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Any cheap small log potmeters <50 ohms?

Discussion in 'Electronic Components' started by Joerg, Jan 24, 2006.

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  1. Joerg

    Joerg Guest

    Hello Folks,

    Are there small and cheap potmeters 50 ohms and lower that are log or
    audio-taper?

    Reason I ask is that I want to make an adjustable current source out of
    just two parts, an LM334 and that potmeter. Well, plus maybe a limit
    resistor for the upper current range (or a mechanical peg on the pot).
    The current shall be adjustable from about 1.5mA to 8mA. No big deal but
    with a linear potmeter the upper part of that range is being squished
    into the last 20% or so of that potmeter, making it kind of fickle to
    adjust. The current setting of an LM334 is proportional to 1/R.

    I am looking for one of those cheap transistor radio type potmeters. The
    alternative would be a more elaborate voltage control circuit but it's
    larger and also I am limited to very small voltage drops. Can't waste
    many battery volts in overhead here.

    Regards, Joerg
     
  2. Pooh Bear

    Pooh Bear Guest

    The lowest value audio taper I've ever seen is 5k.

    Graahm
     
  3. Phil Allison

    Phil Allison Guest

    "Joerg"
    ** No.


    ** To operate in the normal clockwise direction - you need a reverse log
    pot.

    For fine adjustment a 100 ohm, 10 turn WW pot will do.

    Or just use a 12 way switch and some fixed resistors.




    ........... Phil
     
  4. Joerg

    Joerg Guest

    Hello Graham,
    Well, that's where the market is, unfortunately. The lowest log potmeter
    I have used was 1K and that was a rather large hi-rel version.

    Regards, Joerg
     
  5. Joerg

    Joerg Guest

    Hello Phil,
    That would not be an issue here. It'll be thumbwheel and just be mounted
    the other way around.

    Both way too expensive and too large.

    Regards, Joerg
     
  6. JeffM

    JeffM Guest

  7. Joerg

    Joerg Guest

  8. Sjouke Burry

    Sjouke Burry Guest

    If you let the moving part of the pot carry that kind of
    current,it will not live very long.(or work very well).
     
  9. Phil Hobbs

    Phil Hobbs Guest

    Yeah, the only problem in this case is that you want the resistance vs
    shaft angle curve to be concave upward, which means you have to use a
    negative resistance in parallel.

    OTOH, you can do this, e.g. you can linearize a platinum RTD
    *beautifully* with a little positive feedback. Not much use to Joerg,
    but oh well.

    Cheers,

    Phil Hobbs
     
  10. Idon't know about 'cheap', but the Bourn's 81/82 ranges are available in
    log taper, down to 50R value, as are the 85/86 range units, but I'm fairly
    sure these are more expensive.

    Best Wishes
     
  11. In the old days, CTS would make whatever we wanted. Just pop in and
    have a word with their engineers, place a reasonable minimum order and
    the stuff would be ready in a few weeks. You can probably still do
    that- but not in North America.


    Best regards,
    Spehro Pefhany
     

  12. --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  13. Joerg

    Joerg Guest

    Hello Roger,

    I am still waiting for a call back from Bourns Engineering. Hopes aren't
    high though. I remember 50ohm log taper pots for RF attenuators and
    still have one here. But these were rather huge. Also, with the advent
    of chips like the AD603 their market might have shrunk too much by now.

    Regards, Joerg
     
  14. Joerg

    Joerg Guest

    Hello Spehro,
    Haven't found anything under 100ohms at CTS. Long term if this goes in
    series production it will be a custom pot. Probably one of those dual
    paste screening deals like we use with hybrids.

    Regards, Joerg
     
  15. Joerg

    Joerg Guest

    Hello Sjouke,


    Yes. It depends on the type of wiper. In this app it's not going to need
    much precision and would be somewhat of a disposable.

    Regards, Joerg
     
  16. John Fields

    John Fields Guest

    ---
    Can you use a pot with a resistor in parallel with the wiper and one
    of the ends of the pot?:


    +-----+
    | |
    [R] +----->ROUT
    | |
    +-->[POT]
    |
    +----->ROUT
     
  17. Joerg

    Joerg Guest

    Hello John,
    Tried it but with our range it doesn't work. The problem with such
    adjustable current sense resistors is that the resolution for higher
    currents is poor. It all piles into one corner of the pot.

    It looks like a custom potmeter or long term a little ASIC.

    Regards, Joerg
     
  18. Perhaps a cheap low voltage sot-23 opamp will do, howland current source,
    allows you to use any pot you like. LMV321, 21 cents a piece.
     
  19. John Fields

    John Fields Guest

    ---
    I understand.

    What would you like the the "open" resistance of the pot to be and,
    say, its resistance when the shaft was at its halfway point around
    the arc?
     
  20. Joerg

    Joerg Guest

    Hello John,
    Hmm, one of those cases where I couldn't see my own reply. Guess the
    news server swallows some posts again.

    Ideally full open would be 45ohms and minimum 9ohms. But that's not
    critical, I could use a mechanical peg. A halfway point around 15ohms
    would be nice. Or less, since the high current values will still
    continue to cram themselves into the corner from there.

    When using a series resistor of 9ohms to cap the max current the halfway
    point would need to be well below 10ohms.

    Regards, Joerg
     
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