A joule thief that can produce a high voltage output for powering neon bulbs
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At this point I have built this circuit according to the schematic. It works well enough to drive single neon bulbs off a single AA cell, though I have only tested with 1.23V minimum as that is the lowest voltage that my crude bench supply can provide. For phosphor coated bulbs or with multiple bulbs connected it is usually necessary to increase the input to 1.5V or so.
When I increase the input voltage I get an increased HV output, which is nice. However, once I increase the input voltage high enough, the circuit will stop operating correctly and won't provide enough output voltage anymore. My goal right now is to understand why this happens and if possible, find a solution for this problem. It is nice that this circuit could run off a single cell battery, but I would like to be able to run it on better power sources if I want more output. I will try to get more information about this issue with my oscilloscope, and I will post an update with some scope captures.
Here's how I built this circuit. If you wish to make your own, you may need to do things differently depending on what materials you have.
At this point you should be able to power up the circuit and check the voltage of the capacitor on the secondary. You could do this with a multimeter but I prefer to use an oscilloscope to get more information. If your circuit has no output or it is very low, the polarity of the secondary could be incorrect. Swap the ends of the coil around on your breadboard and try again. With my circuit I was getting an output of about 40V for 1.5V input. Interesting, but not enough to drive a neon bulb. There are a couple solutions to this. By tuning the circuit I was able to get the output closer to 70V, still not enough but a big improvement. I solved this issue by winding another coil of fine magnet wire on top of the secondary. This gives you two HV coils, which you can wire up in series to get higher output voltage. I recommend insulation between these two HV coils, such as the Corona Dope coating I mentioned before. With the two HV coils and proper tuning I was about to get up to 200V output using 2.8V input. I will describe the tuning process next.
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