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How to give specific range of resistances?

Discussion in 'Electronic Design' started by Michael, May 6, 2007.

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  1. Michael

    Michael Guest

    Hi there - I need some odd valued potentiometers. I have two different
    setups I need:

    one that ranges from 178.2 - 207.1 ohms

    and

    one that ranges from 96.3 to 168.6 ohms

    So - my first inclination was to just use a single resistor for the
    smaller value and a potentiometer to cover the range. So for the first
    I'd need a 178.2 ohm resistor, and a 28.9 ohm pot. However - 28.9 ohm
    pots seem to be slightly less common than I would like. Same goes for
    the second setup and the desired 72.3 ohm pot. Now - I can fudge
    around on these values a bit - but not too much. These values are
    setting the output voltage of a light, and if that voltage goes too
    high I suspect the magic smoke of the device will be released.

    So - any suggestions? I don't need very linear operation from the
    potentiometer, if that matters. I don't think many pots have
    mechanical stops to stop you from setting the value too high. What do
    I do?

    Thanks!

    -Michael
     
  2. John Larkin

    John Larkin Guest


    Why do you need such precise resistances? What does turning the pot
    control?

    You can do a series-parallel pad on a pot to force the limits, but
    it's mathematically a pain.

    John
     
  3. Charles

    Charles Guest

    Not sure I understand your question. Is a rheostat (2-terminal adjustable
    resistance) OK? If so, find delta R and then pad a higher variable device
    to that value using the familiar Req equation. Then, add a series resistor
    to establish Rmin.
     
  4. Michael

    Michael Guest

    It doesn't need that level of precision - but it needs to be fairly
    close (+-10%).

    What is this series parallel pad you're referring to? Would that just
    be putting a resistor in parallel with the pot?

    -Michael
     
  5. Michael

    Michael Guest

    Can you give an example of the schematic that you're describing? (and
    how to find the values for the resistors)I guess I don't entirely
    follow what you're describing.

    Thanks,

    -Michael
     
  6. Michael

    Michael Guest

    Err, is what you're saying that you'd just have a single resistor in
    series = rmin, then a resistor in series with a pot whose max parallel
    resistance would give rmax - rmin? Are there any better ways?

    Thanks,

    -Michael
     
  7. Charles

    Charles Guest

    Close. The pad is in parallel with the rheostat and decreases its Rmax to
    delta R.

    Better? Hard to tell as I am still not sure what you would like to
    accomplish.
     
  8. Charles

    Charles Guest

    50 ohm rheostat shunted by a 69 ohm resistor gives an Rmax of 29 ohms. Rmin
    is obvious.
    Add a series 178 ohm resistor and you'll have 178 to 207!
     
  9. John Larkin

    John Larkin Guest



    ------+--------r1------------+--------
    | |
    | |
    | |
    +-----r2----pot--------+


    As I said, mathematically a nuisance. If it's an opamp circuit, there
    are usually cleaner ways to do this.

    John
     

  10. o
    |
    |
    +----+---+
    | | |
    .-. .-. |
    Rp | | | |<-+
    | | | |
    '-' '-' Pot varies from Rh ~ Rl (measured)
    | | with wiper tied to element as shown.
    +----
    |
    .-.
    | |
    Rs | |
    '-'
    |
    |
    o


    Say you want a resistance that varies from Rmin to Rmax, and you have
    a pot (of greater nominal value than Rmax - Rmin, of course) that
    varies from Rh to Rl.

    You put a resistor in parallel Rp = 1/(1/(Rmax - Rmin) - 1/(Rh- Rl))

    And a resistor in series of

    Rs = Rmin - 1/(1/Rl + 1/Rp) ~= Rmin - Rl ~= Rmin

    Eg, you have a pot that varies from 3 to 207 ohms and you want to go
    from 70 ohms to 170 ohms.

    Rp = 1/(1/100 - 1/204) ~= 196 ohms

    Rs = 70 - 2.95 ~= 67.0 ohms

    The resulting rheostat will be somewhat nonlinear depending on how
    much you shunt down the element value. Don't use a higher pot value
    than necessary.

    You can use nominal rather than measured values for the pot, and
    assume Rl = 0, if you want to simplify things, but typical tolerance
    on pot elements is fairly wide, and minimum resistance may be
    significant if you're going for values in the hundreds of ohms.


    Best regards,
    Spehro Pefhany
     
  11. whit3rd

    whit3rd Guest

    The range is the specification of a two-terminal device (like
    a potentiometer with wiper connected to one fixed terminal),
    i.e. a rheostat.

    Consider a parallel combination of fixed resistor R1 and
    (series-combination R2 and Rvariable);

    R1//R2 = 178.2 Ohms,
    R1//(R2 + Rvariable) = 207.1 Ohms

    is a pair of equations in two unknowns (R1 and R2) and
    Rvariable :== 100 Ohms is a free chosen quantity.
    That freedom is required because pots aren't available in
    odd values.

    Solve for R1 and R2 and if they aren't negative, that's
    a good solution. Otherwise, go to the next standard Rvariable
    value.

    As others have noted, protection against overvoltage/overcurrent
    usually uses other techniques- a current limit could be
    a LM317 with program resistor, or a voltage limit could be
    a TL431 with program potentiometer. Rheostats are kinda...
    old-school. Sometimes that's good.
     
  12. Tom Bruhns

    Tom Bruhns Guest

    As an alternative to adding a shunt (parallel) resistor, just put
    mechanical stops on the pot.

    If your tolerances are 10%, it's a bit silly to be giving resistor
    values to 0.1% --

    One might suspect there's a better way to do things. Can you tell us
    more about the application?

    One thing to beware of with potentiometers is that the wipers
    sometimes fail to make good contact with the element. In that case,
    the resistance goes to "infinity" or at least some large value. It
    can be worthwhile when you want a rheostat function (a variable single
    resistance) and you have a potentiometer (a resistance with a variable
    tap point) to tie the wiper and one end together. Then the resistance
    will normally only jump to the end-to-end value if the wiper fails to
    make contact properly.

    Beware about DC current through the wiper. Some pots handle that
    gracefully, and others do not. Wire-wound pots are probably OK.

    Cheers,
    Tom
     
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