Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!rpics!yerazuws From: yerazuws@rpics.RPI.EDU (Crah) Newsgroups: sci.electronics Subject: Re: Tesla Coil - broadcast power Message-ID: <553@rpics.RPI.EDU> Date: Fri, 19-Dec-86 15:33:26 EST Article-I.D.: rpics.553 Posted: Fri Dec 19 15:33:26 1986 Date-Received: Sat, 20-Dec-86 02:00:30 EST References: <4815@reed.UUCP> <823@A60.UUCP> <824@A60.UUCP> <904@sfsup.UUCP> <3803@utcsri.UUCP> Organization: RPI CS Dept., Troy NY, USA Lines: 154 Summary: You do need that spark gap... The reason for the spark gap is that it's a device to convert low frequency AC (like 60 Hz) or DC. Otherwise (as previous postings have indicated) all you get is a big step-up coil and the neat Tesla-type discharges don't appear. First principle you need - How a "spark gap" works: Current in a spark gap behaves very differently than current in a resistor. More specifically, it takes about 10,000 volts per centimeter (I know it varies a lot, flames to /dev/null) to throw a spark between two rounded objects. Sharp points, edges, and etching/blast zappings can decrease this, but 10 kV is not a bad estimate. That 10 kV is to START the spark. Once the spark initiates, the ionized air in the region quickly decreases resistance (takes nanoseconds) and the voltage drop across the active arc becomes approximately constant at 25 volts in dry air at sea level, irrespective of the length of the arc. As long as enough current is available to maintain the arc (and the electrodes don't melt :-) ) the voltage will not exceed 25 to 30 volts, no matter how long (or short) the arc is. So, what the spark gap gives us is a switch that in a few nanoseconds can go from an insulating to a conductive state. (Turning the switch off takes significantly longer - because it turns off only when the air in the arc cools and deionizes. So, heavy arcing actually INHIBITS the spark-gap switch from turning off and on rapidly.). So what we want to make the high frequency out of DC or 60Hz is a circuit that works like this- 1) Capacitor charges up - 2) causing spark gap to fire ("switch on") - 3) which rapidly discharges capacitor - 4) falling voltage causes arc to cool and "switch off" - 5) which brings us back to state (1) ...with the whole loop going as rapidly as possible. Note that the 2->3 step should take on the order of 500 nanoseconds or so. A circuit to do this looks like: R ---------------\/\/\/\/----------------- | | | 60 v ------- Hz gap ^ ------- capacitor | | | ---------------------------------------- and the switch-on speed is controlled by the capacitance of the capacitor and the parasitic inductance of the wires between the gap and the cap. The gap firing rate is controlled by 1/RC (assuming that it's much faster than the 60 Hz supply, which we can view as DC because it's "on" for so long compared to the spark-gap part of the circuit. Why the Resistor "R"? It's so that when the spark gap DOES fire, the 60 Hz supply cannot put enough current into the arc to keep the air ionized. If R wasn't there, you'd only get one firing per half-phase of the 60 Hz (or 120 firings per second). We can get cuter and more efficient by replacing R with a coil, such that the time constant of the coil-gap circuit is much longer than the time it takes for the spark gap to fire and quench. Then, voltage builds up, gap fires, large V appears across coil, but because the coil has a decent inductance, the power supply can't put enough current into the gap QUICKLY to keep the arc from quenching. So the arc quenches. But current is now flowing from the power supply thru the coil, charging the cap. As the cap charges, the voltage across the coil drops. At some point in the future (hopefully soon) the current through the coil is again less than what the spark gap needs to arc continuously. At some still LATER point, the voltage on the capacitor is enough to fire the gap - which quickly discharges the capacitor. Cycle repeats. So, that's how the spark gap converts low-frequency AC or DC into high FREQUENCY AC. (note - if the coil doesn't have enough inductance, the arc won't quite quench- and the repeating cycle stops repeating) Now we need to convert the high Freq. AC current to high freq AC volts. That's what the air-core transformer is for. We have this pulsating magnetic field around the coil we put where the resistor was originally. We can put a second coil, magnetically coupled with the first coil, and tap off the high freq. energy directly. So, that's the secondary coil for your Tesla coil. Now, note what happens as you draw energy from the secondary coil (the tower/nail-on-top/whatever: the secondary coil, by coupling energy out to somewhere, impedes the expansion of the magnetic field in the primary coil. If we impede that magnetic field, the current in the primary is also impeded. Which means the whole oscillation slows down. The frequency of oscillation decreases but the power output stays constant. What if you couple the secondary to some other non-resistive load (like a fluorescent tube, or a pinwheel, or the ionosphere? Then the two oscillators (one being the load, one being the spark-gap/coil/cap) will exchange energy. However, the spark/coil/cap oscillator can oscillate over a relatively broad band (as long as the cap recharges before it's called upon to have enough volts to fire the gap) it really doesn't matter how often or slowly the gap fires. So: second important principle: Tesla coils TUNE THEMSELVES TO THE TARGET'S NATURAL RESONANT FREQUENCY (within reasonable bounds). Generally, the coil has to have a natural oscillation frequency somewhat higher than the target's but it works the other way around, too. This tuning action isn't like "we have this small servomotor diddling a potentiometer"- it's that if the target wants to oscillate slower, it will hold the oscillation in the coil/gap/cap circuit back. When it finally lets the spark gap fire, the kick resulting causes the target to oscillate with the coil/gap/cap. Now, a few comments: I've built tesla coils with "habitrail" tubes as the outer coil forms and toilet plungers (the giant suction-cup darts) as inner coil forms. They work. They make sparks that you thought only Lucasfilm could do. They make a lot of ozone (irritant/corrosive to rubber and fabrics) They make a LOT of radio hash. If you build one, be careful (Hopefully it's really oscillating up in the MHz, but I don't know a crafty way to really tell for sure). Don't run it for long periods, or unattended. And realize you are smashing radio communications over a broad bandwidth- this is a moderately antisocial act and can get you in trouble if you act immaturely. Voltages all around the coil/gap/cap/power supply are LETHAL and if they're at low frequency, will really cause your heart to stop and your brain to cold-boot. (I think I got that all right. Anyone have any changes/amplifications?) -Bill Yerazunis { code, encode, decode, DES, NSA, CIA, KGB, technology export, Iran, Nicaraugua, Contra, rebel, terrorist, plutonium, botulism - I realize it's a futile gesture, but it's a gesture nonetheless. Besides, the poor G-3 in Langley who has to screen these messages might want to know how to build a Tesla coil. :-) }