time is about normal
you'll need more test equipment than a meter.
If you just want a go/no go test to see if the
diode is just 'basically' working, then a meter
will help. At least you'll know if the diode
blew out completely or something.
You can also forward bias the diode with a small
battery and a 1k resistor to see if the forward
voltage is about normal, but this also doesnt
tell you anything about the dynamic response
of the diode.


Take care,
Al

<small>[ December 24, 2005, 10:59 AM: Message edited by: MrAl ]</small>