Tesla Coils

<body BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#1010A0"> <P ALIGN=CENTER> <IMG SRC=double_exposure_med.jpg ALT="Double-exposure: Me posing with my Tesla Coil"> </P> <H1 ALIGN=CENTER>Tesla Coils</H1> <hr width=90% size=2 noshade> <table align=center width=90%> <tr align=center> <td><font color=#E00000 size=+1><b>WARNING !!</b></font></td> </tr> <tr align=center> <td><font color=#E00000 size=+1><b> This page deals with equipment and methods that produce LETHAL voltages of electricity. DO NOT attempt to replicate any of the items discussed in this page unless you are an electronics professional and you know what you are doing. I can't stress this enough - one mistake and you will be DEAD! Note that the picture shown above is a DOUBLE EXPOSURE. I was not - in fact - standing anywhere near the device while it was in operation. This image was created for entertainment purposes only and does not represent a recommended operating procedure. The owner of this web site is not responsible for any actions taken by any individuals after having looked at this page. Read at your own risk. </b></font></td> </tr> </table> <br> <hr width=90% size=2 noshade> <table align=center width=90%> <tr> <td align=left> <h2>Introduction</h2> </td> </tr> <tr> <td> <p align=justify> As you may have guessed by now, Tesla coils involve the generation of high-voltage electricity. Invented by the eccentric, but brilliant inventor <a href="http://en.wikipedia.org/wiki/Nikola_Tesla" target=_top>Nikola Tesla.</a>, the Tesla coil is probably his most famous and iconic invention. Other forms of high-voltage generators such as the <a href="http://en.wikipedia.org/wiki/Van_de_Graaff_generator" target=_top>Van De Graaff</a> generator often seen in schools use electrostatic effects and a direct current (DC) to build up a static charge of a constant electrical polarity. These devices are limited in either current delivery or in the voltages they produce. In addition, once they are discharged, a period of time must be allowed for them to charge back up prior to another discharge. </p> </td> </tr> <tr> <td> <p align=justify> <br> The Tesla coil on the other hand, uses resonant energy transfer and alternating current (AC) to turn a low-voltage / high-current radio-frequency signal into a high-voltage / low-current signal. Correctly designed, a Tesla coil can be made to output a near-constant arc of ionised gas in the form of an electrical breakdown from it's top. This makes for a much more impressive demonstration of electrical energy ;-) </p> </td> </tr> <tr> <td align=left> <h2><br>Operation</h2> </td> </tr> <tr>http://www.meade.com/lxd75/index.html <td> <p align=justify> The basic circuit of a Tesla coil is shown in the circuit below. It consists of two parallel-resonant circuits - the primary circuit and the secondary circuit - and a spark gap. Notice that the secondary capacitor C<sub>S</sub> is typically made up of the "stray" capacitance of the electrode at the top of the secondary coil which is traditionally a sphere, though more commonly a torus is used these days. The operation is as follows: </p> </td> </tr> <tr> <td align=center> <img src=tesla_circuit.gif alt="Tesla coil circuit diagram"> </td> </tr> <tr> <td> <br> <table cellpadding=10 align=center width=90%> <tr> <td valign=top>1</td><td><p align=justify> The primary capacitor C<sub>P</sub> charges up from the high-voltage source via the primary inductor L<sub>P</sub>. Since the charging current comes from a DC or low-frequency (50-60Hz) AC source, L<sub>P</sub> - which is very small in value - appears like a short circuit.</p></td> </tr> <tr> <td valign=top>2</td><td><p align=justify> Eventually, the voltage across C<sub>P</sub> - and hence the spark gap - reaches the break-down voltage of the spark gap. The spark gap then "fires" and effectively closes the loop around C<sub>P</sub> and L<sub>P</sub> and forms a resonant circuit.</p></td></tr> </tr> <tr> <td valign=top>3</td><td><p align=justify> When this happens, all the stored energy in C<sub>P</sub> transfers to L<sub>P</sub> and then sloshes back and forth between the two at the resonant frequency of the primary circuit. This causes <i>enormous</i> currents - 100s of Amps - to flow in the primary circuit generating a prodigious alternating magnetic field around the primary inductor.