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From: asd@cbnewsj.att.com (Adam S. Denton)
Newsgroups: sci.electronics
Subject: Re: WAIT! - Re: 12 to 9 volts dc (kinda long)
Message-ID: <1991Feb28.231304.1844@cbnewsj.att.com>
Date: 28 Feb 91 23:13:04 GMT
References: <erk.1356@americ.UUCP> <1991Feb26.235009.1389@cbnewsj.att.com> <5825@optilink.UUCP>
Organization: Analog Innovations, Inc.
Lines: 30

In article <5825@optilink.UUCP> elliott@optilink.UUCP (Paul Elliott x225) writes:
>I understand that it is *not* good practice to boost the output of
>the 78XX-series regulators by putting diodes (Zener or otherwise)
>in the ground lead.  Something to do with the short-circuit protection
>(I haven't analysed it myself).

Sounds reasonable; I have never done this myself.  The data books all
use resistors anyway.  Serves me right for brainstorming...!!

>I prefer the LM317-series of regulators.  These let you set the

So do I!  Why I forgot to mention it, I don't know.  The only concern
is that the 317 (like the 78's) requires a minimum differential voltage (the
value eludes me at the moment) which isn't too small (maybe 2-3V).  On
the other hand, the transistor ckt can technically operate with a very
small differential (~0.4V) but that can throw the various part values
way out of whack.  The LM2931 series are good for low dropouts but
can't hack the current (i.e. 100mA is their limit).  Those new
National mini-switchers look pretty nice...gotta try 'em soon.

[good explanation on using resistors with 78XX deleted]

>All in all, the LM317 is a better bet, though.

Yup.  Although I have a vaccuum-tube circuit that's better :-) :-)

>      Paul M. Elliott      Optilink Corporation     (707) 795-9444

Adam Denton
asd@mtqua.att.com