Jeff Pennoyer's Tesla Coil & Antique Radio Web Page
This web page includes links for documents and images that show my efforts at creating and operating Tesla coils. It also has a link for my collection of antique radios.
Hobbies:
Tesla coils.
Rebuilding antique tube radios.
Basic Stamp applications.
Rock music.
Jeff Pennoyer’s Tesla Coil Rules of Thumb
General:
• Use Teslac.exe DOS program for guidance. It will run under Windows XP. Just put a shortcut to the Teslac.exe file on your desktop. • Make as much of the support structures out of acrylic (plexiglass) as it is easy to cut with a jig saw and is nonconductive. • Use a relatively coarse blade in your jig saw as a fine blade will cause melting of the plastic as you cut it and will “heal” the cuts you make! • Use ShoeGoo or Liquid Nails for Projects (clear) goop glue to put plexiglass together. Just make sure it completely dries before you use your coil as it is flammable. This stuff is great as it makes a great bond but you can still take it apart later if you want to. • Be prepared to experiment! Your coil WILL NOT work optimally the first time you fire it up. You will need to experiment with your spark gap size, top load size on the secondary and will definitely have to tune your system by moving the contact on the primary coil. This allows you to achieve resonance between the primary and secondarysystems. • The primary and secondary systems are actually just inductive – capacitive (LC) tank circuits. Your objective is to make them resonate at the same frequency. When you do this, you will get the biggest sparks out of your Tesla coil. • Use a good Rf ground for the bottom terminal of your secondary coil. This can be simply a wire connecting the bottom or your secondary coil to as many large metal objects near your coil as possible. Do NOT connect the bottom connector of your secondary coil to the mains ground. Not only is this dangerous but it can fry all of the electronic stuff in your house! If you do not use a good Rf ground, you will have many intracoil streamers in the secondary coil and may also get primary to secondary streamers. • If you don’t use a big enough load on your secondary coil, you may get primary to secondary sparks. This can lead to burning of your secondary coil. • Use the Tesla coil circuit where the spark gap is directly across the NST secondary and the primary capacitor and primary coil are hooked up in series. Some schematics show other configurations. The one I described is ideal as it protects your NST from the primary circuit. • Tesla coils generate a LOT of ozone. Ideally, run your coil in an open garage. If not, have a good ventilation source to ventilate the ozone outdoors. I run my coil in my basement and have a high volume exhaust fan that I vent outside. • Make sure you put a strike rail just above and outside your primary coil and that it is not a complete circle (you want about a 1” gap in it as you don’t want it to act as an inductor). This strike rail is connected to your Rf ground. When your coil really starts to put out sparks, some will hit this rail. Without the rail, sparks can hit the primary coil and fry your NST. • Moisture is the enemy. Make sure everything is dry that you construct your coil from. For example, if you use PCV tubing for your secondary coil form and it has been out in the weather, you will want to oven-dry it prior to using it. • When making the adjustable connection to your primary coil, you can use a thick, strong alligator clip. Ideally, attach the clip from below the coil. If you clip from above the coil, this can cause localized fields that can lead to primary-secondary arcing or secondary-secondary strikes that may lead to burning of your secondary coil. Transformer:
• Don’t bother with Ignition transformers. Not enough punch. • Use Neon Sign Transformers (NST) without any Ground Fault Protection. If you buy an NST on eBAY beware that the listing must state that the transformer is not ground fault protected or you must ask the Seller. If you get one with Ground Fault Interruption (GFI) by accident, it may be possible to “depot” the transformer (get it out of the metal case) and disable the GFI but it’s not easy. I did it with an ACTown transformer but it may not be possible with other models or other manufacturer’s NSTs. Don’t believe what you read on the Web when it says you can just drill a hole in the right place and disable the GFI. This would be a sure way to destroy your transformer. • Use only the following Neon Sign Transformers. Any others will not give you enough power and you will get frustrated and end up getting one of these anyway: 9000v – 15000v at 60ma. • Make sure the transformer is “normal” power factor and is not power factor corrected. You will know this if the VA rating equals the product of the expected output. Ex. a 9000V, 60ma transformer should be rated at 540VA. The higher the VA, the higher the power and the longer the sparks you can obtain. Power is the “name of the game” in getting bigger sparks. • Some folks gang two or more Microwave Oven Transformers (MOTs) together. They put the primaries in parallel and the secondaries in series to get desired voltage. Most MOTs put out 2000v at reasonable currents (100ma or more). The only problem with this is the initial current surge upon plugging these in will most often blow your circuit breaker due to the high inductive load. The only way around this is use a power factor correction capacitor in parallel with the primaries. The only problem here is that the capacitor must be on the order of 10-30uF nonpolarized and around 200-300V rated. These aren’t easy to find and are expensive. Primary Coil: • Go to hardware store and buy ¼” soft copper tubing. Buy 20-30 feet in box. This tubing is ideal as it is already shaped in a coil formation and bends easily without kinks. • Make supports by cutting grooves into elongated rectangles of plexiglass. This is most easily done with a ¼” wide grinding bit on a Dremel Moto-tool. Primary Spark Gap: • Don’t bother with multiple gaps. In theory it is better than a single gap but is difficult to make and adjust. You will read about making rotary gaps. That’s fine to experiment with after you make a working coil with a single stationary gap. Rotary gaps are for the hard-core Coilers… • Use a single gap with force air. Use a used hair dryer on cool setting blowing directly on the gap. This gives you a clean spark. If you don’t use force air, you will not get a clean quenching of your spark and your coil will not work very well. • Start with the gap set at about ¼”. Then experiment. The widest gap with a consistent spark is often the best. • Can make your spark gap by grinding the head down on a small bolt and soldering to a ¼” to ½” stainless steel ball bearing. Using forced air cooling prevents the ball bearings from heating up too much and melting the solder. Turn on your forced air before you turn on your transformer. • You can get larger ball bearings from an old car bearing. Soldering to these can be tough. You have to use silver solder with the appropriate flux. Can use a small torch to get enough heat. After you tin a small section of the ball bearing with the silver solder, you can then solder the bolt onto the ball bearing by holding the bolt upside down in a jig pressed against the correct place on the ball bearing and applying heat. Primary Capacitor: • You can make you own capacitor and this can be rewarding but a homemade capacitor will never be as efficient as doorknob capacitors or oil-filled high voltage capacitors. • Make sure your total capacitor voltage rating is above 30kV DC or you will probably fry your capacitor(s). • If you are using a 9000v – 12000v NST at 60ma, then the best capacitor is around 0.01uF rated at 30kV DC or higher. This can be done by putting the right combination of capacitors in parallel and/or in series to get the right capacitance and voltage rating. For example, I put four 0.04uF 15kV oil-filled capacitors in series to give me 0.01uF at 60kV. In my other coil I put nine 0.1uF 3kV capacitors in series to give me 0.011uF at 27kV. • If you make your own capacitor, do not bother using glass as a dialectric. In theory, glass is an ideal dialectric but in practice, there is a lot of loss using glass at Rf frequencies. The less loss in your system, the better your sparks will be. Using a glass dialectric causes a lot of loss… • Use 1/16” thick pieces of polyethylene or thin acrylic. Use thin steel sheets or aluminum roof flashing. Steel is better if you can get it because you can solder to it. • Allow lots of room around all edges of your capacitor as sparks will creep around the edges and fry your capacitor if you don’t. • You can measure the capacitance of a homemade capacitor very simply if you have a decent, high impedance volt meter. Just take a transformer that puts out between around 12 – 50VAC and a 100K ohm ¼ watt resistor. Wire up the capacitor and resistor in series and hook up to the transformer secondary. Measure the voltage across the resistor and then across the capacitor. Use the calculations shown below. This works because the 60hz coming out of the transformer is held at a very constant frequency. Secondary Coil: • The best secondary coils are short and fat, NOT