</p></td></tr> </tr> <tr> <td valign=top>4</td><td><p align=justify> Now, if it were not for the presence of the secondary circuit inside that alternating field, these oscillations would continue until the losses in the primary circuit cause the voltage across the spark gap to fall to the point where it can no longer conduct. Instead, the loosely coupled secondary circuit causes the oscillatory energy in the primary to gradually get transferred to the secondary circuit. Eventually, all the energy in the primary will have been transferred to the secondary.</p></td></tr> </tr> <tr> <td valign=top>5</td><td><p align=justify> This is when the fun starts!! It is a popular misconception that a Tesla coil operates like a regular voltage transformer. This is not the case. While it is true that the turns ratio between the primary and secondary is related to the voltage gain - like a regular transformer - it is actually the ratios of <i>inductances</i> that causes the voltage gain. Since the secondary inductor has a <i>much</i> higher inductance than the primary inductor, the voltage gain from the primary circuit to the secondary circuit can be considerable. This super-high voltage appears on the top electrode (sometimes called a top load) and if the electric field strength around the electrode exceeds the break-down voltage of the air, sparks are formed from the electrode.</p></td></tr> </tr> <tr> <td valign=top>6</td><td><p align=justify> Eventually, all the energy that was originally stored in the primary circuit will be dissipated and the spark gap will quench, at which point the process goes back to step 1. Repeat this several hundred times per second and it should be quite a show. </tr> </table> </td> </tr> <tr> <td> <p align=justify> <br> This is like a "Tesla 101" level description. If you want a full, in-depth analysis of how a Tesla coil works with oscilloscope traces and animations, there is no better place than <a href="http://www.richieburnett.co.uk/tesla.shtml" target=_top>Richie's Tesla Coil Web Page</a>. Just follow the link to the <a href="http://www.richieburnett.co.uk/operation.html#operation" target=_top>Tesla coil operation</a> section. </p> </td> </tr> <tr> <td align=left> <h2><br>An example - my Tesla coil</h2> </td> </tr> <tr> <td> <p align=justify> This is actually the third Tesla coil I've built. The first one I built was made from some old wire wrapped round an empty paper-towel roll with a 100:1 step-down transformer put in reverse attached to a 16V AC train-set controller to act as the high-voltage input. It made inch-long sparks, which I always remember being pleased with for such a crude device. In hindsight, this was probably closer to an air-core transformer than a true Tesla coil.<br><br> The second one was more of joint effort between myself and my father. It was similar to the one described on these pages, but quite a bit smaller and the spark gap was a simple static gap which made an ear-splittingly loud noise. So loud, we had to put it in a box for fear of going deaf. It produced 6-inch long sparks of an altogether more "serious" nature. I used to draw the 1-inch spark from my first Tesla coil out to my finger. There was no way either of us were going near this one.<br><br> I built this third one mainly because the project I was working on at the time was called Tesla and I thought it would be a nice way to celebrate the end of the multi-year project. The secondary reason was simply that I love these things and wanted a decent example to show off to my kids, family and friends. </p> </td> </tr> <tr> <td align=left> <h3><br>Construction</h3> </td> </tr> <tr> <td> <p align=justify> The size and shape of my Tesla coil was as much determined by the materials I could lay my hands on as from my own imagination. I didn't do a lot of planning. I had a vague idea of how big I wanted it to be - physically - so I did some rough calculations based on that to make sure I was going