lm 317 with digital potentiometer

Discussion in 'General Electronics Chat' started by flyer50, Nov 12, 2015.

  1. flyer50

    Thread Starter New Member

    Nov 10, 2015
    5
    0
    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

    [​IMG]
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Looks like an ordinary op amp. Try a TLC271, or TL071,TL081, TL084,
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    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?
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,540
    1,251
    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
     
  5. #12

    Expert

    Nov 30, 2010
    16,298
    6,809
    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.
     
  6. eetech00

    Active Member

    Jun 8, 2013
    650
    112
    What about stepped resistors using a CD4066 analog switch?
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    - 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.
     
  8. flyer50

    Thread Starter New Member

    Nov 10, 2015
    5
    0
    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
     
  9. flyer50

    Thread Starter New Member

    Nov 10, 2015
    5
    0
    you think you can use a digital potentiometer also to adjust a triac?
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    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.
     
  11. flyer50

    Thread Starter New Member

    Nov 10, 2015
    5
    0

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

    Moderator

    Aug 23, 2012
    4,855
    767
    What is the number of digital potentiometer (ad5171?), how many steps?
     
  13. flyer50

    Thread Starter New Member

    Nov 10, 2015
    5
    0
    I have some MCP41100 and a lot of X9c104p
     
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