Building a Mini Coil
Introduction
This mini Tesla Coil was originally designed to run using a previously built high voltage flyback circuit as a power supply. The circuit provided about 45watts at about 10kV. This means that there are limits to the size of arc that can be produced. My coil is capable of 4" arcs. Such a small coil provides a great talking point, and is small enough to be easily portable. Another point is that it is cheap to build.
Please read all of the information on this page before you start building. Also read the safety page.
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Power Supply
The circuit of the power supply is the frequently published two transistor fly-back circuit.
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D1 may be either an EHT rectifier stick from a tv or a string of lower voltage diodes for example 15 x 1N4007s. If you use a string of lower voltage diodes, they must all be of the same type. Despite what is often said in textbooks, don't use equalising resistors across the diodes, modern diodes don't need them, and if one fails then too much stress will be placed on the diodes with spectacular results. Although the supply is shown as 12V, up to 18 is ok, but you will need to use a 25V smoothing capacitor rather than the 16V one. This circuit will draw 3-5A, depending on several factors but chiefly the current taken from the secondary (HT) winding. The 2N3055s need heat-sinks of a good size. The primary coil needs to be wound with thick insulated wire, 18swg is about right.
Baseboard
Various materials can be used for the baseboard, provided that they are good insulators. The coil shown in the picture used varnished MDF. A good alternative would be acrylic sheet (PerspexTM or PlexiglasTM), or polycarbonate (LexanTM).
Capacitor
The capacitor used in this particular coil needed a value of approximately 5nF at about 10-15kV, I have since used it with 8.3nF with appropriate adjustments to the primary coil tapping. There are various ways of obtaining these values. Whilst many coilers would use a string of higher value capacitors in series (an MMC), I used an alternative approach initially, building my own from double sided printed circuit board. For the 8.3nF capacitor I used an MMC, consisting of 12 x 100nF 1k5V rated Philips capacitors. If you go this route, then it is essential that you use the correct type of capacitors, namely foil polypropylene capacitors rated at the appropriate voltage and of the appropriate construction for high discharge rates. A number of sites have details of the construction of MMCs, read what they have to say before going this way, it will save you a lot of time, effort, money and frustration. Other approaches include making rolled polythene or salt water capacitors. Both these latter approaches can be messy, and they end up with very bulky components. Bleeder resistors should be placed across capacitors, these need to be high voltage types, they also need to be of high value, for example for an MMC 10M across each capacitor. Click here for the printed circuit board capacitor.
Spark Gap
Various types of spark gap have been used, the simplest being a static gap consisting of two mild steel brackets with brass bolts forming the actual gap. The preferred gap, and the one shown in the picture is a multiple gap of the RQ type. No forced air cooling is necessary at the power levels of this coil.
3/8" o.d. copper tubing, obtained from a local model shop was used. The tubing was cut into 5 x 11/2" lengths, using a pipe cutter, this has the advantage of slightly radiusing the ends so that no de-burring is necessary, if you use a saw, then the edges of the cuts need to be radiused to reduce corona formation, arcs also form at very much lower voltages from sharp edges so your gap won't quench properly if left. The tubes were drilled and fitted to a piece of perforated srbp (PaxolinTM) board using short self-tapping screws. This board is available from electronics suppliers and is commonly called perf-board. The holes are 0.1" apart, and will need to be enlarged using a small drill bit. Ideally the holes should be slightly larger than the screws to enable minor adjustment. To set the gaps a credit card is ideal. Set each gap in turn, tightening the screws as you go. Connections can be made to the end tubes by soldering thick wires to them, a large soldering iron is necessary. The completed gap can be mounted on the base board using spacers.
Primary Coil
The primary coil consists of 12 turns of 18swg tinned copper wire, wound as a flat or Archimedes spiral. The coil is supported using four strips of the same perf-board as used for the Spark Gap. Again the holes have to be enlarged slightly, as they are too close a fit for easy winding. The easiest way of making the winding is to glue the strips in place using epoxy. Remember that they need to be staggered slightly as the wire is wound as a spiral. Personally I fixed the first in position and spaced the opposite one 0.05" further out, then adjusted the second and third to give a smooth curve. It is easier to wind the wire if it is first at least partially pre-formed by winding the wire round a cylinder of approximately the mean diameter of the coil. The best adhesive that I have found for fixing the perf-board strips in place is hot-melt glue, it has the advantage of being flexible even when set; I have also used a slow-cure epoxy resin the fast cure tends to be far more brittle. Cyano glues work, the best being the gel type as it has some gap-filling properties, but it is rather brittle when set.
