Erratic Potentiometers

Discussion in 'The Projects Forum' started by jwilk13, Jul 27, 2011.

  1. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    Hey all,

    I've got a few 5 kohm potentiometers that I'm messing around with, but their behavior is really strange. If I start at 0 ohms and slowly increase the potentiometer resistance, the resistance goes up just fine until about 100 ohms, then it jumps drastically up to about 1.5 kohm, then increases fine after that. I have a few of these pots and all behave the same way, so I'm pretty sure it's not just a bad pot.

    I'm looking at using these in voltage divider circuits to provide analog inputs for various uC projects, but if I'm going to want some sort of precision (I always do :p), that jump from 100 to 1000 ohms really bothers me. For example, if I'm creating a voltage divider circuit with an output (across the pot) that sweeps from 0-5V with a 15V input and 10kohm resistor as the other element, my voltage would jump from approximately 0.15V to 1.36V across the potentiometer when the pot is barely turned. Not good enough for me :rolleyes:

    I'm wondering if there's a way to compensate for this abrupt change and get a more "linear" voltage sweep. I can't think of anything right now besides lookup tables in the uC, but that would require setting the lookup tables for each pot individually. Maybe a resistive voltage divider isn't the way I should be looking at it, but I'm not sure. Any thoughts?
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I don't you should go this way.
    If these are normal potentiometers they are either dirty inside oder defective.
    Strange thing is, you say they all behave in exactly the same way... :confused:

    Are these new potentiometers? What type?
     
  3. JingleJoe

    Member

    Jul 23, 2011
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    What are the 5K pots from? My first thought is that they have some kind of weird custom track resistance configuration.
     
  4. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Are these multiturn pots? If so, I've learned long ago that "precision" multiturn pots are not stable.

    They suffer from hysteresis (due to lag in the wiper assembly) and instability due to mechanical shock.

    IMHO, only single turn pots should ever be used for precision adjustments. The circuit needs to be optimized to provide the best setability while providing enough range to accommodate all the tolerances which you are trying to adjust out.

    I uploaded some software last week to assist in designing optimal resistor dividers using pots:

    http://forum.allaboutcircuits.com/showthread.php?t=56960
     
  5. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    Thanks for the quick responses. To try to answer everyone's questions, I am not sure what these came out of or what they were used for. I tried using one for my DC motor PWM controller project, but the abrupt change in resistance didn't provide good control. They're actually 4-pin potentiometers, and from what I've measured out, the fourth pin is a center tap halfway between the ends of the internal "resistor".

    I thought they were pretty slick because I could start it at the center and get (almost) zero resistance between the center tap and the wiper and then turn it either direction and get the same resistance both ways. It looks like what I thought was "slick" is turning out to be garbage :p. The only marking on them is COB70024, which only told me they were a Clarostat product (via Google search).

    I like the "bi-directional" feature that it seemed like I could get out of them. I'm not entirely sure what I'd use it for yet, but I liked the concept.

    As far as the turns go, they don't even make one full revolution from what I can tell. They're not trimmers. They're the big beefy kind with a shaft.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    If they are not physically damaged it may help to spray contact spray (for electronics) if there is an opening somewhere. Then turn it a few times. If the problem is dirt you should get rid of it this way...

    If not, as you said : garbage :D
     
  7. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    I'm chalking them up to garbage...oh well. Thanks for the input :)
     
  8. debe

    Well-Known Member

    Sep 21, 2010
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    There are 2 types of pots Linear taper B type. Then theres Log taper A type (these are used in audio aps). You should be using linear in your aplication.
     
  9. JingleJoe

    Member

    Jul 23, 2011
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    of course! how did I forget that, modern "logarithmic" pots which you can buy in electronics shops tend not to be acctually logarithmic, they have a low resistance bit of track and them a high resistance section (or vice versa) usually they join together at the same resistance value.
    How far around the turn does the erratic jump in resistance occur? If it's right at the start I'd say dodgy pot, if it's further along, I'd say crappy modern "logarithmic" pots.
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Doesn't really sound like a logarithmic potentiometer.
     
  11. KJ6EAD

    Senior Member

    Apr 30, 2011
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  12. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    That's interesting. What kind of application would that be useful in? And how would rotating the pot 15° actuate a switch? Not sure I'll ever do that, but I like learning.
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Is it possible that these are fader pots? Like from a car radio that lets you change sound level from front to back or left to right.
     
  14. KJ6EAD

    Senior Member

    Apr 30, 2011
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    That series is configurable into multiple pots on the same shaft, switches, with and without center detents, etc. Yours doesn't have a switch but may have been intended for assembly with one. The common type clicks on in the first 15° of CW rotation from the CCW stop like old car radios that used the volume control as the power switch. If your pot was intended for such an assembly, the resistance in the first 15° of rotation would be ignorable since there'd be no power to the circuit until the switch was closed.
     
  15. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    @KJ6EAD If it was a model like you said, could it have the described effect? Increasing slowly, then jumps to 1.5k then continues to increase?
     
  16. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    In case anyone's interested, I played around a bit more with those potentiometers (I really don't enjoy throwing stuff away, especially when I don't understand it :p). When I was using them previously, I was making a voltage divider with an external resistor and trying to adjust the voltage that way. Once I actually drew up a schematic symbol for the pot and looked closer, I realized that I could essentially just make a voltage divider out of itself (sorta). So, I connected it like the attached image, and as it turns out, you can start the pot in the middle (wiper at the tap) and turn it either direction to sweep from 0 volts to whatever your input is (almost perfectly linear).

    Not sure what I'll use them for yet, but it looks like I'll be holding on to these for a little while longer :)
    [​IMG]
     
  17. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok, but can you explain where the "jumping" effect from your first post does come from? 0 to 100 linear then it jumps to 1.5k then linear again?:confused:
     
  18. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    I don't understand it either. It doesn't jump like I described when wired the way I showed in the image. Your guess is as good as mine...:confused:
     
  19. praondevou

    AAC Fanatic!

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    If you were really curious, you could open one of them and find out.:D but after that it may be really garbage...
     
  20. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    Not curious enough to destroy them, especially after I figured out a way to use them productively :)
     
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