Microwave excitation of Neon
This experiment was initially built as an entry for the "Flashing Light Prize 2018". There are a couple of points worth explaining in more detail. Firstly, on safety, microwave oven magnetrons generally operate around 850W and there are a number of risks with this energy. It is possible to reduce the output but there is a limit where the emissions stop abruptly rather than reduce. One of the thresholds is the plate voltage must remain around 4kV. Using the doubler circuit and smaller capacitor reduces the energy available which does appear to work. The magnetron is supplied from a 12V supply via a ZVS oscillator and homemade transformer. From a safety point it is worth avoiding big MOTs for this experiment. The 12V supply is loaded by 2A when microwaves are being radiated. The circuit still draws almost 1A when not resonating so the maximum output energy is going to be less than the 24W suggested. There is a link to the video at the bottom of the page.
The feedback control for this experiment used the ionised gas resistance and proved to be an excellent feedback mechanism. Even though the gas is ionised by a strong signal it is possible to pass a small DC current through the ionised gas making use of the fact it is a conductor. In the case of the neon lamp being used here a resistance of 100k can be measured when glowing. The 12v feed from the DC power supply is use, via the neon plasma, to operate a relay on the heater driven by the MOSFET. Adjusting the distance from the wave guide and the voltage applied to the heater can give enough variables to set the flashing rate and duty cycle.
Finally, it is quite fitting that a neon lamp was used as an energy absorber. A similar device (ATR) is used to prevent transmitted high energy pulses being reflected into receivers in radar aerials.
