"Don Bowey" <dbowey@[EMAIL PROTECTED]
> wrote in message
news:C4916A23.BD130%dbowey@[EMAIL PROTECTED]
> On 7/2/08 4:14 PM, in article XXTak.3268$vn7.2496@[EMAIL PROTECTED]
"Bob
> Eld" <nsmontassoc@[EMAIL PROTECTED]
> wrote:
>
> >
> > <jalbers@[EMAIL PROTECTED]
> wrote in message
> >
news:2f8acf53-1cda-424c-9073-6706a1eb55ac@[EMAIL PROTECTED]
> >> I am experimenting with a relaxation oscillator circuit consisting of
> >> a NE-2 bulb wired parallel to a capacitor and this pair is wired in
> >> series with a resistor and connected across a 150 V DC power source
(a
> >> bunch of DC wallwarts connected in series). The resistor is variable
> >> 0-1 Meg Ohm, the capacitor is a 1uF electrolytic rated at 160V. I
> >> don't have any capacitors on hand with a higher working voltage.
> >>
> >> The circuit seems to work. I can get the bulb to blink around 3
times
> >> a second but I am wanting a higher flash rate and I am not getting
> >> it. Lowering the resistance makes the bulb turn on continuously. I
> >> don't think that the bulb is fla****ng faster than the eye can
> >> distinguish. I conneded the circuit to an oscilliscope and when the
> >> bulfb is visually fla****ng I see the RC discharge curve but lowering
R
> >> until the bulf truns on continuously pretty much produces a flat line
> >> on the scope.
> >>
> >> I was expecting to maybe be able to get around 2-100 hz with a NE-2
> >> relaxation oscillator. Is this possible or am I expecting too much
> >> from this type of circuit? And if so, why?
> >>
> >> Any help would be greatly appreciated. Thanks
> >
> > As mentioned above, use a much smaller capacitor. Also, never allow
the
> > resistance to go near zero ohms as this can burn out the NE-2.
> >
> > The Neon lamp is a negative resistance device. To oscillate it has to
stay
> > in the negative resistance region. The pot is a positive or normal
resistor.
> > If it's value is adjusted too low, the overall combination resistance
> > becomes positive, oscillation stops and the lamp turns full on. That's
what
> > you are observing. Keep the resistor value high and the capacitor
small.
> >
> >
>
> Actually, it's a relaxation oscillator. The lamp is off when the
voltage
> across the cap is less than the firing voltage (about 67 Volts for a
NE2).
> When the cap charges to around 67V, the neon ignites and the voltage
across
> the cap falls from the low resistance "short" and the cycle of
> charge-fire-charge-fire continues, creating the typical sawtooth
waveform.
That's the definition of negative resistance. When the voltage is high the
current is low or off and when the voltage is low, the current is high.
That's the opposite of a normal resistor where current goes up with
voltage.
Relaxation cannot occur without negative resistance.
BTW it's possible to make a similar device with two transistors that will
work on a few volts.
Another ancient device that exhibits this characteristic is a tunnel
diode.
GHz oscillators can be made with them.
Look up unijunction trasistor and diac, other devices that exhibit this
characteristic.
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