Intro: Leyden Jar Array for Tesla Coils
A number of people have asked how I made the Leyden jar array used with the "Tabletop Tesla Coil." In this separate Instructable I offer a detailed explanation of how the battery was made and what it can do.
Historical footnote: Ben Franklin created the term "Leyden battery" to describe grouping a number of Leyden jars together. He made the analogy to a battery of cannons. More cannons=more boom, more Leyden jars=more zap.
Leyden jars are the oldest form of capacitor. Basically they capture and store electricity, releasing it for use by other components in he circuit. The Leyden jar was invented in 1744-45 by two men, working independently: Ewald von Kleist and Pieter van Musschenbroek. Their original version used a glass jar filled with water. Basically a capacitor consists of two conductive surfaces separated by a dielectric (an insulator). The early Leyden jar was a glass jar filled with a saline or acid solution. A metal terminal passed through the top of the jar into the water; the outside of the jar was coated with metal foil.
In 1899, Nikola Tesla used banks of liquid-filled mineral water jars as the capacitor array for his high voltage radio and power transmission experiments in Colorado Springs. Modern Tesla coil builders often use homemade capacitors in their projects, usually made from beer bottles, aluminum foil, and salt water. There are other kinds of homemade capacitors--glass plates sandwiched with layers of foil, rolled sheets of polyethylene and foil, etc. The trend among coilers seems to be away from homemade capacitors and using instead "MMCs," or Multiple-Mini-Capacitors. These are arrays of high voltage commercial capacitors arranged in series and parallel to give the desired capacitance and voltage rating. Problem is, high voltage capacitors can be expensive and hard to find. When you do find high voltage caps, they may not be suitable for Tesla coil use, as the high frequency pulsations of power through a Tesla circuit impose severe stress on the components.
Because I'm cheap and have a long standing interest in old-fashioned technology, I decided to develop a dry, non-liquid Leyden jar that would work with a Tesla coil. I wanted a capacitor made of easily obtained materials that was sturdy, effective, and cheap. What follows is the design I've come up with so far.
Step 1: The Materials
Modern experimenters are lucky. We have access to materials early electricians could not imagine. Our households are populated with all sorts of interesting plastics, metals, and artifacts useful for high voltage experiments. In an earlier Instructable I described my Soda Can Leyden Jar. The basic unit of the Tesla Coil Leyden Jar array is very similar to the previous design.
To start with, you will need:
-a number of powdered drink mix canisters (Crystal Light or its generic equivalent)
-an equal number of aluminum soda cans (brand does not matter)
-aluminum ductwork tape (do not use silver fabric duct tape--it will not work!)
-plain, uncolored and unscented wax or paraffin; a pound will seal 6 of these jars
-16 gauge (or better) electrical wire; about 6 inches per jar
-large rubber bands
-a polyethylene storage box large enough to hold the array.
Everything needs to be clean, dry, and label-free. The exception are the soda cans. Soft drink logos and other graphics are usually applied as sleeves to the sides of the can. The top and bottom of the cans are bare metal. They require only a quick wipe with a solvent like acetone or xylene to get them clean and ready for electrical connection.
The drink mix canisters are made of polypropylene (PP). They measure 6.5 inches high ( about 16.5 cm) and 2.625 inches in diameter ( 65mm). They are straight side cylinders. The plastic is fairly thin and quite strong.
There are a number of brands of aluminum tape. For ductility and strength I like Henkel's Duck Brand HVAC Metal Repair tape. I prefer the two inch wide tape. You can find aluminum tape in 3 inch widths, but 2 inch gives you greater flexibility in how much surface you cover.
The 16 gauge wire is rated for 600 volts. I salvage all I need from old microwave ovens, CRT monitors, and old TVs. All the lengths you will use will be pretty short, so all those odd pieces you salvage will be useful. You can buy wire, of course. Get at least 16 gauge, at least 600 volts.
Wax is used to insulate the top of the soda can after the jar is assembled. Be sure to use plain unscented wax or paraffin. Wax is a good insulator, but dyes and scented oils may not mix well with high voltage electricity.
Rubber bands help bind the cluster of jars together. It is important the outside foil coatings of the jars make good contact with each other.
Since you will be running anywhere from 6,000 volts and up from a transformer through the Leyden jars, you'd better have a good insulated container for them. Storage boxes made of HDPE (high density polyethylene plastic) work well.
Step 2: Material Preparation
Remove the drink mix tubs from the canister. (Drink the lemonade at your leisure!) Discard the plastic cap--we won't need it. Remove the paper label from the canister. This is usually pretty easy, as glue does not stick to polypropylene too well. Labels often just peel off. Wash and dry the canister thoroughly.
Choose a soda can that is free of dents or creases. Physical damage can distort the flow of electricity in and around the Leyden jar. Remove and discard the pull tab on the soda can. Wash and dry the can well. I bake the soda cans in the oven for 10-15 minutes at 300 degrees F. to insure they are completely dry. When you are certain the interior of the can is dry, seal the opening with a square of aluminum tape. Cover the hole completely and smooth the tape down, avoiding wrinkles.
