Thursday, July 27, 2017

Flashing Light Prize 2017: "Discharge-tube oscillator with transformer"

I'm working on a project this summer with an excellent student name Barbara. We are getting some real results that I expect to post about soon. However, this first post on the project is just a silly spin off: our entry for the Flashing Light Prize 2017:

We had characterized a number of different neon glow lamps by measuring the voltage V across them as a function of the current I pushed through them. All glow discharges, including the ubiquitous neon glow lamps, have an interesting V(I) graph with a region known as the subnormal regime where the differential resistance is negative: dV/dI < 0. With the operating point in the subnormal regime a glow discharge can be used in a Pearson-Anson oscillator (I show a print out of that Wikipedia page in the video). We had a breadboard with a working oscillator circuit when I heard about the geeky fun known as the Flashing Light Prize and I thought we could spend an afternoon to modify the oscillator circuit to make an incandescent bulb blink. Here's what I came up with:

The circuit does work and is the one used for our entry. However, we first had to go through a frustrating number of different glow discharges, transformers and incandescent bulbs. In the video I mention the severe current limitation imposed by keeping the operating point in the subnormal glow regime and how poor a fit that is for an incandescent bulb. I forgot to mention how hard it is to make a transformer perform acceptably at a frequency of one Hertz, which makes the inductive reactance awfully small.

The resistor R1 is a 720 kOhm one, rated at 2 W, in series with a smaller high-power resistor (inside the glass jar in the video). We think the latter is a couple of hundred kOhms. The capacitor is one microFarad, rated at 1500 V. The glow discharge is a discharge tube with adjustable pressure and distance between the electrodes. The working gas is air and we pumped the pressure down to about 400 mTorr. The final adjustment we made was to increase the electrode distance to move out on the right side of the Paschen minimum to increase the breakdown voltage from about 400 V to 700 V. The transformer is just a power transformer with about 30:1 turn ratio. Finding a small enough incandescent bulb was also hard until we found a parts number in a comment on one of the other entries. It's supposed to operate at 1.5 V and 15 mA, but we could not get the secondary current that high, but apparently we got close enough! Search for "grain of wheat light bulb" on AliExpress or similar site. I found a bag of 50 for $20.

More work than expected to finally get steady and reliable blinking, but it was a sight to behold! The video does not do the colors justice. The discharge tube glow was a beautiful blueish purple and the incandescent bulb a yellowish orange.