On 7/3/08 7:53 AM, in article 8spp64hncghn88d0gqbensnerupo39h4k0@[EMAIL PROTECTED]
"John Fields" <jfields@[EMAIL PROTECTED]
> wrote:
> On Wed, 02 Jul 2008 22:44:53 -0700, Don Bowey <dbowey@[EMAIL PROTECTED]
>
> wrote:
>
>> 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?
>
> ---
> news:lnpp64t1q9ohk1d1ar57dhd673pmaif3g6@[EMAIL PROTECTED]
>
> JF
Thanks.
I had already thought more about the neon lamp and decided it could have,
or
appear to have, a negative resistance region, so I did some google work
and
found several trustworthy articles sup****ting it. I also found one
credible
article saying the effect is not negative resistance, but...... If it
quacks
like a duck it's probably a duck.
IMHO the typical neon tube relaxation "oscillator" does not use that
effect,
however, but uses the gross cap discharge to begin a next charge cycle.
The
external R must be high enough to allow the voltage across the tube to
fall
far enough to extinguish the lamp.
This will be a good slow-day plaything. I think I'll put a parallel
resonant circuit in series with the lamp and variable voltage
source.......
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