My NE-2 Relaxation Oscillator is Too Relaxed ???

[[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 flashing faster than the eye can
distinguish. I conneded the circuit to an oscilliscope and when the
bulfb is visually flashing 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

 

[Andrew Holme]

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 flashing faster than the eye can
distinguish. I conneded the circuit to an oscilliscope and when the
bulfb is visually flashing 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

Try a smaller capacitor e.g. 0.47uF

 

[Bob Eld]

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 flashing faster than the eye can
distinguish. I conneded the circuit to an oscilliscope and when the
bulfb is visually flashing 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.

 

[Bob Eld]

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.

 

[Phil Allison]

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?

** The neon's conduction current must be able to reduce the voltage on the
cap to below the neon's extinguishing voltage in order for there to be
oscillations.

If the series resistor is too low in value - this will not happen.

So use a small cap - like 10nF and try again.



....... Phil

 

[petrus bitbyter]

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 flashing faster than the eye can
distinguish. I conneded the circuit to an oscilliscope and when the
bulfb is visually flashing 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

 

[neon]

it takes 90v to break it down. keep that in mind

 

[Mark Zenier]

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 flashing faster than the eye can
distinguish. I conneded the circuit to an oscilliscope and when the
bulfb is visually flashing 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

Digging out my GE Glow Lamp Manual, for a 5AB (the tight spec.
equivalent to an NE-2) the resistance range for oscillators runs
from .5 meg up to 15 meg. To get 100 Hz, you need to run with
a .01 uF capacitor and 1 to 5 megohm, depending on voltage
(100-150 volts). The maximum frequency, with a capacitor in the
10-30 pf range, is 3 to 30 kilohertz.

Mark Zenier [email protected]
Googleproofaddress(account:mzenier provider:eskimo domain:com)

 

[Rich Grise]

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.

I also vote "smaller cap", but I'd add to this: There should be a 220K or
so (I wouldn't go below about 180K) in series with the pot so that when
you turn the pot down, you don't burn out the bulb. I guess you'd size the
R so that with your supply you'd want to limit the neon current to the
~1-2 mA range.

Have Fun!
Rich