[Elecraft] Negative Resistance????

[Don Brown]

Hi All

Many of the older Textronix (465, 475, etc) scopes have tunnel diodes in =
the trigger circuits. But the reason a meter will read a negative =
resistance has to do with the way DVM's read resistance. A DVM forces a =
constant current across the component under test and reads the voltage =
drop with the voltmeter. If the circuit is powered or there is a charge =
on a capacitor then the voltage may overwhelm the constant current and =
cause the meter to read a negative resistance (voltage). This is common =
with in-circuit measurements. Try reversing the leads and see if the =
reading is better or change to a lower range. Remember the resistance =
measurements are ball park only. The voltage readings are more useful.

Don Brown
KD5NDB


----- Original Message -----=20
  From: Ron D'Eau Claire=20
  To: 'Lee Buller' ; 'Elecraft Reflector'=20
  Sent: Thursday, July 17, 2003 5:15 PM
  Subject: RE: [Elecraft] Negative Resistance????


  Negative resistance is a very common phenomena in electronics. Someone
  mentioned "tunnel diodes" which use negative resistance to oscillate =
in the
  microwave range. But even much more mundane devices such as a common =
"neon"
  bulb exhibit negative resistance.=20

  First, let's define negative resistance starting with er.... =
"positive"
  resistance. In normal of "positive" resistance, the current varies =
directly
  with the voltage applied to the resistor according to ohms law. You =
increase
  the voltage and the current increases. You decrease the voltage and =
the
  current decreases. It's a linear function.=20

  "Negative" resistance is a situation in which when you reduce the =
voltage
  the current goes up (and vice versa). Take a common neon bulb - a =
glass
  envelope with two electrodes that come out to leads. The envelope is =
filled
  with neon gas.=20

  As you increase the voltage across it, the current increases very =
slightly
  until the "ignition" voltage of the bulb is reached. At ignition, the
  current jumps up very fast, even if the voltage stays exactly the =
same! That
  is, it exhibits "negative resistance". Even if you reduce the voltage,
  within limits, the current increases just the same. That's because =
once the
  neon ionizes (and starts producing light) it's resistance drops
  dramatically.=20

  A common oscillator circuit used for many years produced a nice =
sawtooth
  wave using just such a bulb. You hook a capacitor across the bulb. One =
end
  of the bulb/capacitor goes to the negative terminal of a battery with =
enough
  voltage to light the bulb (usually 40 to 60 vdc). The other =
bulb/capacitor
  terminal goes through a BIG resistor (100 k or so typically, depending =
upon
  the size of the capacitor) to the + side of the battery. The bulb is =
an open
  circuit at first. The battery charges up the capacitor through the =
resistor
  until the ignition voltage of the bulb is reached.=20

  WHANG! The bulb lights and starts drawing a LOT of current, =
discharging the
  capacitor. The resistor in series with the battery is too large for it =
to
  supply enough current, so the voltage across the capacitor starts =
dropping.
  But because the bulb, once lit stays lit about 50 volts typically =
stays lit
  until the voltage across it drops to about 20 volts, the current keeps
  flowing even though the voltage is dropping (there's the "negative
  resistance" effect). Finally, the voltage drops too low and the neon
  extinguishes, stopping the heavy current drain and the battery starts
  recharging the capacitor again.

  More exotic circuits using tubes produce oscillations with "negative"
  resistance. One of those was the "klystron" oscillator - a key element =
in
  the early Radar sets from the WWII period. In those cases the =
"negative
  resistance" effect comes from the manipulation of clouds of electrons =
in the
  spaces between the elements of a tube, much in the same way a tunnel =
diode
  achieves it manipulating carriers in a solid state material.=20

  Ron AC7AC
  K2 # 1289


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