>> http://www.electro-tech-online.com/attachments/microcontrollers/32974d12=
51452611-digital-control-voltage-using-lm317-adjusting-regulator-output-dig=
itally.gif

This circuit is flawed.
It is semi oen loop.
It relies on the FET having an approximately constant Vturnon, so that
Vout =3D 1.25 + 2 x Vturnon + Vadc.
Minimum voltage is limited to 2 x Vturnon + 1.25.
Vtturnon (aka ~=3D Vth)(as at 10 mA) will vary between FETs and more.

> http://www.electro-tech-online.com/attachments/electronic-projects-design=
-ideas-reviews/18890d1209004583-lm317-control-pic-mcu-lm317.png

Looks entirely workable - essentially what I described. Some playing
will be needed. eg may oscillate as is. Adding a transistor output
driver will allow Vins > opamp Vdd max if desired.

Drive something like circuit 2 with the ADC or equivalent and 'you
have a ball game".
Circuit 2 Voutmax is limited by the op amp's Vmaxout but if you
operate the opamp Vdd supply from LM317 Vin you get almost max
possible swing as O amp max output needs only be
Vin - 1v* - 1.25V **  or 2.25V or more below Vin. Check Max Vdd wrt
out  of opamp used.


  R
--=20
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