Spehro Pefhany wrote:
> Sure, at the expense of greatly increasing the Pd of Q1

But Q1 is only seeing the base current of Q2 plus the 750mV or so drop
accross R2, so this would be well within its capabilities.  Let's say you
want to make sure Q2 is solidly on (and perhaps use a differnt transistor)
and give it 1/20 the base current of the maximum collector current.  250mA /
20 = 12.5mA.  Let's say R2 is 1Kohms, so that's another 750uA for a total of
13.3mA.  When on, Q1 would have about 7V accross it, for a dissipation of
93mW.  That's well within the safe range of a MMBT4401.

> and exposing
> the PIC to damage from transients on the +12V power rail.

I don't recall Neil saying anything about transients or automotive use.  But
in any case Q1 is operating as a controlled current sink when on, so can
take momentary collector voltages up to its max collector breakdown spec,
which is around 40V if I remember right.  None of this is going to get back
to the PIC.


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