At 02:52 PM 2/25/2003 +1300, you wrote:
> > >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.

He did specify a relay. 20kHz is a *bit* fast for a relay. ;-)

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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