Pot divider problems

Discussion in 'General Electronics Chat' started by clintonb, Mar 30, 2015.

  1. clintonb

    Thread Starter Active Member

    Mar 6, 2011
    I bought a 5k potentiometer that is unusual in that it rotates 360 degrees continuously.

    I hooked this up to an atmega32 adc port and it reads the voltage. It works as expected except that it shows 1023(max 10. Bit value) over a 20-30 degree range. I hooked it up to a voltmeter to find out why but the pot is in fact continuous from 0 ohm to 5 kOhm seperated by a small notch that shows open line.

    I expected half position which has 2.5kOhm to show half voltage and provide the value 511.

    Can anyone think of a reason why there is this unresponsive zone and why this pushes position away from where you would expect them?
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Post a schematic showing how/where the pot was hooked up. What supply voltage?
  3. crutschow


    Mar 14, 2008
    Reason 1: The pot is incorrectly connected.

    Reason 2: You have a non-linear pot.
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
    Single turn pots typically rotate 270° or so from stop to stop but there are other variations. One notable exception is a pot with 330° of rotation and no stops designed to be turned by a servo motor as part of it's position control.

    Some military specification potentiometers are designed so that if they are forced past their end stops the wiper and resistive element remain undamaged and functional as a kind of fail safe feature.

    If your pot is logarithmic, not linear, the midpoint value would typically be somewhere near 10% or 90% of the full scale value depending on which end of the taper you were reading.

    Those are some possible explanations for the mechanical and electrical properties of the potentiometer but I can't comment on the correctness or lack thereof of your ADC readings.
    Last edited: Mar 30, 2015
  5. clintonb

    Thread Starter Active Member

    Mar 6, 2011
    I was hoping to use it with a type of servo motor but I got the data sheet this morning. It might have a 360 degree rotation but only effective electrical characteristic of 340 degrees. The 20 degree still gives a voltage but the resistance reading is in fact 340 odd ohm over the entire 20 degree band.