<jalbers@[EMAIL PROTECTED]
> schreef in bericht
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
>
You'd first of all read the specifications of the NE-2. Like a lot of neon
bulbs it ignites at about 90V. It is the rating for DC but that's half the
truth. It will ignite when the voltage accross its leads reaches that 90V.
That's where the 65Vac comes from. A 65Vac RMS sine will peak at about
90V.
Once the bulb has ignited, the voltage accross its leads sinks to about
60V,
regardless of the current. So you have to make a provision to keep the
average current at or below 0.5mA where it is rated for. More current will
reduce the lifetime of the bulb or may even destroy it. The bulb will
extinguish when the voltage across its leads sinks below 60V. You may not
find this 60V in the specifications as this value varies. Most of the
times
it is not specified at all.
So the mecanism of a relaxation oscillator will be clear. The capacitor is
charged via the resistor. Once the voltage reaches 90V the bulb ignites
and
discharges the capacitor until the voltages sinks to about 60V. Then the
bulb extinguishes and the capacitor will start to charge again.
But... once the bulb has ignited, the current through the resistor adds up
with the discharge current of the capacitor. If that "resistor" current is
enough to keep the bulb going, it will not extinguish. So you cannot lower
the resistor too much as it controlls the current.
The frequency of your oscillator depends on the resistor and the capacitor
value. They determine the time required to charge the capacitor from 60V
to
90V. Once the bulb ignites, discharging goes fast. This time can be
neglected with respect to the charging time.
So once you reaches the lowest value of the resistor, you can only lower
the
capacitor value to raise the frequency. Using the right components,
frequencies of several kHz should be possible.
petrus bitbyter
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