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negative resistance and constant current source

Discussion in 'Electronic Design' started by Abstract Dissonance, Jan 29, 2006.

  1. I messing around reviewing the basics and I came up with some idea about
    using a voltage divider to run a load and negative resistance to cancel out
    the loading effects... the idea follows

    R1
    |
    +----+
    | |
    R2 RL
    | |
    | |
    G G


    If, say, we let R2 = -R1*RL/(R1+RL) then the total resistance is 0... we
    could then put in a series resistance with R1, say, Rs that fixes the
    current as V/Rs.

    I'm curious as to how pratical this is? Is it possible to "sense" the
    resistance RL and to have some device that can make negative resistance
    following that equation for R2? I don't know much about negative resistance
    except that tunnel diodes exhibit it for certain ranges and stuff.


    Now that I think about it you don't even need to use a voltage divider ;)
    just having R2 in series with RL and R2 = -RL would do the same.

    Any ciruits that use this "principle"?

    Thanks,
    Jon
     
  2. Sure, any electronics book has a negative resistor,
    usually with an operational amplifier.
    Limitations are the bandwidth and the usual
    amplifier limitations.

    Rene
     
  3. John  Larkin

    John Larkin Guest

    Sure. This can be done to improve at-load power supply regulation, to
    compensate for wire losses, although 4-wire remote sensing is better,
    because it doesn't need to know the wire resistance.

    Linear oscillators use negative resistance to cancel real resistive
    losses.

    Interestingly, connecting a negative resistor in parallel with a
    platinum RTD makes the net resistance linear on temperature. For a 100
    ohm RTD, the value is - don't quote me, it's been a while - something
    like -2.5K.

    Simple hysteretic buck switching regulators often have a negative
    output resistance... I've never figured out why.

    You can build a 2-terminal 1K negative resistor with a couple
    batteries, an opamp, and three 1K resistors. Then you can build fun
    things: voltage dividers with gain, interesting R-C circuits,
    oscillators.

    John
     
  4. Don Foreman

    Don Foreman Guest

    Some DC motor control circuits use this principle to cancel the
    armature resistance, thus providing constant speed with varying load
    for given supply voltage. I think some model train supplies work
    this way so the trains don't slow down when going up hills. Some
    Minarik controllers also use this principle.
     
  5. Jim Thompson

    Jim Thompson Guest

    Sort of.

    You insert a resistor in series with motor (in ground lead) equal to
    armature resistance.

    Measure this voltage and add TWICE this value to control voltage for
    the amount applied to motor.

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    "Don't mess with my toot toot!", Antoine 'Fats' Domino
     
  6. Joerg

    Joerg Guest

    Hello Jon,

    Since you mentioned tunnel diodes: These were in almost any electronics
    project book when I was young. It all looked so deceptively easy. What
    the authors did not tell their fellow readers was that it was next to
    impossible to obtain any. At least not unless you name was Rockefeller.

    Regards, Joerg
     
  7. John  Larkin

    John Larkin Guest


    Some of the GE germanium TDs used to cost around $1.50, as I recall. I
    did a lot of TD circuits when I was a kid... bought them from Allied.
    My senior paper at Tulane was "The Tunnel Diode Slideback Sampling
    Oscilloscope."

    I think that GPD still makes fast - 20 ps - TDs, but the last time I
    bought one, some years ago, they were about $80 each. They acquired
    the old GE mesh-etch process, truly astounding.

    John
     
  8. Joerg

    Joerg Guest

    Hello John,
    I lived in Europe in those days. Maybe the project books consisted in
    part of schematics from the US. I had tried to buy tunnel diodes a few
    times and the electronics stores always had the same answer. Too
    expensive, nobody would buy them so they didn't stock them. There was no
    such thing as Digikey back then and a special order would have required
    me to ask dad for a substantial loan :-(

    BTW it was the same with unijunction transistors. Lots of project ideas
    published but the things were unobtanium. Oh well, that way we all
    learned how to design with what's there, no with what's the rage.

    Regards, Joerg
     
  9. John Larkin

    John Larkin Guest

    I wonder if the post-war boom in electronics in the USA was partly
    driven by the ability of kids to get parts. I used to buy surplus
    radars and stuff from Fair Radio Sales, for pennies per pound, and I
    had an infinite supply of old tube-type TV sets to scavenge. I think
    it was deliberate government policy to dump all that WWII electronics
    on the surplus market.

    When semiconductors arrived, you could buy a CK722 or a 2N107 and more
    exotic stuff from Allied.

    The Soviets probably damaged themselves by being so restrictive about
    ham radio. It's best to learn anything, tennis or dancing or
    electronics, when you're young.

    John
     
  10. Joerg

    Joerg Guest

    Hello John,
    Europe wasn't too bad either. WWII surplus was available from both
    sides, mostly at hamfests. Old Ge transistors (OC series) could be
    scavenged from discarded hearing aid amplifiers or radios. Scratch off
    the lacquer and you had a photo sensor. But I build more with tubes.
    They had better oomph and were usually free.

    They had ham radio but they had to be careful what they said. Big
    brother was omnipotent. Their skills might have even been honed better
    than in the west since they really had to live on scarce supplies.
    Everybody over there must have known how to create things like a stable
    voltage without ever dreaming of laying their hands on a zener diode.
    The purchase of something like the uA723 was unthinkable.

    Regards, Joerg
     
  11. Oy, my aching wallet! Years ago around our area you could get old
    CDC 3200 computer cards for 5 cents each. Each card had about 6 TO-18
    transistors, a few 1/8 watt resistors, and often a tunnel diode or two.
    Back then we'd use to salvage the "useful" components, like the 2N706
    transistors, and toss away everything else. Wish I'd saved the tunnel
    diodes!
     
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