Just remember to look carefully at your steady-state power=
 dissipation
in your pass transistor or regulator. This is probably the most=
 common
problem I see with new or inexperienced designer's power=
 circuits.
"Gee, the transistor is rated for 5A, but it gets HOT!". Basic=
 laws of
physics are always at work and you can't dissipate 1W in a=
 SOT-23, SOT
-89 or TO-92 package without it getting very hot and (probably)
destroying it.

Also remember the hotter you run a particular semiconductor the=
 sooner
it will fail. Cool running silicon works virtually forever (all=
 other
possible stresses aside). Most of my designs are conservative=
 and
generally, no semiconductor runs hotter than you can continuously=
 touch
with your finger (but use a temp. probe to check if you're not=
 sure!).
Many of those designs are still running in commercial and=
 industrial
applications after 15-20 of years continuous use too. =A0

Just my thoughts on the subject.

Matt Pobursky
Maximum Performance Systems

On Fri, 12 Jul 2002 21:40:58 -0700, Jim wrote:
>Thanks for the fast response.
>The reason I ask was I just got a bunch of 78L05's 100ma from=
 ebay.
>But then again I also got a bunch of smt pnp's @ 600ma so maybe=
 I'll
>go the transistor route. I tryed a pnp with an lm317 a year or=
 so ago
>and had some problems with it like smoking transistors or=
 somthing I
>don't remember. With a schematic that I found on the web on=
 several
>sites.
>
>Thanks Jim

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