On 7/2/08 7:06 PM, in article zuWak.31133$ZE5.5572@[EMAIL PROTECTED]
"Bob Eld" <nsmontassoc@[EMAIL PROTECTED]
> wrote:
>
> "Don Bowey" <dbowey@[EMAIL PROTECTED]
> wrote in message
> news:C4916A23.BD130%dbowey@[EMAIL PROTECTED]
>> On 7/2/08 4:14 PM, in article XXTak.3268$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.
>
>
Perhaps you could post a link that plots the negative resistance region
of
a neon lamp?
|
