lm 317 with digital potentiometer

Thread Starter

flyer50

Joined Nov 10, 2015
5
greetings to all
I need to adjust 20 circuits with lm317 and I planned to use a digital potentiometer as suggested in the attached diagram

The chip AD711AP is hard to find and expensive, especially because I have to buy 20 units.
Can I replace it with a component cheaper and easier to find?

Thanks for your help
Giorgio

 

crutschow

Joined Mar 14, 2008
23,314
Why not eliminate the op amp and just use the digital pot in place of R4 to ground?
Do all 20 outputs need to be adjusted to different voltages at the same time?
What is the voltage adjustment range desired?
 

AnalogKid

Joined Aug 1, 2013
8,102
At 23.67, why is the opamp gain so high? Whatever Vcc is at the top of RV1, the pot output will drive the opamp to saturation with very little adjustment above GND.

ak
 

#12

Joined Nov 30, 2010
18,076
What voltages do you need and how accurate?

I can see some basic limitations with a quick search: 5V max on the pot, 3ma to 6ma current max through the pot, only 64 steps available in the pot I looked up first, but without anything except, "I need a different op-amp" you aren't going to get anything except a different part number for a circuit that looks like it won't work in the first place.

Better questions get better answers.
 

GopherT

Joined Nov 23, 2012
7,983
greetings to all
I need to adjust 20 circuits with lm317 and I planned to use a digital potentiometer as suggested in the attached diagram

The chip AD711AP is hard to find and expensive, especially because I have to buy 20 units.
Can I replace it with a component cheaper and easier to find?

Thanks for your help
Giorgio

- You need a 5v regulator on your potentiometer (max 5v).
- You need a rail-to-rail op amp so you can get the op amp output all the way down to ground.
- Your feedback resistance at the op amp is way to high (gain is too high). If your digital pot is 0-5 v, then you only need a gain of 4x that to get the op amp to go all the way up to 20V. Your R4/R1 ratio sets the minimum voltage out of the regulator at about 5V (when the digital pot is at zero volts). You can get lower (down to 1.25v) by eliminating R4 and replacing with a plain wire. Current into the op amp will be only 10 mA at all voltages. Note: increase feed back resistor to allow op amp to reach at least 23 volts if r4 is eliminated.

I kind of like this circuit. Controlling with a digital pot is a good idea. I hope other point out something I missed.
 

Thread Starter

flyer50

Joined Nov 10, 2015
5
thank you all

The 20 cards are regulated individually one at a time using the 20 digital potentiometers.

for not having random outputs from the lm 317 , the 5 volts and the controls for the potentiometers are produced first in order to set the references for the lm317 and then is supplied the voltage to the regulator

the voltage requested varies between 8 and 18 or 20 volts not very important the linearity
 

GopherT

Joined Nov 23, 2012
7,983
you think you can use a digital potentiometer also to adjust a triac?
Yes. Make sure it is fully on with the 5V available from the wiper of potentiometer (or at least on for enough of the phase that you need). There are "sensitive gate" triacs that work at low current and can be driven by low current.
 

Thread Starter

flyer50

Joined Nov 10, 2015
5
Yes. Make sure it is fully on with the 5V available from the wiper of potentiometer (or at least on for enough of the phase that you need). There are "sensitive gate" triacs that work at low current and can be driven by low current.

thanks, I'll do the tests when I finish with lm317
 
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