> >I've always thought (but might be mistaken) that "fast" diodes just
> >turned OFF faster than regular diodes, and they don't necessarily
> >turn ON any faster. For a catch diode accros a relay the most
> >important is turn ON time, right?, so there is no benefit in using
> >anything more expensive than a 1N400x?

> You are correct.

Agree. BUT the turn off losses in the 1N400x will be higher. Often this is
unimportant. But if you repeatedly switch an inductor off at a high rate the
dissipated energy may exceed the 1N400x's thermal ratings. I have a 20 kHZ
PWM application which uses a protection diode to snub a nominally "non
inductive" winding which in fact has a small inductance. A 1N400X lasts
about 20 seconds there despite being nominally rated for the voltage and
currents involved. An equivalent high speed diode (eg BYV26c) runs cool in
the same application.



        Russell McMahon

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