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basic properties of a trimmer pot

Discussion in 'Electronic Basics' started by Jim Alexander, Aug 24, 2005.

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  1. I've never actually designed a circuit that used a trimmer pot, and I'm
    a bit ignorant of their basic properties, and I do not have any samples
    on hand I could just hook up to my DMM. From the way their specs are
    specified, it sounds like they really live up to their name: the range
    of resistance they offer is very narrow. For instance, a 1M trimmer
    pot would offer resistances from about .9M to 1.1M over its range of
    adjustment. Contrast this with a regular 1M potentiometer which can offer
    resistances from near 0 all the way up to its 1M spec. Is this correct?

    I want to use a potentiometer in the RC circuit input of a monostable
    multivibrator in order to adjust the output pulse length. The pulse
    length will need to be somewhere between 4 and 8 seconds, and I want
    fine control over that whole range. I was considering the use of a
    trimmer pot, mostly just because they are so small - the final device
    will have 7 of these, so space is an issue. This time constant will
    only be set once for each multivibrator, so the durability of trimmer
    pots isn't a problem. However, if the resistance range of a typical
    trimmer pot is only 10%, though, then I don't see how I could obtain
    a time constant over such a wide range. Are there variable resistance
    devices available with a wider resistance range, and big enough values
    to obtain time constants in the seconds range, but with the small footprint
    of a trimmer pot?
     
  2. John  Larkin

    John Larkin Guest

    Never seen such. Trimpots usually span their full spec'd range.

    John
     
  3. Andrew Holme

    Andrew Holme Guest

    No. Where did you get that idea from? Trimmer pots go from zero to max.
    resistance like any other pot. If you've seen one specified with a 10%
    tolerance, that relates to the max. resistance not the adjustment range.
    All resistors, fixed and variable, are manufactured to a specified
    tolerance.
     
  4. John Fields

    John Fields Guest

    ---
    No, a trimpot is the same as a regular pot, but smaller and
    sometimes with a greater mechanical range of adjustment, say ten or
    twenty turns instead of less than one for a regular pot. What
    you're thinking about is the total resistance a 1 megohm trimpot
    with a tolerance of +/-10%.
     
  5. ]> I've never actually designed a circuit that used a trimmer pot, and
    ]> I'm
    ]> a bit ignorant of their basic properties, and I do not have any
    ]> samples
    ]> on hand I could just hook up to my DMM. From the way their specs are
    ]> specified, it sounds like they really live up to their name: the range
    ]> of resistance they offer is very narrow. For instance, a 1M trimmer
    ]> pot would offer resistances from about .9M to 1.1M over its range of
    ]> adjustment. Contrast this with a regular 1M potentiometer which can
    ]> offer resistances from near 0 all the way up to its 1M spec. Is this
    ]> correct?
    ]
    ]No. Where did you get that idea from? Trimmer pots go from zero to max.
    ]resistance like any other pot. If you've seen one specified with a 10%
    ]tolerance, that relates to the max. resistance not the adjustment range.
    ]All resistors, fixed and variable, are manufactured to a specified
    ]tolerance.

    As I said, ignorance ;-) I was led down this incorrect path by the name
    "trim pot" combined with some ambigous wording in a data sheet I
    downloaded - it could have meant tolerance across samples or a small
    resistance range in each part. I am glad to be informed that it is the
    former.
     
  6. No, but a 100k trimmer on a 1M resistor will give you 1M to 1.1M.

    Tom
     
  7. Jasen Betts

    Jasen Betts Guest

    trimmer pots are just like regular pots only without the spindle for
    mounting a knob. they are used to save space and to save money where
    adjustment is a once only or infrequent thing.
    I think that 0.9 to 1.1 you see in the specs is the variation in total range
    of the pots that come from that manufacturer.

    IE you order a 1.0M pot and you might really get a 900k or a 1.1M
    but the pots will be adjustable from somewhere near 1m right down to 0
    7 RC ooscilators - are you tring to generate DTMF signals... there's cheap
    chips built just for that...
    if it's not DTMF something needing 7 timers looks like a good task for a
    small mictorontroller and you get a crystal reference for your timebase
    which should make it more reliable in harsh environments etc...

    Bye.
    Jasen
     
  8. Bob Myers

    Bob Myers Guest

    Or a 200k trimmer on the 900k resistor would give you the originally-
    mentioned 0.9 - 1.1M range, of course.

    Bob M.
     
  9. ehsjr

    ehsjr Guest

    But he wants an RC of anywhere from 4 to 8 seconds:
    "I want to use a potentiometer in the RC circuit input of a monostable
    multivibrator in order to adjust the output pulse length. The pulse
    length will need to be somewhere between 4 and 8 seconds, and I want
    fine control over that whole range. "

    A series 900K resistor and 200K trimmer won't do it.

    Ed
     
  10. John Fields

    John Fields Guest

     
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