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Re: [Electronics_101] High resistance materials

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  • Roy J. Tellason
    ... Have you ever looked at some really high-powered potentiometers or rheostats and seen how they were constructed? Basically there s nichrome wire (or
    Message 1 of 7 , Mar 1 3:28 PM
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      On Wednesday 01 March 2006 06:09 pm, anarchestra wrote:
      > HI,
      >
      > This may be stupid. I'm fairly ignorant about all this.
      >
      > What I'm hoping to do is replace 100k or 250k or 500k pots (in musical
      > devices) with very long (4-6 foot) faders. The idea is to make knob
      > turning more of a physical act.
      >
      > Is this doable? How would I find out what resistance material to use?
      > Durability would be important, I don't need to save weight.
      >
      > Any suggestions?

      Have you ever looked at some really high-powered potentiometers or "rheostats"
      and seen how they were constructed? Basically there's nichrome wire (or
      similar resistance wire) wound around a form and you change the resistance
      overall by the size of the wire and the profile of the form, the bigger the
      form the more wire for each turn. You change the resistance you're tapping
      off by moving a sliding contact along the form (in either a linear or a
      rotary fashion) until you get to where you want to be.

      I used to have a set of load resistors that were like that, these suckers
      went most of the way across a 19" rack panel that consisted of perforated
      metal, were (apparently) 10 ohm resistors that had been tapped back to 8
      ohms, and from what I could find on them I could've probably connected them
      in series across the power line and cooked on top of that rack without too
      much in the way of problems. At times I used to test amplifiers putting out
      300 Watts per channel, 2 channels, into 4 ohms, which got that rack pretty
      warm anyway.

      Given all of that, to get the kind of resistance values you're talking about
      here you're going to need some very fine wire, and you're going to need a
      lot of it, meaning many turns, which means this is going to be *real*
      tedious to make. What I'd do instead is to come up with some sort of
      physical actuator, say a nice lever or whatever (thinking here of the
      lighting board I saw in my high school once), and then couple that to the
      pots being moved, but also some stuff to take up some of the physical force
      that's being exerted and give the desired effect. I'll probably turn out to
      be easier in the long run.

      --
      Member of the toughest, meanest, deadliest, most unrelenting -- and
      ablest -- form of life in this section of space, a critter that can
      be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
      -
      Information is more dangerous than cannon to a society ruled by lies. --James
      M Dakin
    • Stefan Trethan
      ... Great idea! Alternatively to a chain and sprocket a string and pullies would probably do (like the mechanism in an old radio moving the pointer). I was
      Message 2 of 7 , Mar 1 3:33 PM
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        On Thu, 02 Mar 2006 00:23:04 +0100, rtstofer <rstofer@...> wrote:

        > So, do it mechanically. The slide moves a chain that turns a couple
        >
        > of sprockets. One of the sprockets turns a multi-turn pot. Try to
        >
        > keep the ratio of slide length : sprocket diameters such that it won't
        >
        > overrun a 10 turn pot.
        >
        >
        > There are some very nice, high precision, 10 turn pots.
        >
        >
        > Richard


        Great idea!
        Alternatively to a chain and sprocket a string and pullies would probably
        do (like the mechanism in an old radio moving the pointer).

        I was thinking of putting a huge lever on a normal pot, but didn't think
        of other mechanical possibilities.

        ST
      • anarchestra
        ... Thanks. I didn t know there were 10 turn pots. I tried belts and pulleys but the reduction was too much and very sloppy, with a multi-turn pot a rack and
        Message 3 of 7 , Mar 1 4:16 PM
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          --- In Electronics_101@yahoogroups.com, "Stefan Trethan" <stefan_trethan@...>
          wrote:
          >
          > On Thu, 02 Mar 2006 00:23:04 +0100, rtstofer <rstofer@...> wrote:
          >
          > > So, do it mechanically. The slide moves a chain that turns a couple
          > >
          > > of sprockets. One of the sprockets turns a multi-turn pot. Try to
          > >
          > > keep the ratio of slide length : sprocket diameters such that it won't
          > >
          > > overrun a 10 turn pot.
          > >
          > >
          > > There are some very nice, high precision, 10 turn pots.
          > >
          > >
          > > Richard
          >
          >


          Thanks.

          I didn't know there were 10 turn pots. I tried belts and pulleys but the reduction was
          too much and very sloppy, with a multi-turn pot a rack and pinion arrangement
          would work.

          I'm a better mechanic than electronicist anyway.

          Thanks again,

          Alex.





          > Great idea!
          > Alternatively to a chain and sprocket a string and pullies would probably
          > do (like the mechanism in an old radio moving the pointer).
          >
          > I was thinking of putting a huge lever on a normal pot, but didn't think
          > of other mechanical possibilities.
          >
          > ST
          >
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