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I'm trying to cancel wiper resistance of a 10 kΩ DiGiPot that controls output voltage of a switching power supply. I'm using MCP4131-103 that has 120 Ω wiper resistance @0% pot position. This wiper resistance limits the max voltage that I can get from power supply. In this post there is a method that I have simulated it in EasyEDA: enter image description here But there is an issue! I want to use LM358 that is not a bipolar op amp both because I have it in my disposal and also it needs a single power supply (0-12 V) instead of (-6 to +6 V). So I tried this sketch that apparently is not right: enter image description here

Can please someone tell me how can I reach a solution using LM358?

ANSWER:

THANKS to Andy aka for providing the solution, just adding the final sketch for documentation: enter image description here

This sketch gives me all I want:

  • POT@0% -> probe 3V
  • POT@50% -> probe 1.5V
  • POT@100% -> probe 0V
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    \$\begingroup\$ Why not simply use a unity gain follower with very high input impedance (JFET input OpAmp; input resistance is in the GOhm range)? Additional 120 Ohms wiper resistance in series to the GOhm input resistance don't matter at all. \$\endgroup\$
    – Curd
    Dec 21, 2022 at 11:29
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    \$\begingroup\$ Unrelated to your question, but "LM358 that is not a bipolar op amp ... also it needs a single power supply (0-12 V) instead of (-6 to +6 V)." is wildly incorrect. an LM358 will work just fine from a bipolar supply. Since your opamp doesn't have or need a direct connection to Ground it never "knows" whether the circuit's supply is bipolar or unipolar. The LM358 is often used in 'single-supply' circuits because it's capable of operating all the way down to its negative rail - but there's not requirement that you do so. \$\endgroup\$
    – brhans
    Dec 21, 2022 at 14:10
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1 Answer 1

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Can please someone tell me how can I reach a solution using LM358?

It seems you are going to a lot of trouble trying to make a negative impedance in order to cancel the wiper resistance when, in fact, you could just buffer the wiper voltage with er... a buffer amplifier. It can be a unity voltage gain buffer amplifier made from an LM358 too: -

enter image description here

Picture from here.

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  • \$\begingroup\$ Thank you, I was commenting your answer to ask for a sketch. Thanks for providing it. I'll try it right now and will give a feedback soon. \$\endgroup\$
    – AKTanara
    Dec 21, 2022 at 11:35
  • \$\begingroup\$ Thanks again, at least in simulation I got what I needed. To be honest, I don't get hot it works, but it works... \$\endgroup\$
    – AKTanara
    Dec 21, 2022 at 12:08
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    \$\begingroup\$ @AKTanara because the input to the op-amp can be regarded as very high impedance, any current flowing into the op-amp input from the wiper will be very, very tiny and drop only a few microvolts across the wiper resistance. That volt drop (micro-volts) is so low that you don't notice the error i.e. the error is very, very tiny. \$\endgroup\$
    – Andy aka
    Dec 21, 2022 at 17:43
  • \$\begingroup\$ AHA! now it makes sense... THANKS AGAIN. In simulation I got what I wanted but when implemented, it didn't work properly. I thought it might be from POT wires that I rose from power supply that is my black box! But I also found this electronics.stackexchange.com/a/476266/301585 that is exactly what I'm trying to do and mentions that had no success with none of the op amps except for LM311. So, Probably the issue is incompatibility between my application and LM358, I'll try to do it with LM311 and report the results... \$\endgroup\$
    – AKTanara
    Dec 22, 2022 at 11:25
  • \$\begingroup\$ "it didn't work properly" <-- doesn't really help diagnose what the issue might be @AKTanara \$\endgroup\$
    – Andy aka
    Dec 22, 2022 at 11:30

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