TWO DOLLAR TESLA Just a quick post that, as has been the case on many days for months, I was thinking about yesterday while working but too lazy to actually write once the evening arrived. I was babbling on about buying things in my last post and how I hadn't bought any non-consumables for my own use for a couple of months, but I didn't mention that what I actually bought back then was a Tesla coil kit! Last weekend I finally got around to building it and having a play. A Tesla coil is basically an air-core transformer with a stupidly large number of secondary turns so that a little AC on the primary generates a voltage high enough to get you a reputation for being completely bonkers, as Mr Tesla found out himself. If you understood that description then you probably already knew about Tesla coils anyway, but what the hell. Buying a kit is really pretty wimpy as it doesn't take much to make your own so long as you've got the patience and dexterity to wind the tiny wire extremely neatly. I really don't though, having stuffed up much simpler coil winding attempts in the past, hence the kit. But as if that weren't wimpy enough I bought a tiny cheap one from the Chinese sellers on Aliexpress, actually combined with some other (more practical) parts from the same seller, so it worked out only costing about $2AUD. It doesn't have many parts and construction was very easy except for trying to get the tiny wire of the Tesla coil to stay in place while soldering it to the board. The only really interesting thing about the circuit design, which is based around the most common cheap Tesla driver/oscillator circuit, is that the primary isn't actually a wire that you loop around the outside of the coil, but a track on the circuit board that you glue the coil on top of. I'm guessing that a wire might work a little better, but it saves a part and I'm surprised that it works at all. In spite of the unimpressive size, it does all the standard Tesla coil tricks with the exception of making any visible sparks through the air of interesting length (the current is too low, though that comes with the advantage that it can't kill you). The kit came with a neon bulb which dutifully lights up when you wave it nearby, plus CFL light bulbs spring into life and small fluorescent tubes turn in to something you immediately identify as a magic wand. But I had most fun with my collection of Xenon flash tubes, inside which plasma arcs like to form and can be manipulated by moving the tube around, as well as an odd specimen from my small incandescent light bulb box which has something of an onion shape and (unlike any other bulb I tried) exhibits a wide bright purple arc from the filament to the top of the glass envelope when sat in the top of the Tesla coil. The really neat photo that I took of this with a long exposure on my digital camera (showing its age (huh, 18 years old - it's an adult!) with a fair few dead pixels visible in the Black) is my key incentive for talking about all this: gopher://aussies.space/I/%7efreet/photos/tesla_bulb.jpg It doesn't actually look nearly so good on the 16bit colour display of this mid 90s PC though (too many colours are substituted for Black), but I was definately impressed when looking at it on my laptop. Here's a less-good shot of the arcing in a Xenon flash bulb, which also reveals the wimpyness of my Tesla coil: gopher://aussies.space/I/%7efreet/photos/tesla_xenon.jpg As reported by other buyers, the transistor gets quite hot in spite of its little heat sink. So I think you'd want to mount the transistor off-board on a more serious heat sink if you wanted to run it for a while without frequent 'rests'. Bigger coils are available from China as well, both as kits and also large coils on their own, but the pricing ramps up significantly. It occoured to me that it would be cheaper to just buy ten or twenty of these kits for the mini coil in them, and wire them all in series, or eg. three stacks of three in series connected in parallel, or something like that. So you'd build up the equivalent of one big tesla coil, and maybe even discover some new effects. But besides making decent sparks, and maybe inefficiently playing music, there's not a great deal more to do with a Tesla coil anyway. The lack of a real practical application sort-of dampens my interest in the whole thing, which I acknowledge clearly isn't a problem for a whole lot of other people (eg. see http://www.electricstuff.co.uk/ for some 'real' Tesla coils). On the topic of cheap alternative approaches, there's also this neat design which goes beyond just putting the primary winding on the PCB, and actually does the secondary as a long spiral PCB track as well: https://www.tindie.com/products/kitsforkids/pcb-etched-tesla-coil-v3b/ So to all you spare-change mad scientists out there, you've now got no excuse - get zapping! - The Free Thinker.