It was clear that the soda bottle didn't perform all too well. So I went down to the hardware store to get some 80 mm PVC drainage pipe. I also unearthed my coil winding jig. As it used a rubber belt, and I had just taken apart that crappy Chinese stereo, I replaced it with the much more powerful tray motor/CD changer assembly:
About halfway I ran out of wire, but fortunately I still had the old 0.2 mm enamel copper wired bottle lying around so I unwound it and used the wire to finish the new secondary. The whole process took about two grueling hours...
Then the coil had to be varnished. I had an old can of "Plastik" spray from Kontakt Chemie. There was no indication on the can of the type of plastic, but I think it's PU or something. It mentioned protection against corona, so that's OK then. As all the propellant had escaped to damage the ozone layer I had to pry open the can and drip the liquid onto the coil. I spun it real fast to get a good even cover, and let it dry while spinning for a few hours:
I put everything back together and fired it up. It turned out that the new frequency was not 400 kHz as initially measured but only 300 kHz. So I had to change the primary: fewer turns and another capacitor. Oh well... Now I had a very feeble spark, just a few millimeters. I also noticed two plate current dips. Some more reading showed that this phenomenon, according to Richie Burnett's excellent primer, is known as frequency splitting, which occurs when the two resonant coils are too strongly coupled. So I raised the secondary a bit, and this improved things a lot. After raising it some more I finally got crackling streamers:
Note the glowing LEDs in the magnifying lamp in the background. The 300 kHz field is so strong that my counter will display the frequency without being connected! Also it gives nice sparks when tuned carefully:
Spark length is about 2-3 cm, the pin (LED lead) is 20 mm long. I also found that quite a bit of energy ends up in the coil, as I can melt the tip of the pin...
Still, these results are by no means anywhere near satisfying. I've come across reports about PL519 VTTCs generating 20 cm sparks, so this won't do. First of all I'll experiment some more with the primary. Different shapes, wider coils, maybe pancake-style. Currently the capacitance is 5300 pF and the coil is 48 µH. The secondary I dunno, it's 80 mm diameter and 33 cm long with 0.2 mm enameled copper wire, giving about 1.0-1.2k turns.
In the meantime I did solve the ringing in the previous installment, this was simply caused by a long test lead leading from the anode tank to the decoupling capacitor. Silly me...
8 April 2009