to be in the ball-park of where I wanted to be at the end. Here are my initial calculations:<br><br> <b>Primary coil inductance:</b><br> L<sub>P</sub> = (r^2.N^2) / (228 r + 254 h)<br> r = radius = 0.205 m<br> h = height = 0.180 m<br> N = number of turns = 9<br> Therefore:<br> <b>L<sub>P</sub> = 36.8 uH</b><br><br> <b>Secondary coil inductance:</b><br> L<sub>S</sub> = (r^2.N^2) / (228 r + 254 h)<br> r = radius = 0.0575 m<br> h = height = 0.8 m<br> N = number of turns = 1000 (approximate)<br> Therefore:<br> <b>L<sub>S</sub> = 15.3 mH</b><br><br> <b>Top electrode capacitance (torus)</b><br> C<sub>S</sub> = 0.37 D + 0.23 d<br> d = diameter of the torus cross-section = 10 cm<br> D = total diameter of the torus = 40 cm<br> Therefore:<br> <b>C<sub>S</sub> = 17 pF</b><br><br> <b>Primary capacitance</b><br> C<sub>P</sub> = (L<sub>S</sub>.C<sub>S</sub>) / L<sub>P</sub><br> Therefore:<br> <b>C<sub>P</sub> = 7 nF</b><br><br> <b>Resonant Frequency</b><br> F<sub>r</sub> = 1 / (2 pi sqrt(L<sub>S</sub>.C<sub>S</sub>))<br> Therefore:<br> <b>F<sub>r</sub> = 312 KHz</b><br><br> From the ratio of L<sub>S</sub>:L<sub>P</sub> (or, C<sub>P</sub>:C<sub>S</sub>), I should be getting a voltage gain of about 400:1. Note that this is greater than the 111:1 ratio of secondary to primary turns. As I said before - it doesn't work like a transformer. That ratio means that for a (conservative) input voltage of 1 KVolt, I should be getting about 400 KVolt out the top!!<br><br> The construction of the various elements pretty much went from the bottom up. That is, the order in which I constructed the various parts was: Spark gap, Primary coil, Secondary Coil, top electrode. Only once I had all these pieces together could I accurately determine the actual value of the primary capacitance required and hence order the necessary parts.<br><br> <b>Spark gap</b><br> For the spark gap - I used a bench grinder with the grinding wheels removed. I replaced one grinding wheel with the rotating armature of the spark gap which I constructed from a four-sparred engine support frame for a model aeroplane found at my local hobby shop. The other grinding wheel was replaced with the fan blades from a 7 inch equipment ventilation fan. This provides the forced-air to help both quench the spark gap and keep the electrodes cool. I put a "spinner" for a model aeroplane propeller on the front of the fan blades which makes them look like a jet-engine intake. Very cool ... until you accidentally brush your knee up against the rotating blades and have to take yourself off to hospital to get seven stitches. Don't try this at home ... Anyway, I shrouded the whole thing in a section of ABS sewer pipe to prevent any shrapnel from killing anybody, should anything disintegrate in a spectacular way while spinning at 3600 rpm.<br><br> <b>Primary coil</b><br> Altogether less dangerous. Simply made from a length of 1/4 inch "micro" copper tubing from the local hardware store. The supports were made out of ABS pipe. I made a special jig so I could use a 1/4 inch drill to make the notches in the sides of the supports.<br><br> </p> </td> </tr> <tr> <td> <br> <p align=center> <img src=primary_coil.jpg><br><br> Primary coil. </p> </td> </tr> <tr> <td> <br> <p align=justify> <b>Secondary coil</b><br> Despite being one of the most time-consuming parts of the project, it was actually relatively easy to make. However - this is only true if you automate the winding process. I had to wind about 1550 turns onto the 4 inch PVC pipe I used as a form. Doing that by hand was not an option. I picked up a second hand synchronous motor with a gearbox attached from the local electrical surplus store. The output shaft rotated at 1 revolution per second and I reduced this to 0.5 revolutions per second using a couple of pullies and some string (seriously!). I then knocked up a make-shift lathe (see pictures below). So that I could pull off the wire from the spool in one long continuous operation. I put the spool on a "lazy Susan", which has very low friction. I tried just sticking a wooden dowel through the spool and orienting the axle horizontally at first, but this had too