LM317 question

[Mark Amundson]

Arthur Harrison wrote:
> The critical specification here is "dropout voltage....
> ..Since you have a 3v difference available, you should be okay
> with this part.  

J. Larry Hendry wrote:
> >I am wanting to supply 9 +VDC @ 1 amp from a 12 VDC supply.  Would a LM317
> >be a good choice for this application?  All the schematics I have found
> >using a LM317 to supply an adjustable output voltage use more than 12 VDC
> >as the source voltage to the LM317. Thanks, Larry Hendry

The 3 volt difference between supplied and regulated output is
acceptable under conditions Mr. Harrison and others mentioned. If it
were me, I always allocate at least a 3 volt margin for early regulators
like the 78XX/79XX, and 317/337 types. Thus I would choose if possible,
a more modern low dropout version of the LM317.

Things like AC supply sags (brownouts) and wall wart supplies always
make me pad the worst case input voltage number on the supply. Unless
battery powered, I am more than willing to deal with the extra heat
dissipation on the regulator.

Mark Amundson,