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Potentiometer terms

Discussion in 'Electronic Basics' started by Ron J, Feb 6, 2006.

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  1. Ron J

    Ron J Guest

    Hi all,

    I was reading the datasheet for a potentiometer and came across this
    listed feature:
    "High resolution 25-turns enables precision adjustment easily."

    Does this mean that there are only 25 available resistance?

    Thanks!
     
  2. Dan Hollands

    Dan Hollands Guest

    No - it means you have to turn the shaft 25 complete revolutions to go from
    one end of the pot to the other.

    Dan


    --
    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
    585-872-2606

    www.QuickScoreRace.com
     
  3. Ron J

    Ron J Guest

    Hi Dan,

    Thanks! Ahh.. okay. So a 25 turn compared to a single turn translates
    to a higher resistance resolution, right?

    I thought it was 25 resistance values because the price was cheaper
    than a single turn.
     
  4. John  Larkin

    John Larkin Guest

    Not really. Most multi-turn trimpots have enough backlash in the drive
    that their resolution is no better than a single-turn pot... just a
    lot more tedious to adjust.

    John
     
  5. Don Bowey

    Don Bowey Guest

    Go get him Phil. You can't let him get away with this misinformation.
     
  6. John Fields

    John Fields Guest

    ---
    Resolution refers to the smallest increment of resistance achievable
    in the pot, and has nothing to do with backlash. Of course, if you
    go too far and then have to go even farther forward to back up to
    whre you need to be, it's inconvenient, but the resolution of the
    pot is still the the resolution of the pot,. A wirewound pot,for
    example, will have a resolution depending on the pitch of the helix
    the resistance wire is wound to, while the resistive element of a
    cermet pot will have essentially infinitesimal resolution. Whether
    that resolution can be achieved will be largely determined by the
    'stiction' of the drive, but there will be no fixed resolution limit
    as exists in a normal wirewound pot.

    I think what the OP is referring to is the _sensitivity_ of the pot,
    which can be described as the total resistance of the pot divided by
    the total angle of rotation of the shaft. That is, a single turn
    10k pot with 300 degees between stops will have a sensitivity of:

    Rt 10kR
    S = ------- = ------ = 33.33 ohms per degree
    theta 300°


    While a 25 turn 10k pot will have a sensitivity of


    Rt 10kR
    S = ------- = ------- = 1.11 ohm per degree
    theta 9000°


    Which means that if the best you can do, by hand, is a one degree
    crank, with a 25 turn pot (with a resolution great enough to allow
    it) you'll be able to resolve 1 ohm out of 10k, which is 0.0111%,
    while with a single turn pot with 33.3 ohms out of 10k, that's
    0.333%.

    So, for the same angle of rotation, the 25 turn pot will have a
    sensitivity 30 times greater than that of the single turn pot.
     
  7. John Larkin

    John Larkin Guest


    Your calculations are impeccable. But the pots don't work that way. I
    just tried a couple of Bourns trimpots, single and 20-turn.
    Settability was similar; both have hysteresis and both have stiction.
    Both are fairly easily set to 0.2%, and to 0.1% with some reasonable
    teasing. The 20t is actually a bit better if you're willing to
    *really* futz with it, by maybe as much as 2:1, but nothing like 25:1.

    But no circuit should require a trimpot to be set to 0.1%. Trimpots
    aren't stable enough to justify setting them this close.

    John
     
  8. John Fields

    John Fields Guest

    ---
    I wasn't arguing the actual settability of the pots as much as I was
    the difference between 'resolution' and 'sensitivity', and I agree
    with you, generally, on the stability matter.

    However, once the pot is adjusted and the leadscrew glyptalled down,
    there's not much which is going to affect the pot but temperature,
    and if it's being used as a voltage divider unto itself, it's
    internals will be pretty nearly isothermal, so the ratio of the two
    resistances on either side of the wiper will stay constant, as will
    the putput voltage.

    Especially if it's a _good_ pot, like some of the Vishays with a
    tempco of 5PPM/C°

    Don't get me wrong; I don't care much for pots, but they _do_ have
    their place.
     
  9. Pooh Bear

    Pooh Bear Guest

    Multi-turn pots aren't normally cheaper than single turn ! Surplus
    parts
    ?

    Graham
     
  10. Bob Masta

    Bob Masta Guest

    Maybe he's comparing a cheap multi-turn trimpot with a
    single-turn panel-mount pot?


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  11. Ron J

    Ron J Guest

    Thanks for the reply everyone!

    I'm not sure about Surplus, but I saw it on Digikey.
     
  12. Rich Grise

    Rich Grise Guest

    No, it means that you have to turn the knob 25 whole 360 degree turns to
    get from one end of its range to the other. You need a special,
    gear-driven, turns-counting knob to use it effectively.

    Hope This Helps!
    Rich
     
  13. Rich Grise

    Rich Grise Guest

    Oh! A TRIMPOT! In my other answer, I thought you were talking about a
    PRECISION pot! What this is sounding like - is it about 3mm by 6mm by
    about 20 mm long, with a screw head at one end? In that case, the screw
    head is actually connected to a lead screw, that moves the wiper from one
    end of the element to the other, like a FDD indexing leadscrew.

    Hope This Helps!
    Rich
     
  14. Rich Grise

    Rich Grise Guest

    Maybe not, but it sure is fun adjusting them while watching your output on
    a spectrum analyzer! ;-)

    Cheers!
    Rich
     
  15. John  Larkin

    John Larkin Guest


    Bob Pease (ever the voice of moderation) in his "Analog
    Troubleshooting" book, claims that multiturn pots are worse longterm
    than single-turns, by a factor of two or three, something about
    storing stress and jumping when exposed to shock. He suggests 0.2% as
    a good working resolution for trimpots.

    But a 60-cent eeprom can replace 500 pots.

    John
     
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