long. This is counter-intuitive as you would think you would get longer sparks out of a longer coil. The reason this is not true is because the best coupling between primary and secondary coils occurs when the secondary coil is not so long. I recommend a ratio of about 3:1 to 4:1. For example, if you want to use a secondary coil form that is four inches in diameter, the height of the coil should be about 16”. I made a better coil with a 6” diameter base and only 14” high. The coupling is great and the sparks really fly! • Do not use wire that is too thin. Thinner wire will give you more inductance BUT it will also give you greater losses due to wire resistance. The “Q” of the coil, or it’s efficiency is determined by having the most inductance with the least resistance. This is accomplished by using lower gauge wire. For example, I got best results when I used 22 gauge wire on my 4” coil form than when I used 28 gauge wire. • The wire should be a single piece of enameled wire. You can get spools of this type of wire on eBAY for around $30.00. • Wrap the windings on the coil and manually move them as closely as possible by pushing with your thumbs. • It is a LOT easier to wind a coil if you use a motorized device to wind it. After putting a bolt in the middle, I epoxied circular plexiglass pieces to the ends of my primary. Attached an electric drill hooked up to a variac to a jack stand. At the other end, had a piece of tubing in my vice so that the support at the other side of the coil could spin freely. Slowly applied voltage to the variac to get the form spinning slowly and started winding. • During winding, use small pieces of tape to hold the previously wound windings in place. Do this every couple of inches. If you don’t, you will regret it as the windings you already wound may unwind. • Avoid any kinks or crossovers. If you get one or two, don’t get too excited but if you have several, this will decrease your coil efficiency and be a focus for intracoil breakouts. • Use a plastic coil form if possible. Preferably acrylic (plexiglass) or a hard, thin plastic. Cardboard tubes are not as good as it can retain moisture. You can buy polyethylene pitchers that have pretty good measurements. I wound one coil on a coil form made from a pitcher that tapered from about 4.5 to about 6 inches. Put the fatter end at the bottom. Note that this tapered design closely approximates some of the original coils that Tesla built. The only problem with tapered coils is that they are harder to wind due to the taper. Need lots of little tape bits to keep your coil from shifting and unwinding from the large end toward the smaller end. • Another good coil form is 4-6” diameter PCV piping. It’s easy to work with. Buy it new if possible as it will be less likely to have retained any moisture. • After winding the coil you will need to coat the coil in a few coats of polyurethane. I leave the coil form on the winding device and spin it slowly while I’m painting on the polyurethane. If you use a fan and gently blow over the coil, a quick-drying polyurethane will dry in a couple of hours. Then apply another coat. I have had coils I have put 10 coats of polyurethane on. Besides acting as insulation, a glossy polyurethane makes your coil look nice. Top Load: • An easy way to set up a topload is to purchase 4” or 6” diameter flexible aluminum tubing used to vent clothes driers. • Drill a hole through the tubing and you can attach a connector with a small bolt or a rivet. • You can connect the two ends to form a toroid by using some aluminum duct repair tape. This is not the usual cloth duct tape but is metal tape. Can be purchase from your local hardware store.
Capacitance Determination: 1. Put transformer secondary, resistor and capacitor all in series. 2. Measure the voltage across the resistor, than across the capacitor. 3. For capacitance on the order of 0.005uF, us a 100K resistor. Use a high input impedance meter. 4. Transformer secondary voltage doesn't matter. In this case, used a 51V secondary transformer. 5. Transformer secondary voltage: 53.2 6. Calculate circuit current ( I ) by measuring the voltage across the resistor: I = (V / R). 7. Calculate capacitor reactance Xc by measuring the voltage across the capacitor: Xc = V / I 8. Calculate unknown capacitance by using equation C = 1 / (2 x pi x f x Xc). f will be 60 Hz Example: 1. Measured 18.2V across the 100Kohm resistor. I = V/R = 18.2 / 100,000 = 0.000182A 2. Measured 49.1V across my capacitor. Xc = V/I = 49.1 / 0.000182 = 269,780 3. C = 1 / (2 x pi x f x Xc) = 1 / (2 x 3.14 x 60 x 269,780) = 1 / 101,653,104 = 9.83 x 10e-9 = 9.83nF = 0.00983uF.
