Potentiometer with min max settings

Discussion in 'General Electronics Chat' started by Sambo1969, Mar 29, 2018.

  1. Sambo1969

    Thread Starter New Member

    Mar 29, 2018
    6
    0
    Hi, I am looking at modifying my foot pedal to a welding tig machine to include a min and max settings to the linear 10k pot currently in the pedal.

    Would anyone have a circuit diagram required to connect another 2 pots for the min and max settings of the existing pot.

    The footpedal controls the amps on the tig welding machine. Ranges from 5A to 200A.

    Any assistance would be greatly appreciated.

    Regards
    Sam
     
  2. crutschow

    Expert

    Mar 14, 2008
    22,553
    6,623
    You could put the added pots in series with each end of the original pot (perhaps 1kΩ to 5kΩ, depending upon how much you want to move the high and low settings).
    The added pots would be configured as variable resistors (wiper connected to one end of the pot).
     
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,947
    1,961
    That's assuming that the pot is wired as a potential divider.
    It gets more complicated if it's wired as a rheostat.

    How many wires are there from the pedal to the machine?
     
  4. Sambo1969

    Thread Starter New Member

    Mar 29, 2018
    6
    0
    Hi Albert, 5 wires from the pedal to the machine. Two for the microswitch to start the gas solenoid and hf frequency ignition for the arc and three for the pot.

    Regards
    Sam
     
  5. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,947
    1,961
    In that case Crutschow's plan is a runner.
     
  6. Sambo1969

    Thread Starter New Member

    Mar 29, 2018
    6
    0
    I have added a circuit diagram
    Hi crutschow, thanks for your advice. I have a 10k pot(linear) for the pedal, and another 2 more 10k pots for the min max settings (linear). I have connected on the breadboard and machine starts at 100A and only the min pot works, but only moves 10A. I think it needs more components to work. I have attached a diagram for a similar pedal, but looks a bit more complicated. Rgds sam
     
  7. crutschow

    Expert

    Mar 14, 2008
    22,553
    6,623
    This is how the pots should be connected?
    Is that how you did it?
    upload_2018-3-30_2-1-59.png
     
  8. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,947
    1,961
    The Max and Min pots will interact so you will need to go back and forth between them.
    Set the pedal to minimum and adjust the Min set pot.
    Set the pedal to maximum and adjust the Max set pot.
    Repeat these two steps until it is adjusted to your satisfaction.
     
  9. AfdhalAtiffTan

    Senior Member

    Nov 20, 2010
    120
    11
    [crutschow]'s solution is fine, but, adjusting the min will affect the max and vice versa.
    Assuming the bias and the input impedance are appropriate, you can try the following attached circuit.

    The min and the max can be independently adjusted without affecting the other.
     
    • pot.PNG
      pot.PNG
      File size:
      17.1 KB
      Views:
      15
  10. dendad

    Distinguished Member

    Feb 20, 2016
    2,829
    764
    Your circuit is a pretty good way to go I think. But a mod would be to add a resistor to the top of the Min pot and another to the bottom of the max pot to limit their ranges. otherwise, the setting pot could work in reverse if the Min is set above the Max setting.
     
  11. AfdhalAtiffTan

    Senior Member

    Nov 20, 2010
    120
    11
    That is actually a very good idea!
     
  12. Sambo1969

    Thread Starter New Member

    Mar 29, 2018
    6
    0
    Hi crutschow, not quite. I am a newbie to breadboarding and probably need a few more YouTube lessons. I will try your suggestion and afdhalattif. A bit of trial and error from my end with circuitary.

    This is a great forum. Thanks all for your advice.

    Regards
    Sam
     
  13. crutschow

    Expert

    Mar 14, 2008
    22,553
    6,623
    That's not strictly true.
    The interaction may not be as large as with the pots in series, but one limit adjustment will still affect the other.
    However the interaction may be small enough to not be a problem.

    Using smaller resistance limit pots will reduce the interaction.
    To get no interaction you would need to buffer the output of the limit pots with an op amp so that the voltage to the control pot has zero impedance.
     
  14. Sambo1969

    Thread Starter New Member

    Mar 29, 2018
    6
    0
    Hi crutcshow, pots in series is the go. getting there with the min max settings. I can get the minimum to work well, which is a good help. At least not starting the welding at 5Amps (straight to say 60Amps). But by moving the other pot, it does change the range of amperage but strangely. What did you mean by op amp in your comments.

    Regards
    Sam
     
  15. crutschow

    Expert

    Mar 14, 2008
    22,553
    6,623
    That was referring to AfdhalAtiffTan's circuit in post #9.
     
Loading...