Variable resistor / trim pot wiring

Discussion in 'The Projects Forum' started by ebeowulf17, Apr 9, 2015.

  1. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
    678
    79
    I've needed a variable resistor several times recently and used two legs of a pot to get the job done, leaving the other end disconnected. So far these circuits have all worked as expected.

    More recently, however, it seems like when I see pots used as variable resistors in other people's schematics, the unused side is always connected to the wiper. Is there a reason for this extra connection?

    As far as I can tell, it would have no effect electrically, unless maybe in the case of a badly worn wiper the extra connection guarantees some sort of continuity even when the wiper isn't working well... dunno, grasping at straws here. What's the purpose of the extra connection?

    Below is an example of a circuit I made without the extra connection between the wiper and the left side of the pot(VR1.)

    [​IMG]
     
  2. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    You're absolutely right in thinking that it has no effect, but, as you venture into, real world mechanical potentiometers and trimmers aren't exactly built to resist a nuke.

    A wiper separating from the resistance track will, if the "open" end is actually left open, result in an infinite (well, apart from board leakage) resistance and in many applications this will have adverse effects, ranging from slight to catastrophic failure. By connecting the wiper to the open end, the max resistance is then limited to the value of the pot and in a well designed circuit, the guy with the pencil has made sure (if at all possible) that this value won't harm anything.

    Always tie down loose ends and that goes for many other situations, like eg. unused op-amps or CMOS gates in a more-than-one-package.

    When you're breadboarding, it's rarely that critical, as it only has to hold up until eventual wrinkles have been ironed out.

    Edit: If eg. your VR1 goes open, the oscillator will stop. Whether that would be catastrophic depends on what it drives, like a flashing LED or a respirator.
     
    Last edited: Apr 9, 2015
    ebeowulf17 likes this.
  3. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
    678
    79
    Excellent explanation. Thanks!

    I'll try to develop the habit of tying up those loose connections now, so that I'm already used to it by the time I'm working on something more critical.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,651
    632
    Yep. And sometimes pots develop high resistance spots within their normal range of travel (like those "scratchy" volume controls of the old days). Sometimes a resistor going to near-infinite resistance can cause disaster.
     
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