# negative resistance and constant current source

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

1. ### Abstract DissonanceGuest

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

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. ### Rene TschaggelarGuest

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 LarkinGuest

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 ForemanGuest

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 ThompsonGuest

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/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

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

6. ### JoergGuest

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 LarkinGuest

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. ### JoergGuest

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
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

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 LarkinGuest

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. ### JoergGuest

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
the lacquer and you had a photo sensor. But I build more with tubes.
They had better oomph and were usually free.

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. ### Ancient_HackerGuest

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!