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Potentiometer/voltage divider?

  1. Sep 13, 2006 #1
    Potentiometer/voltage divider?!??

    Hey,
    We've been taking electronics in class and among the things we've taken are "potentiometers", as the teacher calls them...
    From our labs, all I know is that it's a device that has a slider thing on a rail... and apparently it somehow controls the current or something? It has an input and two other holes which somehow connect to the circuit, like so:
    [​IMG]

    It's illustrated in the following circuit diagram as the big box in parallel:
    [​IMG]

    Can someone explain exactly how this thing works? (The big picture, I'm not so much interested in the complete details)
    Also, if it's just a voltage divider, why are there three holes? Why couldn't it just be considered a variable resistor?

    I hope someone can clear this up for me.

    Thank you.
     
  2. jcsd
  3. Sep 13, 2006 #2
    Pot Explanation

    Pseudo Statistic, a pot IS just a variable resistor - actually, it's two. The resistance between the outer two terminals will be a constant value, let's say 10K ohms for example. By moving the slider, you effectively place a tap somewhere in the middle of that resistance, anywhere from 0 to 10K for this example. So if R1 is measured from left tap to center, and R2 is measured from center to right tap, you will have R1+R2=10K. If you ground the right tap and connect the left to some voltage V1, then the center tap can be measured to be at V2 = R2/(R1+R2). If you realize that R2 varies from 0 to 10K, and (R1+R2) equals 10K, then you can see that V2 varies from zero to V1 volts. Ohm's law rules.
    Best regards,
    Mike
    http://www.eehomepage.com
     
  4. Sep 13, 2006 #3
    But exactly how does the circuit work through it?
    I mean, what's the point of having two variable resistors there? Exactly how is this of any significance in the above circuit?! I mean, what's it doing in the above circuit diagram and what does the arrow mean?
    Thank you.
     
  5. Sep 13, 2006 #4
    Pseudo Statistic, time to do a bit of reading. You might start at http://en.wikipedia.org/wiki/Ohm's_law, or simply Google "Ohm's Law" and you'll find plenty of background - which you MUST read if you want to understand your coursework.
     
  6. Sep 13, 2006 #5
    I understand Ohm's Law, Kirchoff's rules and all that other stuff. :)
    I just don't understand the circuit diagram, with respect to the arrow being drawn to that resistor and where exactly on the diagram it shows where each of the outputs of the potentiometer would connect to..
    That's what's confusing me; and sadly for me, my textbook doesn't explain it.
     
  7. Sep 13, 2006 #6

    berkeman

    User Avatar

    Staff: Mentor

    As Mike said, the pot has resistive material between the ends, and a mechanical wiper contact that moves from one end of the material to the other as you turn the pot handle. The two ends of the pot are generally labeled CW and CCW, to indicate which end the wiper is at when the control is fully clockwise or counter-clockwise.

    So for a 1kOhm pot, when the control is CW, the resistance from the wiper contact to the CW end is zero, and to the CCW end is 1kOhm. When the control is mid-way, then the wiper is touching the resistive material half-way between the two ends, so you would measure 500 Ohms from the wiper pin to each of the CW and CCW pins on the pot.

    You use potentiometers in circuits where you need to vary a voltage or a resistance. A common application would be a volume control, for example, where you vary how much signal you feed from one stage to another. If you ground the CCW pin of the pot and feed the signal into the CW end, then the wiper can tap off a varying amount of signal as you turn the control. You can also use a pot for DC bias control, as shown in the circuit you linked in your OP.

    Note that you should pay attention when connecting up a pot to ensure that the CW/CCW directions make sense in your circuit. That's why in my volume control example, I said to ground the CCW pin. Most people expect the volume to increase when they turn a control knob CW.
     
    Last edited: Sep 13, 2006
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