Transistor not being turned on / closed by 555 timer

[Keith42]

Hi, i am trying to build a power supply for a GE ne-42 neon lamp. All it really needs to operate is about 60v, however, only the negative electrode of the lamp glows, so i am trying to generate some AC power from AA batteries so that both electrodes will glow.

I tried to do this using a 555-timer circuit, two transistors, and a center tapped transformer. The timer turns on/off the transistors at around 60hz, the transistors being opposites (NPN vs PNP) so that when one is on, the other is off. The center line from the transformer is attached to positive power terminal, and then each of the two end lines of the transformer run to one of the transistors.
My power source is 8 AA batteries, or about 12v at maximum.
My problem is that the NPN transistor never seems to allow power to flow. The PNP one works just fine, however with only one of them ever passing power, my transformer generates pulsed DC power instead of AC. As such, only one terminal of the lamp is lighting. I have tried the PNP transistor in both its spot and the spot where the NPN normally resides, it works in both, so that is not the problem.
On both transistors, the base is connected to the output of the 555, the collector to one of the end wires of the transformer, and the emitter to ground.
Using the diagram Here i have:
Resistor one: 1 kOhm
Resistor two: 100 kOhm
Capacitor: 0.1 microFarad
The two transistors i got from radio shack are This and This.
As best i can tell, the timer circuit works correctly. If i double resister two to take it down to 30 hz, the lamp visually flickers.

This confounded NPN transistor has me hung up though. Ive already replaced it once, no change. I read that you need a minimum 0.7 volt differential between base and emitter for the transistor to close, if i hold multimeter to those pins it reads about 0.55 volts. I don't know if that is really worth anything since the base pin's power is being switched on and off at 60ish hz. If i connect the NPN's base directly to +v it does close and let power flow.



Now, i really have minimal idea what i am doing here. Ive already exploded one battery when i didn't put any resistor in line with the transformer's ground (100 ohm there now). I hadn't the foggiest idea what a transistor was until i wiki'd it, and my understanding is still basically "whatever power you put to base allows power to flow from collector to emitter proportionally". I only partially understand how the capacitor makes the 555 timer set up work, but hey, it IS working. I basically found these neon lamps on some steampunk instructables, thought they were cool, ordered a few, and then got to thinking how the heck to get both terminals to glow.

Thanks in advance for any help, i hope this was sufficiently descriptive, and i pray that those who actually know what they are doing with transistors will be able to bonk me on the head and say "well there's your problem!" and illuminate what silly thing i have done wrong.

 

[Reloadron]

I have done this using a 555 and a small audio transformer just like this one. You drive the primary with the 555 and on the secondary I used a diode and a cap (.1 uF 250 volt). I should have the circuit somewhere just not sure where but it is a very common circuit. I have also used a 6.3 volt small filament transformer but the 8 Ohm : 1 K Ohm audio transformer is real common and works well. You do not use the C/T. Off the transformer connect the green lead to the anode of a 1N914 and the cap goes from the 1N914 cathode to the blue lead (across the secondary after the diode). Then a 10 K resistor in series with the neon lamp.

Ron

 

[audioguru]

Please post a complete schematic of how you connected the transistors and transformer so we can see how you connected them wrong.

In one sentence you connected the "center-line" of the transformer to the positive power terminal (+9V?) but in another sentence you connected the "ground terminal" of the transformer to ground but a center-tapped transformer does not have a ground terminal.

The base-emitter junction of the NPN transistor is shorting the output of the 555 and one of them will blow up (or the battery will blow up).
The PNP transistor will never turn on (never "close"?)

 

[Boncuk]

Hi Keith,

this simulated circuit might give you an idea how to achieve your goal.

The timer circuit generates an almost 50% duty cycle square wave of 130Hz. You might use a variable resistor in series with R1 if accurate frequency is an issue.

In that case reduce the value to 3.9K or lower than 5.6K. Use a potentiometer to gain about 10K total resistance.

The timer output controls an RS-Flipflop which divides the input frequency by two activating Q2/Q3 and Q4/Q5 alternating.

The transistor pairs control the power MosFet transistors Q6 and Q7.

Q1 is turned on when the switch is closed causing the transformer to generate AC voltage with a square shape.

If you build the circuit use the same value for R2 and R3. They are now 4.7K and 10K. Use 4.7K for both.

Boncuk

 

[Reloadron]

Attached is the circuit that I had in mind. It is just a simple 555 timer configured as an astable multivibrator (the oscillator) driving a little audio transformer from the secondary side. The output is taken off the primary side. Reducing the 47K resistor a little will result in a slightly increased output voltage. As drawn the circuit will illuminate just about any common neon lamp. The transformer is any 8 Ohm / 1K Ohm audio transformer and I have also used small 120 volt / 6.3 volt filament transformers with the same result.

The merit to using transistors is that you would get more drive current. If you just want to illuminate a single lamp or two the circuit will work fine. If you want more drive current then transistors round things out. Just a matter of how much output power you want or need.

Ron

 

[audioguru]

Attached is the circuit that I had in mind. It is just a simple 555 timer configured as an astable multivibrator (the oscillator) driving a little audio transformer from the secondary side.

The transformer's primary is not driven with AC, it is driven with DC pulses that might saturate its core.
Drive the transformer through a coupling capacitor.

 

[Reloadron]

The transformer's primary is not driven with AC, it is driven with DC pulses that might saturate its core.
Drive the transformer through a coupling capacitor.

Hi AG

The circuit I posted works and works just fine. Hell no, it is not driven with AC and is, as you point out driven with a pulse, no screaming secret there. I never said it was AC driven did I? Now if you have a better suggestion to illuminate a single friggin neon lamp toss it out there. Beyond that, shut the hell up as to my suggestion. The DC pulses will not saturate the core, I built the circuit many times using the transformer I called out. It works just fine. I made that quite clear. While your comments are appreciated I suggest you actually build the circuit. It does work just fine as drawn. It works fine with the transformer I suggested as well as using an off the shelf few hundred mA 6.3 volt filament transformer as I also mentioned. There is no need at all to drive the transformer using a coupling transformer.

Sorry if I seem a bit testy but the circuit works as I posted it. If it did not work or was not tested I would not have posted it.

Ron

 

[Keith42]

Attached diagram for what i have.



To Reloadron and his Circuit: wouldn't the timer with those resistor values be running like a 98% duty cycle, almost always high? I also do not entirely understand how that's going to make AC, i'm guessing something to do with the capacitor supplying lamp power when the 555 output is low, and happening to supply it in opposite direction. Again, my knowledge here is quite limited.

 

[Keith42]

Attached is a diagram of my circuit, with the trouble transistor labeled. I am for the moment more interested in whats wrong with mine than a whole new one.

To Reloadron -
Wouldn't the resistor values for your timer give it near constant on 98% duty cycle?
I also do not entirely understand how that would make AC power... something like the capacitor in the lamp's circuit supplying current in the reverse direction when the transformer is not supplying any (timer low) ? I will take that it works on faith.

 

[audioguru]

I corrected your circuit and explained why.
The Neon bulb should have a series resistor to limit its current.

EDIT: My circuit also will not work. I changed it so now it will work.