Re: gEDA-user: Power relay question

[Bob Paddock <graceindustries@xxxxxxxxx>]

> Even if I ran a 4000 right on the 12v, I wouldn't count on that 12v to
> be very clean if it has motors running off it.  Electrically braking a
> motor with an H-bridge can spike the power rails.

> And a pic + regulator (sot-26 + sot-323)...

Fast transients will go through the typical regulator as if it was not
there at all.  Some have better rejection ratios than others.

A Depletion FET can be put before a regulator, driving the regulator's
Vin, in a noisy environment, or where the regulator itself does not go
as high as Vin.  Drive its gate with the output of the regulator, it
then becomes a variable resistor to absorb the transient, be sure to
account for power dissipation, to keep the transient  out of the
regulator in the first place.  SuperTex has an application note
covering this, which Ixys copied sections of; "Depletion-Mode Power
MOSFETs and Applications" by Abdus Sattar of Ixys.  Ixys makes higher
power Depletion FETs than SuperTex.


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