much friction and the spool jiggled about too much. Before I started winding, I coated the form inside and out with polyurethane varnish and let it dry thoroughly. Apparently, PVC absorbs moisture and that's the last thing you want near a radio frequency inductor. The winding itself actually took place very quickly. At 0.5 revolutions per second, it only takes 2 seconds to wind one turn. Multiply this by 1550 and you get 3100 seconds - or about 50 minutes. It actually took me closer to 2 hours from start to finish including all the stops and starts along the way, but that's really not that long. Finally, I coated the entire coil with about 4 thick coats of polyurethane varnish. I ran the lathe while I was doing this so I could put on quite thick coats without it running and it made the varnish level to a glass-smooth finish. Hopefully - with the wire entirely encapsulated in a thick layer of insulation - it will keep arcs to the secondary to a minimum.<br><br> </p> </td> </tr> <tr> <td> <br> <table align=center width=1030> <tr> <td> <p align=center> <img src=coil_lathe_1.jpg><br><br> Coil winding lathe showing spool of wire sitting on a "lazy Susan". </p> </td> <td> <p align=center> <img src=coil_lathe_2.jpg><br><br> Overview of the coil winding lathe. </p> </td> </tr> </table><br> </td> </tr> <tr> <td> <p align=justify> <b>Top electrode</b><br> This was the one part I agonised over the longest. Most people go for a toroidal design. However - it's the thing everyone looks at since the sparks are being emitted from it, so I wanted it to look nice. You can buy specially made spun-aluminium torii, but they are hard to come buy and expensive. At a complete loss as to what to do and almost being resigned to making a torus from chicken wire and foil wrap, I stumbled on the answer during an un-related shopping trip to IKEA. They do these almost perfectly hemispherical stainless steel salad bowls in various sizes, and they're cheap. So I bought a bunch in all three sizes. I took two medium-sized ones, made some fixing hardware and joined them together into a sphere. You can still see the "IKEA" logo etched into the steel if you look at the top of the ball. </p> </td> </tr> <tr> <td align=left> <h3><br>Testing and operation</h3> </td> </tr> <tr> <td> <p align=justify> Having constructed three out of the four main components of the Tesla coil, it only remained for me to test the actual components and measure their properties so that I could refine the value of the primary capacitor. To do this, I first measured the actual inductance of the secondary coil. I did this by measuring the resonant frequency of the inductor when combined with a known value of capacitance. I repeated this exercise with the primary coil and got the following results:<br><br> <b>L<sub>P</sub> = 40.0 uH</b> (actual)<br><br> <b>L<sub>S</sub> = 28.5 mH</b> (actual)<br><br> As can be seen - the value of the primary inductance agrees quite well with my original estimate. The secondary inductance is quite a bit bigger, but that is mostly down to the fact that I wound about 1.5 times as many turns onto the coil than in my original calculations.<br><br> Next, I placed the top electrode on the secondary coil and stimulated it into resonating (just like hitting a bell with a hammer, only with electricity). This allowed me to determine the resonant frequency and hence the capacitance of the secondary. The results were as follows:<br><br> <b>F<sub>r</sub> = 180 KHz</b> (actual)<br><br> <b>C<sub>S</sub> = 27 pF</b> (actual)<br><br> This allowed me to compute the value of C<sub>P</sub> by using the secondary:primary inductance ratio of about 700:1 ...<br><br> <b>C<sub>P</sub> = 19 nF</b> (actual)<br><br> Finally, I made up a 19 nF capacitor from a couple of standard-value capacitors and connected it in place of where the real primary capacitor was going to go and then took a 9V "PP3" battery and some crocodile clips to make the tesla coil circuit. Obviously, a 9V battery isn't going to break-down the air-gap in my spark gap, so I simply shorted the croc' clips by hand to mimic the action of the spark gap. I then attached one channel of my oscilloscope to the primary circuit and the second 'scope channel I just hung in the air attached to a length of wire. Sure enough - I could see the resonant transfer of energy from the primary to the secondary and back again. Even though the second 'scope probe was just hanging in the breeze, it registered about 50V!! The actual voltage on the top electrode must have been several thousand volts.