measuring pot ohms

Discussion in 'General Electronics Chat' started by atlantis43, May 1, 2014.

  1. atlantis43

    Thread Starter New Member

    Apr 30, 2014
    15
    0
    I'm trying to measure resistance of an unmarked pot used as a variable resistor in a circuit (only two terminals present). When I connect a multimeter to the terminals (after disconnecting the pot), I'm getting very inconsistent values. As I turn the wiper, the measured values seem to raise & lower erratically.
    I've checked the meter against a known resistance, and it's accurate & stable.
    Any explanation for the erratic behavior on the pot? Is there anything else that I should do?
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Depends on the meter.

    Try getting the value from outer terminals. From that, set meter to correct range.
     
  3. ramancini8

    Member

    Jul 18, 2012
    442
    118
    The pot may be burnt or have worn out surfaces yielding erratic measurements. If you show the schematic so we can see what application it is used in we might be more help.
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Sounds like a Bad pot.
     
  5. atlantis43

    Thread Starter New Member

    Apr 30, 2014
    15
    0
    Thanks to all responses.
    The pot is a dimmer switch contol from a dimmer transformer for a 12vac halogen bulb. Actually, the dimmer does work, but the switch portion of the ganged unit is failing to turn off the bulb. So---the pot/switch is bad, but with the dimmer portion working properly, I'd expect that the rheostat should still be giving me graduated resistance values.
    [My multimeter is limited to one resistance setting (K-ohm), and can't be calibrated.]
    I'll try to figure out a schematic of the unit, and, when completed, I'll add it to this thread. I should note that some of the items on the transformer portion I've never seen (there are two items with coiled wire wrapped around them which I can't identify), so I'm not sure if I'll be able to provide a useful schematic.
     
  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
    762
    If the multimeter is stable and the pot is OK, what else is wrong to give erratic results? The person in charge of the measurements? :p

    Start by deciding which one is good and go from there.

    Any chance to get another multimeter?
     
  7. atlantis43

    Thread Starter New Member

    Apr 30, 2014
    15
    0
    atferrari:
    please await my attempt at a schematic. Nowhere did I say "the pot was good". I said it worked properly as part of a dimmer, until it reached the "off" position.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,010
    3,785
    I have a case of pots from Panasonic - fader sliders - that work as expected when connected as a voltage divider (very nice and linear). However, when connected as a rheostat (or when measuring with an ohm meter connected to one leg and wiper) I get a log response that was completely unexpected.

    These pots seem to have two sets of logarithmic resistance material placed in opposing directions and wiper connects to both resistive elements.

    In other words, try connecting the outside legs of the pot across a voltage source and measuring voltage from wiper to ground as you adjust the pot. Hopefully you will get a reasonable result instead of the unexpected result you have.
     
  9. #12

    Expert

    Nov 30, 2010
    16,269
    6,781
    A photo of the unidentified parts might help.
    We have lots of smart people here.
     
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