Potentiometer as variable resistor vs voltage divider

Discussion in 'General Electronics Chat' started by SPQR, Nov 24, 2012.

  1. SPQR

    Thread Starter Member

    Nov 4, 2011
    Hello all,
    Simple question for you.

    I am building a "generalized" astable 555 circuit where the two resistors and one capacitor that control the frequency and duty cycle are infinitely variable
    (the capacitor is not infinitely variable, but can have 256 values).

    I'm using potentiometers for the resistors.

    But I can connect the potentiometers as variable resistors or voltage dividers.

    So I'll ask you a general question:
    In a digital circuit, when a potentiometer is used, should it be connected as a variable resistor, or as a voltage divider?
    What are the pros and cons of a variable resistor vs a voltage divider in a digital circuit?

    And I thank you in advance.
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    It is entirely design specific... If you want to command a control parameter via a potentiometer, use it as a divider. If you need to adjust the gain on an amplifier that feeds an ADC, use it as a variable resistor....

    again, though it is design specific..
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    I should point out that a pot in potentiometer mode can be used to adjust the gain of an op amp.
    I would say that, if you need a variable resistance (such as in a 555), use a pot as a variable resistor (duh). If you need a variable resistor ratio, use a pot as a potentiometer.
  4. MrChips


    Oct 2, 2009
    The answer should be obvious from the question.
    If you need to divide the voltage, use a voltage divider.
    If you need a variable resistor, use a variable resistor.

    The proper way to convert a potentiometer into a variable resistor is to wire the center wiper to one of the arms of the pot.

    btw, it makes no difference whether the circuit is analog or digital.
  5. SPQR

    Thread Starter Member

    Nov 4, 2011
    Hmmm...then I guess I'm misunderstanding.

    So is there any difference between
    1 - using the middle terminal and one of the outside terminals to make a "variable resistor"
    2. - connecting the middle terminal to one of the outside terminals, and use the outside terminals as a "variable resistor"?
  6. KrisBlueNZ


    Oct 17, 2012
    No, they both do the same thing.

    When you connect the middle terminal to one of the end terminals, the part of the carbon track between the wiper and that end is shorted out by the connection, and has no effect. You end up using only the section of carbon track between the wiper and the other end, the same as you would in option 1.

    In my experience, option 2 is more often used, since it avoids confusion over "is that pin supposed to be connected to something or not?" and avoids unconnected nets which can confuse a design rule check. And it just looks a bit tidier.
  7. kubeek


    Sep 20, 2005
    Actually the main reason is that if the viper looses contact with the resistive track, you don´t end up with open resistor but just the max resistance.
    For example if you use it in the feedback resistor of opamp, then when it looses contact your gain suddenly jumps into tens of thousands, which obivously is not very good.
  8. SPQR

    Thread Starter Member

    Nov 4, 2011
    Now ALL is clear!!!
    Thanks very much to everyone!!