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Bought a board that has an adjustable pot - how to measure ohm vals

Discussion in 'General Electronics Discussion' started by pityocamptes, Sep 25, 2012.

  1. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    I bought a cheap booster board that has an adj pot for voltage output. I want to remove the pot and instead run it to a resistor box for hardwired (switched) voltage outputs. Can I measure the pot resistance for a specific voltage while the pot is on the board or does the pot have to be removed, once voltage output value is reached and then measure the ohm value on the pot for a comparable resistor value? Oh, and one more thing, the pot has three legs, what should I do with the 3rd leg when I hard wire in a resistor selection box? Thanks!
     
  2. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Most likely you can measure it in circuit, since it would likely just be setting a voltage for a comparator, which should not draw much current. If the 3 legs all have separate connections you would need 2 resistors to replace the pot. If one of the outer legs is connected to the middle leg (or not connected) you could do it with one.

    Bob
     
  3. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012

    So how would that be in a circuit - where I need two resistors? Like this:



    http://diyaudioprojects.com/Technical/Voltage-Regulator/


    When I get the board in the mail I can take some pics...
     
  4. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    No, in that circuit, the pot is used as a variable resistor, i.e. only two legs are used.

    Let's say you have a 10K pot. And at some position, the center leg to the left leg reads 3K. The the center to the right leg would read 7K, and you could replace the pot with those two resistors wiring one end together with that junction replacing the center leg on the pot.

    Bob
     
  5. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    How would that work though if I want a resistor box that allows me to switch to multiple FIXED voltage outputs?
     
  6. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    You're getting ahead of yourself. It would depend on how the pot is connected, which you don't seem to know yet. It is most likely used as a variable resistor (like the LM317) example you showed, and you would only need one resistor.

    If it is used a a potentiometer, then most likely one leg is to ground. For each switched pair of resistors you would connect the ground sides together and then uses a 2-pole switch to swith the top of the dividers to the the where the other outer leg is connected and the middle of the dividers to where the center lug was connected.'

    Do you have a schematic for the converter board?

    Bob
     
  7. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    No I do not have a schematic of the board, I will wait until I receive it... thanks.
     
  8. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    OK, just got the board. The side of the adjustable pot says:

    BAOTER
    3296
    xchc electron
    _________1__________
    3 V^V^V^V^V^V^V^V^V^V 1
    _________2__________

    CW

    The top of the pot says:

    194 T
    W 103


    The bottom the pot has all three legs in line if that makes a difference. Thanks!
     
  9. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    [​IMG]

    [​IMG]
     
  10. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    It's a 10K pot, but you still need to figure out how it's hooked aka what legs go where in the circuit...
     
  11. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    So then I need to remove it?
     
  12. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    Maybe or take an real educated guess based on what you can see and test with a continuity tester...
     
  13. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    [​IMG]

    [​IMG]
     
  14. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    Ok it's being used as a variable resistor, you can even see where they left a place for a fixed resistor if you wanted to go that route...

    Measure between these two highlighted points...
     

    Attached Files:

  15. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    Ok, so I just need to come off those two pads to my adjustable resistor box?
     
  16. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    Also, how would be the best way to test the var pot resistor so that I can match the ohm value to a fixed resistor, etc. so that I can make a resistor box that will have a switch for different fixed outputs? Thanks!
     
  17. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    Best way socket that pot or make it removable, but keep the two pins on the right that are shorted when in circuit also shorted when removed, dial it in on the circuit remove from circuit and simply measure resistance between the two pins that are not shorted...
     
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