Reduce sensitivity of potentiometer?

Discussion in 'The Projects Forum' started by diafygi, Oct 7, 2008.

  1. diafygi

    Thread Starter New Member

    Oct 7, 2008
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    0
    Hey all,

    I recently got an oxygen sensor kit for some engine tests. It has a knob that you can turn to calibrate the output. The knob is a simple potentiometer with only two terminals used (so I guess it's just a variable resistor or rheostat). Everything works fine, but my problem is that the bloody knob is too sensitive!

    I barely turn it and the oxygen reading goes up by like 1+ percent. I have to fiddle with it for five minutes before I can get it to settle in on 20.9%.

    Does anyone know how I can reduce the sensitivity of this potentiometer? I tried just adding some more resistance to the calibration circuit, but that just reduced the overall reading (which makes sense).

    Will I need to get a different potentiometer? How can I tell what type I have now?

    Thanks in advance. I'm very new to circuits, so any advice would be appreciated.
     
  2. diafygi

    Thread Starter New Member

    Oct 7, 2008
    2
    0
    Okay, I tested the potentiometer with my multimeter, and it has a range from 0.3-480 ohms (I'm assuming the label would say 0-500).
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Do you have a schematic of the circuit?

    If you use an ohmmeter to measure resistance between the two terminals of the pot that are the furthest apart, you should get the resistance of the pot. Verify that by measuring between one of the furthest apart terminals and the middle terminal; it should be less than the reading you got between the furthest apart terminals.

    Bourns used to make 10-turn "knobpots", which are quite linear across their entire adjustment range; but are pricey to buy them new.

    A less expensive option would be to use a pot of much lower resistance in series with your current pot. To set the pair of them, first adjust the low-resistance pot to the middle of it's range, then use the original pot to get the "coarse" setting, and the low-resistance pot for the fine adjustment.

    [eta]
    Gee, 480 Ohms and it's touchy? Well, let me think about this for a few...
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, here's a part for you:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=3590S-2-501L-ND
    $10.02 is a lot cheaper than a knobpot; you didn't really need the dial anyway. The part is a 10-turn wirewound pot. This is the least expensive 500 Ohm 10-turn pot that Digikey currently carries.

    A standard potentiometer has about 270° of travel, this one has 3600°, or about 13.3 times as much resolution. You'll be able to "tweak" your setting relatively easily.
     
  5. neon9

    Member

    Oct 8, 2008
    15
    0
    just add a resistor in parallel with it or adda 500 ohm resistor in series but that will restrict your range. either way changing resistance is the way to go. of course the better sugestion is turns per ohm as sugested allready $10
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Neon9, I'd already considered using a couple of pots in series, one with a parallel resistor. The pot with the parallel resistor would be very non-linear in it's response. Our OP would have the additional problem of where to mount the 2nd pot; usually kits don't have room for additional controls.

    The one caveat with the part I suggested is that it's wirewound. If the signal applied across it is AC, the wirewound pot will act as an inductor. At low frequencies (<100Hz), it's not likely to be much of a problem. However, at higher frequencies the inductance could be problematic.
     
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