Neon lamps are undeniably cool. These days, everyone seems to have forgotten them in favor of LEDs. However, for certain aesthetics, LEDs simply won't do. Neon produces a warm, glowing light; LEDs are rather harsh and sterile in comparison. Unfortunately, it takes a little bit of work to power a neon lamp -- they require a voltage source above 80V (exact number depends on the lamp), but very little current. The NE-2 bulb is common; it requires about 75V to start, and has a rated current of 0.3mA. They're readily available -- AllSpectrum stocks them at $0.35 (less in bulk).
By way of example, here are some goggles I built using neon lamps for aesthetic effect. The blinking is achieved with neon as well, no transistors or integrated circuits required -- but that's a subject for a later post. The subject of this post is just the power supply. A typical NE-2 power supply would be a 90V source, and a 100k resistor in series with the bulb. Neon lamps exhibit an odd characteristic -- though it takes 75V or so to strike the bulb and turn it on, once on it only takes 55V or so to keep it on. If you applied 75V once it was on, the current would rise rapidly until the bulb exploded. The 100k ballast resistor keeps the current to roughly the 0.3mA rated current once the bulb is on.
For the goggles, I wanted to power them from CR2032 coin cell batteries -- cheap, small, lightweight, and readily available. I ended up using two, for a 6V supply. Turning that into 90V takes some work -- a job for a boost converter. In serious design, a flyback converter with a transformer would be used, but those are hard for the hobbyist to get with appropriate specs. You can salvage them from disposable cameras, but I have a personal preference for parts I can order from Digikey and get a datasheet for. For this circuit, a boost converter is just fine.
In order to keep the circuit simple, I run the converter in discontinuous mode. That means inductor current falls to zero on each cycle. Timing is controlled by the venerable 555 timer IC. Any 555 will work, but the standard ones draw several mA -- a significant load for those poor coin cells. The TLC555 cmos part is much lower power draw, slightly faster, and higher output drive — resulting in faster switching and higher efficiency.
Q1 is the main switch for the boost converter; any small n-channel mosfet with appropriate ratings will do. The BS107A is a cheap, good option. Similarly, any fast diode will do for D1 — the MUR160 or its relative the MUR120 are good choices. 1N4004 types will die in a hurry. The 555 is arranged in normal astable mode. Q2 provides output regulation by varying the control voltage. Each cycle has constant off time; reducing the voltage on the control pin lowers the on time, reducing the current in the inductor, and hence the output current. As the output voltage rises above the setpoint, Q2 starts to turn on, reducing the control voltage and thus the output voltage. R3, R4, and Q2's Vbe control the output voltage. In my circuit, R3 is series combination of 100k and another NE-2 bulb. (R5 and N1 are replaced by the oscillator circuit.) The use of the NE-2 in the regulator divider is mostly for convenience and aesthetic effect, but it will also stabilize the circuit against variation in Q2's Vbe with temperature.
The circuit, completed, on perfboard:
The finished circuit gets good efficiency — about 80%. It fits in one eyepiece of the goggles. The pair of coin cells last for many hours; I think about 15, but I haven't actually measured. It's more than one evening wearing them, though.
Construction tips: This should be a very forgiving circuit. It's tuned to the load intended (roughly one neon lamp in the feedback path, and another on the output). It won't work well far outside that range. Adding a third lamp will work, too many more won't (though appropriate adjustment of component values, and higher current batteries, would fix this). It will work just fine on a breadboard. The only important consideration is that the loop formed by Q1-D1-C3, which is where all the fast switching events occur, should be kept as small as possible. Because the output is current limited to fairly low values (thanks to the discontinuous mode) it should be relatively safe — touching the output should tingle but not cause injury. That said, it is a high voltage, and those can always be dangerous, so use caution. Please don't kill yourself with my circuits. In general, part substitutions should be fine, but pay attention to ratings. Q1 and D1 should be fast; aside from that, details aren't critical (though changing Q2 will likely require changing the feedback divider).
Parts list (Digikey PNs):
I discovered that the 12v CCFL inverters sold for PC case mods works very well to light one or many neon bulbs. Using 120v radio shack panel mount neon bulbs, a single one will start barely glowing at 1.1v input, a second at 1.3v, and a third bulb at 2v. At 9v all three bulbs are plenty bright.
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