John Berry wrote: > I made a simple so-called "Tesla Coil", the secondary is a pancake coil > without top electrode and has a resonant frequency of 760khz, the > primary circuit is a NST and I have used microwave diodes to rectify it > so the coil is run on DC. > > Most of the time I have the Variac set to severely limit the output > voltage/power and I keep the gap small. > > Anyway I was tuning the primary tap point and I was drawing an arc to a > screwdriver and I had decided that at the modest power levels I'd be Ok > touching it, after all with the inductive field put out there would be a > displacement current in my body for sure anyway if I touched it or not. > > So I drew arcs and I noted that as the arc hit there was a stinging > sensation and the muscles in my hand would contract, of course this > shouldn't really be the case when the coil is oscillating near a megahertz. > > I also found that the arc wouldn't create a stinging sensation if it > landed on the tip of a screwdriver, indeed I tried many objects to draw > the arc and it never worked if the arc hit an edge or a point.
Of course, as I'm sure you're well aware, a pointed electrode reduces the capacitance across the gap by a lot. Consequently, though the voltage goes just as high before the arc forms, the total charge which flows across the gap before the arc extinguishes will be far lower with pointed electrodes. And you'll get a higher frequency output with pointed probes, too, as the voltage will rise a lot faster, with (presumably) many more make/break cycles on the arc (same total number of electrons crossing the gap but they do it in smaller bunches). That doesn't explain where the current's coming from to start with, but it seems like it explains why the effect goes away when the electrodes are pointed. > > It is worth noting that the primary circuit does not have (much) high > voltage on it and can be touched with relative safety since I have caps > on either side although the impedance of the primary and possible > imbalance between the home made caps stops this from being strictly true. > > I would wonder if it could be a static capacitive discharge as a VDG > terminal can pack a bit on a punch however there is no terminal only a wire. > > So how is .75mhz from a Tesla Coil without any top loading causing > muscle contractions and a shocking sensation? How sure are you that you're actually seeing 0.75 mHz? Could you put a scope across it? I realize you can't do that directly (if you're fond of the scope) but how about with a high value resistor or two (like 10M and 1K maybe) to form a voltage divider, and put a scope probe on the divided down voltage? Won't be very accurate but it might at least give you some confirmation of the actual frequencies which are coming out of the thing. You never know what's really there until you look, after all. > > My leading theory _would_ be capacitive coupling from the primary but as > stated above it's not possible, the end of the primary coil that the > secondary would be influenced by happens to be at zero volts and I can > touch it without any sensation, the other end is at a slight voltage but > very little and can be touched with no issues. > > I have used wires to conduct this current to my hand and as long as the > arc doesn't fall on a point/wire it can be conducted by a wire just fine > so it is current and not something along the lines of Teslas radiant > stinging effect he observed. > > If anyone can think of a test, or a way to eliminate different > possibilities I'd be interested, also I'd be interested in any way to > measure this besides my hand getting shocked if anyone has and ideas. >
