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I'm looking for a schematic to control an LM317 and an LM337 from an Arduino using MCP42010 digital pots. I'd like to build this schematic (input dual 30 V, 500 mA transformer per coil):

this schematic

I want to use digital pots instead, to control the voltage from a microcontroller. However, I must add an op-amp in order to use digital pots and I can do it only for the positive voltage.

enter image description here

For the negative voltage, I didn't find any schematic yet. Could anyone post me a link to a schematic that lets me control an LM337 and an LM317 at once with digital pots or op-amps?

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  • \$\begingroup\$ An opamp using V- as its -ve supply and V+ as its +ve supply can accept inputs and provide outputs across most of the total supply range (limited by input common mode voltage.) Dpot Vout that is +ve wrt ground can be amplified - still positive and then either translated by a fixed amount to below ground (subtract a constant voltage) OR (better) invert and set relative to ground so for Vpot = 0 to +Vpmax wrt ground the opamp outputs 0 to -Vpmax below ground. This takes a single differential amplifier with unity gain. OR you can roll the gain stage and inverting amp into one. \$\endgroup\$
    – Russell McMahon
    Commented Oct 12, 2016 at 11:45
  • \$\begingroup\$ @Russell McMahon - As I understand, you offer to set inverting opAmp for negative reference ping? Would it be hard for you to paint schematic by hand or provide a link from in internet? I would appreciate it. Thank you \$\endgroup\$ Commented Oct 12, 2016 at 11:57
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    \$\begingroup\$ You might look here: hackaday.io/project/4154-bench-power-supply/log/… as it's not too difficult to follow. Just keep in mind that his schematic shows a (+) regulator, but that the design can be flipped to use the LM337 instead for a negative supply rail. There's some text there explaining that fact, but you do have to use your head as well. \$\endgroup\$
    – jonk
    Commented Oct 13, 2016 at 9:13
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    \$\begingroup\$ If you're interested in a start to end video tutorial on basically this exact project, EEVBlog Power Supply Design #1-8 might be a good resource if you run into any "traps for young players." \$\endgroup\$
    – Phil C
    Commented Apr 26, 2018 at 4:14
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    \$\begingroup\$ Just so that you know, there is a much better way to do this which is to use an adjustable SMPS IC like LM2586. Although even the simplest designs are potentially difficult to get right as these are essentially high frequency power circuits which are sensitive. \$\endgroup\$
    – squarewav
    Commented Jun 30, 2018 at 5:00

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@Russell McMahon - Circuit with inverted opAmp worked. At the moment I can't do full test, but with voltage converter (+30 and -7, basically no load only oscilloscope test), values moves full range up and down. I need to set proper resistor values and use proper transformer. But I think it will work anyway. And I came to idea to use as reference source digital pots them-self. So simply replace opAmp outputs to digital pots. But this needs to be tested later on. If you'll get other useful links to related topic, please post those as well. Thank you. :)

enter image description here

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  • \$\begingroup\$ Just comment about Digital potentiometers. They have limitation for 5V. So I can't use it as ADJ reference more than 5V. (tested yesterday) This means, that I need use digital pot, gain it via opAmp up to 30V, and then apply to ADJ. for negative voltage I must use inverting gain and apply it to LM337. \$\endgroup\$ Commented Oct 14, 2016 at 7:33
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    \$\begingroup\$ LM358 running from 60V total supply? I spy Magic Smoke! You need something like LM344, and even then there's not much safety margin. \$\endgroup\$
    – user131342
    Commented Nov 5, 2018 at 9:18
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    \$\begingroup\$ Digital potentiometers [are limited to] 5V - not true; refer to the MCP41HVX1 for instance - ww1.microchip.com/downloads/en/DeviceDoc/20005207B.pdf \$\endgroup\$
    – Reinderien
    Commented Apr 26, 2021 at 13:22

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