Secondary Coil
The secondary coil former used a vitamin tablet tube with a screw cap, slightly over 1" in diameter and 5" long. The wire used was actually of an unknown size, as it was taken from a surplus relay coil, but was about 40swg (38awg) which gives 178tpi (turns per inch). Starting at one end the free end of the wire was taped down, leaving about1/8" of space at the end of the former, and about 2" of wire free for connections. The wire was wound carefully in a single layer along the tube (remember this wire is very fragile, and you may well break it, in which case the only thing to do is take it all off and start again). This cannot be hurried. Kep the wire under constant tension, make sure the turns touch each other and that no turns ride back over a previous turn. Every 50 or so turns tape the wire down. There is no point in counting the total number of turns, in any case it is very frustrating to do so, you will be well within range of the final number of turns. Stop when you are about 1/8" from the end of the former and tape the wire down, remember to leave a couple of inches of wire for connection purposes.
Next the secondary was varnished with three coats of clear polyurethane. The first coat was diluted 50/50 with white spirit. This first coat will be easiest if you have kept all the taping down of the wire on one side of the former, because you can put on the first coat of varnish on the opposite side, then allowing it to dry before removing the tape and doing the other side.
The advantage of the screw cap, is that it makes it easy to fix the secondary to the base. You can either glue it to your base or fix it with a plastic nut and bolt (the type for fitting car number plates is ideal).
There are various ways of making the connections to the coil. The best I have found is to use copper foil, or thin brass . I cut two pieces, a disk for the top, the same size as the end of the tube, and a strip about 3/4" long and 1/4" wide for the bottom connection. The ends of the coil need to be tinned, modern wire is self-fluxing so this shouldn't be a problem. The points on the brass or copper where connections are to be made are also tinned prior to making the actual connections. The connection pieces were then glued in place. The secondary was then mounted and the base connection made.
Toroid
The topload consists of a toroid made from an expanded polystyrene (Styrofoam) ring obtained from a local craft shop. Various sizes were available, I chose one of 5.75" diameter with a 1.25" thick ring. A disk of stiff card was force fitted into the centre of the ring, and the whole covered with aluminium foil. Two methods have been used, the first was to use double-sided adhesive tape cut into narrow strips, the other method was to use aluminium alarm tape. I have not found a suitable adhesive for sticking foil to foam, solvent based ones often dissolve the foam, and water based ones react with the foil. Keep the foil as smooth as possible, and when the foam is completely covered burnish it with the back of a spoon. The toroid can simply be rested on top of the secondary.
Tuning
Tuning the coil is something that takes time and a bit of fiddling with. There are several ways of going about it. The basic method is to adjust the conections to the primary coil, in order to adjust its inductance, and hence the resonant frequency of the primary circuit comprising the high voltage or tank capacitor and the primary coil. The problem with a mini-coil such as this is the need to make a temporary connection to the coil, and the need to move it many times during tuning. There is also the difficulty that the circuit needs to be energised to see the effects of the adjustments made. Probably the simplest way is to use the same gauge of wire to make the moveable tap as used for winding the primary. If this is longer than necessary, then it can be lightly soldered in place each time you move it.
First thing to do is arrange a suitable grounded point for your arc, a length of stff wire or rod mounted on a heavy base will do, ground it with a length of flexible wire to the ground of the secondary. Now arrange things so the end of the grounded point is about .5" from the toroid. Getting a .5" arc should be easy!
Now start the tuning process. Remember to switch off power before making any adjustments, and short the capacitor to discharge it, using a grounded probe. Start by making your connection to the end of the outermost turn of the primary. Switch on the power, you may or may not get an arc, if not don't worry. Was the spark gap firing? If not reduce the number of gaps if using the RQ type of gap, or if you are using the single bolt type gap, close it until it fires regularly. Now move your primary tap so only 3 turns are in circuit, switch on the power, are you now getting arcs at the toroid? If so, move the earthed point away from the toroid until you no longer get arcs. Move the tapping point outwards by one turn and repeat above steps you will find one or possibly two points where you get far longer arcs. If you get two points, then your final tapping point will be somewhere between the two. Now you can fiddle with things a little until you get the longest arcs. Finally adjust your spark gap to give you the longest good arc. You have now tuned your coil.
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The above picture (sorry about the quality) shows the coil described, in action prior to final tuning, it was producing sparks of about 3".
Problems
If you have any problems building this project, please E-mail me.
