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.

-bruce