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Multimeter and can tell which 2 pins to use to vary the resistance

  1. Jan 8, 2010 #1
    Dear experts,

    I am a novice of engineering.
    So, pardon my basic question about potentiometers.

    I have a 3 pin pot.
    I used my multimeter and can tell which 2 pins to use to vary the resistance.
    To me, i see that i only need 2 pins out of the 3. I looked at the internet and it shows that the 3rd pin is connected to ground.

    I am not sure must that pin be used? If not , have i wrongly used the pot?

    I did not see the need for ground as a resistor has only 'in' (+) and 'out' (-) terminals.

    I noticed also that i could use either of the 2 pins with the middle pin called the sweep to vary the resistance.

    May i know which 2 pins to use and must the 3 pins be used if i just want to drop resistance to lower it so that the load can get the proper voltage?

    sincerely
    Ramone
     
    Last edited: Jan 8, 2010
  2. jcsd
  3. Jan 8, 2010 #2

    vk6kro

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    Science Advisor

    Re: potentiometer

    https://www.physicsforums.com/attachment.php?attachmentid=22936&d=1262955931
    Pot vs rheostat.PNG
    A potentiometer gives you much greater control than a rheostat. Not that this is always a good thing.

    In the diagram above, the rheostat on the left, with the control at the top of the 10 K rheostat, the voltage across the 100 K resistor is 10 volts but at the bottom of the travel it is 9.09 volts. (100 K / 110 K ) * 10.

    In the potentiometer at the right, the travel of the pot gives a range from 10 volts to zero volts.
    It is a potentiometer because of the earth connection at the bottom of the 10 K pot.
     
    Last edited: Jan 8, 2010
  4. Jan 8, 2010 #3
    Re: potentiometer

    Hello ramonegumpert,

    Most small variable resistors are made up as three-terminal potentiometers. This enables them to be used in more different ways, in two or three terminal applications.

    Note however that if your circuit does not show a connection to ground from a variable resistor, you must not connect the third terminal to ground. Doing that can cause a short-circuit.

    On the other hand, where only a 2-terminal variable resistor is required, it is normally OK to connect the third pin to the wiper (= slider = sweep pin). This is sometimes recommended to help reduce problems caused by poor slider contact.

    Finally, do you know how that the word potentiometer cones from a kind of slide-wire variable resistor used in a method for measuring voltages? Try looking that up on the web.
     
  5. Jan 8, 2010 #4

    berkeman

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    Staff: Mentor

    Re: potentiometer

    Here's a very good (classic) primer on potentiometers from Bourns:

    http://www.bourns.com/pdfs/trmrpmr.pdf [Broken]

    .
     
    Last edited by a moderator: May 4, 2017
  6. Jan 8, 2010 #5
    Re: potentiometer

    a Big thanks to all of you :
    vk6kro : excellent explaination. could not be clearer, as always :) I have never knew the difference between a rheostat and a pot nor used a rheostat before.

    Adjuster: great tips.

    berkeman : good stuff!

    Kudos!
     
    Last edited: Jan 8, 2010
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