On Dec 3, 1:08*am, isw wrote:
In article ,
*Bruce Esquibel wrote:



Michael A. Terrell wrote:

* *Here is the datasheet on that family of regulators. the first page
shows 'Adjustable Output Regulator' and that you need a fixed resistor
from output to common, along with the pot from common to ground.

http://www.national.com/ds/LM/LM340.pdf

Yeah but I think that is specific to the LM340, on the 78xx series you just
need a resistor in the path to ground.

This is more what I was talking about:

http://www.fairchildsemi.com/ds/LM/LM7805.pdf

It's a bit messy with the 741 in circuit (fig 13), but allows you to use
higher value pots. The single resistor method works but may require a
multi-turn pot to dial it in with precision or a need for odd values to hit
the target.

Just play around with it, use a 7805 and Vcc around 24-28v for the best
range and just experiment with different values to ground. Just tie the
wiper to one of the ends, one end to ground and the other to pin 2.

You need that cap on the output to ground to snuff oscillation and something
on the input for ripple but thats about it. That 78xx family is forgiving
but if it's too hot to touch, you are doing something wrong.

Maybe not. The temperature where most folks say "too hot to touch" (too
hot to keep your finger on) is around 130 F; the maximum junction
temperature rating for those regulators is around 300 F.

Isaac

Thanks for all the replies guys. While I can certainly appreciate not
having to screw around klugeing a circuit it is interesting to know
that I can play with this basic design as it was suggested if I should
ever need to. And many times over the years I've had to do things like
this to get the job done. When you're 75 miles from home on a Friday
afternoon and "Miller time" is fast approaching it's nice to be able
to make something work that otherwise might require another service
call on Monday. Not to mention the added inconvenience this saves the
customer. BTW, I was always taught that good design practice dictates
that if a semiconductor is too hot to keep your finger on, (greater
than around 55.0 degrees C), then it's running too hot. However I
remember some military aircraft power supplies I worked on in the 70's
that ran a lot hotter then that on a regular basis. It always seemed
like poor practice to me but I never heard of an F15 going down
because of one, so I guess it was OK. Lenny