Example output and coil plans from Teslac.exe:
The number of turns is: 555.5555 closewound with 22 gauge wire The secondary winding length is 15 inches The secondary winding diameter is 5.5 inches (note that this is average. Bottom is about 6” and top is about 4.5”) The resonant frequency is approximately 469.9952 Khz. The required wire length is 799.537 feet The winding aspect ratio (L/D) is 2.727273 The RF resistance of the secondary conductor is approximately 217.5128 ohms. The inductive reactance (XL) is 39423.6 ohms at 469.9952 Khz. The maximum UNloaded Q is 181.2473 at 469.9952 Khz. The secondary inductance is approximately 13.35682 millihenry The secondary distributed/stray capacitance is approximately 8.59574 p.f. The estimated LOADed Q will be approx. 64.73117 depending on the coupling and coil form losses, the size of the discharge terminal, and other factors. The terminal capacitance should at a minimum, be large enough to cause the resonant frequency to drop to approximately 332.2866 Khz. This will require a terminal capacity of at least 8.59574 p.f. TRANSFORMER/CAPACITOR MATCHING What is your transformer secondary R.M.S. voltage: 12000
Is the spark gap a (1) rotary or (2) fixed: 2 What type transformer? 1=neon (shunted), 2=pole & potential (nonshunted):1 What is your transformer(s) maximum secondary current in m.a.: 60 The maximum primary capacitance you can use is 0.0094 u.f.d. It should have a voltage rating (sinewave) of at least 30000 volts A.C. to insure reliability! TRANSFORMER/CAPACITOR MATCHING What is your transformer secondary R.M.S. voltage: 9000 Is the spark gap a (1) rotary or (2) fixed: 2 What type transformer? 1=neon (shunted), 2=pole & potential (nonshunted):1 What is your transformer(s) maximum secondary current in m.a.: 60 The maximum primary capacitance you can use is 0.0125 u.f.d. It should have a voltage rating (sinewave) of at least 22500 volts A.C. to insure reliability! FLAT SPIRAL PRIMARY CALCULATION Enter the frequency in Khz. of the secondary coil with terminal according to program #1 or your actual measurements: 332.2866 Enter in u.f.d. the value of the primary capacitor: 0.0094 Enter the diameter of the secondary coil in inches: 3.75 Enter the diameter of the primary conductor in inches: 0.25 Enter the turn to turn spacing of the primary in inches (usually equal to the conductor diameter): 0.25 Enter the spacing in inches between the secondary and the primary inside turn(usually 2-3 times the primary conductor diameter): 0.75 With a primary capacitance of .0094 u.f.d. you will need 24.4 uh. to resonate at 332 Khz. The inductance at turn 3 is approximately 2.4 uh. The inductance at turn 4 is approximately 4.1 uh. The inductance at turn 5 is approximately 6.4 uh. The inductance at turn 6 is approximately 9.3 uh. The inductance at turn 7 is approximately 12.7 uh. The inductance at turn 8 is approximately 16.8 uh. The inductance at turn 9 is approximately 21.6 uh. The inductance at turn 10 is approximately 27.1 uh. The primary o.d. is 14.75 inches, the primary i.d. is 5.25 inches. It will have 10 turns of .25 inch diameter conductor The turn to turn spacing will be .25 inch(s) The approximate primary tap point will be between turns 9 and 10
STANDARD SIZE TOROID REFERENCE CHART
TORUS SIZE TORUS CAPAC. SUG. SEC. DIA. SUG. POWER LEVEL 1 X 6 in. 5.10 PF 2 - 4 in. 200 - 400 W 2 X 8 in. 6.80 PF 3 - 5 in. 400 - 800 W 3 X 12 in. 12.95 PF 4 - 6 in. 700 - 1500 W 5 X 14 in. 16.08 PF 6 - 8 in. 1200 - 2500 W 5 X 20 in. 21.58 PF 8 - 12 in. 1800 - 4000 W 7 X 30 in. 32.17 PF 10 - 15 in. 3500 - 6000 W 6 X 36 in. 37.18 PF 12 - 18 in. 5000 - 7500 W 12 X 36 in. 40.84 PF 16 - 20 in. 6500 - 10000 W 8 X 48 in. 49.60 PF 18 - 22 in. 8000 - 12000 W 12 X 48 in. 51.80 PF 21 - 25 in. 10000 - 15000 W 12 X 60 in. 63.33 PF 23 - 28 in. 13000 - 18000 W 20 X 60 in. 68.10 PF 26 - 30 in. 15000 - 20000 W SPHERE TERMINAL CAPACITANCE CHART Sphere diameter Sphere capacitance 2 inch 2.8 P.F. 4 inch 5.6 P.F. 6 inch 8.4 P.F. 8 inch 11.2 P.F. 10 inch 14.0 P.F. 12 inch 16.8 P.F. 15 inch 21.0 P.F. 18 inch 25.2 P.F. 20 inch 28.0 P.F. 25 inch 35.0 P.F. 30 inch 42.0 P.F. 35 inch 49.0 P.F. 40 inch 56.0 P.F.
APPROXIMATE SPARK LENGTH VRS POWER INPUT CHART SPARK LENGTH POWER INPUT 6 - 8 INCHES 300 W 8 - 12 INCHES 500 W 12 - 18 INCHES 700 W 15 - 20 INCHES 1000 W 18 - 24 INCHES 1200 W 24 - 30 INCHES 1500 W 30 - 36 INCHES 2000 W 36 - 45 INCHES 2500 W 45 - 56 INCHES 3000 W 56 - 64 INCHES 3500 W 64 - 72 INCHES 4000 W 72 - 80 INCHES 4500 W 80 - 90 INCHES 5000 W