<br><br> Knowing the required value of the primary capacitor then allowed me to construct the primary capacitor. It should be understood that a capacitor of the correct value and voltage rating is not something you can simply order from an ordinary electronics component supplier. You will not find a 15kV rated 19nF capacitor in the pages of a catalogue.<br><br> By utilising the most basic rules of parallel and serial connected capacitors, I was able to construct just such a capacitor using 96 100nF capacitors connected as 4 parallel "strings" of 24 capacitors each. This results in a total capacitance of 100 x 4 / 24 = 16.7 nF. Note that this is a little smaller in value than the 19 nF stated above. This is because I decided to use only 9 turns of the primary as my "nominal" inductor. Thus, the value required is 9/10 x 19 = 17 nF. Not utilising all the turns of the primary inductor will allow me to adjust the inductance by +/- 1 turn for fine tuning. </p> </td> </tr> <tr> <td> <br> <table align=center width=1020> <tr> <td width=33%> <p align=center> <img src=resonance_test.jpg><br><br> Resonance testing using a 9V PP3 battery, some croc' clips and an oscilloscope. </p> </td> <td width=33%> <p align=center> <img src=tesla_finished.jpg><br><br> Finished Tesla coil, only missing a couple of connections. </p> </td> <td width=33%> <p align=center> <img src=tesla_working.jpg><br><br> Tesla coil in operation. Not very impressive yet, but I'll get there. </p> </td> </tr> </table> </td> </tr> <tr> <td align=left> <h3><br>Experience</h3> </td> </tr> <tr> <td> <p align=justify> The sparks emitted from the top electrode are a little disappointing considering the size and potential power of the Tesla coil. I think this is mainly down to dielectric losses in the primary capacitor. At the operational frequency, the losses in the dielectric are considerable. In order to rectify this, I am going to use the larger IKEA salad bowls to make a bigger top electrode. This will increase the secondary capacitance and hence lower the resonant frequency. This will - in turn - significantly reduce the losses in the capacitors. The only problem with doing this is that the value of the primary capacitance will have to increase by the same amount - about 65%. This will involve buying more capacitors and re-configuring them into the correct topology to obtain the right value of capacitance. Hopefully, I'll get round to doing this one day and I'll post the results. </p> </td> </tr> </table> <br> <A HREF=index.htm>Back to the ganimede home page...</A> </P> <hr width=50% size=2 noshade> <P ALIGN=CENTER> <A HREF=index.htm> <IMG SRC=small_back.jpg ALIGN=CENTER BORDER=0 ALT="Back to index page" width=57 height=60></A> <A HREF=ganimede.htm> <IMG SRC=small_gan.jpg ALIGN=CENTER BORDER=0 ALT="Ganimede page" width=57 height=60></A> <A HREF=cgi.htm> <IMG SRC=small_ball.jpg ALIGN=CENTER BORDER=0 ALT="Computer Graphics page" width=57 height=60></A> <A HREF=ballooning.htm> <IMG SRC=small_bbac.jpg ALIGN=CENTER BORDER=0 ALT="Hot Air Ballooning" width=57 height=60></A> <A HREF=quake.htm> <IMG SRC=small_q.jpg ALIGN=CENTER BORDER=0 ALT="QUAKE!" width=57 height=60></a> <IMG SRC=grey_unreal.gif align=center BORDER=0 ALT="Unreal" width=57 height=60> <A HREF=cat7.htm> <IMG SRC=small_7_logo.jpg align=center BORDER=0 ALT="My dream car" width=57 height=60></A> <A HREF=mars.htm> <IMG SRC=small_mars.jpg align=center BORDER=0 ALT="Mars page" width=57 height=60></A> <A HREF=profile.htm> <IMG SRC=small_prof.jpg ALIGN=CENTER BORDER=0 ALT="About me" width=57 height=60></A> </P> <hr width=50% size=2 noshade> <P ALIGN=CENTER> Maintenance of these pages is attempted by <A HREF=mailto:webmaster@ganimede.demon.co.uk><i>webmaster@ganimede.demon.co.uk</i> </A> </body>