Step 3: Insert the Soda Can
A standard size soda can will not fit into the lemonade canister as-is. You must prep the canister by poking a small hole in the bottom. This will let out the air trapped by the can as it goes in. Next, use a hair dryer on the High setting (1500 to 1875 watts) and blow hot air into the canister. Heat the outside as well, slowly rotating the canister to evenly distribute the heat. Don't overdo it, or you'll warp the plastic. When the canister feels fairly pliable, insert the soda can top side down. Push firmly and slowly to allow the air to escape. If the can gets stuck, don't force it. Warm the canister with the hair dryer until you can get the soda can pushed all the way down inside. When the plastic cools you will have a tight fitting interior electrode.
Patch the air hole you punched with silicone caulk (I use GE Silicone II, but any vinegar-smelling silicone sealer will do).
Step 4: The Outside Coating
With the paper backing still on, wrap aluminum tape around the canister and measure for length. Overlap the ends seam by half an inch. Cut two identical lengths of metal tape for each Leyden jar you are making. (You can make square cuts on the tape by folding the tape at the point you wish to cut, making sure the side edges of the tape align. Then pull a sharp knife through the fold for a straight cut. Don't use scissors; you'll get crooked cuts every time.) Peel off the paper backing and apply the first strip around the bottom of the canister.
Polish out the wrinkles in the tape with a wooden dowel, round pencil, or even a round chopstick. Don't use a metal tool or you will tear the foil tape. With light but even pressure rub the dowel back and forth over the tape, smoothing out all wrinkles and bubbles. Keep your edges straight and avoid creases as much as possible. Thin, sharp edged conductors tend to radiate high voltage electricity as "corona." This will rob your efficiency and may lead to short circuits and arcing.
When the first piece is smoothed, remove the paper backing and apply the second. Overlap the edges of the first and second pieces of tape about 1/4 inch. This will insure good contact between the pieces and cover the canister to a height of 3.75 inches.
Step 5: Linking the Jars Together
Now that you have made one jar, make a bunch more. If you made the jar as described, you will find you have a single Leyden jar with a capacitance of between .45 to .50 nano-farads (450 to 500 pico-farads). I have made almost 50 of these and measured every one with digital hand meter and always got the capacitance described.
The voltage rating is more of a guess. I connect these jars to my Wimshurst static machine and crank it until the jar spontaneously discharges between the electrodes. I estimate, based on the length of the sparks emitted, that the jar is good for at least 100,000 volts DC. For AC you have to reckon on half that value or less, but 40 to 50,000 volts AC is pretty good.
Group your jars together in a convenient shape. You can nestle them like this
or you can square them up
Either way is fine. Nestling insures good contact and results in a stronger physical structure.
Once you decide on the form, cut a bunch foil tape strips about five inches long. Before you remove the paper, crease them in the middle with the foil on the inside; this makes a V. Peel off the tape and careful spread the tape V over two of the Leyden jars. They should be close, but they don't have to be too rigidly joined. The tape will tear unless you incorporate some give. Do this again and again until all the jars' outside coatings are linked by strips of tape. Use your dowel to smooth down the tape strips. At some point clean the foil surfaces with a light solvent cleaner like Goo Gone to get rid of the excess mastic from the tape. If you leave the stickum on, it will attract dust and arc dangerously at high voltages.
Now you will need lengths of 16 gauge (or better) stranded wire. The wire has to reach from the top of one soda can to the next, so an 8 inch length is about right. Strip 1 inch of insulation off both ends of the wire and fan out the strands. Take a 1 inch square piece of foil tape and tape the fanned wire to the top of the soda can. Smooth the tape down firmly. Repeat until all the jars are linked by wires. Cut a second length of wire at some convenient length--10, 12 inches and fan just one end. Tape that to one of the Leyden jar tops at the end of your array. This is your high voltage output. Take a second wire of equal and fan one end. Cover it on both sides with a 2 inch square of foil tape (sticky side to sticky side). This will be your high voltage input, which we will attach later.
Step 6: Wax Insulation, and Rubber Bands
Get a handful of good quality rubber bands. Size #64 is good. Loop them together into a long strand that will fit around the Leyden battery, holding it together snugly but no so tight it deforms the jars or tears the foil. Make two identical strands, one for the top and one for the bottom of the array. Once the rubber bands are in place, slip the input lead you made (the one with the foil tape applied to the fanned wire) through the rubber band strands, making sure the foil pad is in good contact with the aluminum coating on the outside of the jars.
Put the Leyden battery in a suitably sized insulating box. I use a polyethylene garment box. Your Leyden battery is now ready for use. Depending on how many jars you made, your capacitance should come close to these values:
For my 18 jar battery I get a value of 8.05 nF.
For my 16 jar battery I get a value of 6.23 nF
My 6 jar battery made with peroxide bottles is good for 2.29 nF
I have connected the first two batteries together and gotten over 14 nF.
That's my Leyden jar array. It works well with my small Tesla coils. They're kind of bulky, but lightweight and very cheap. With recycled lemonade canisters, soda cans, and wire salvaged from microwave ovens, these high voltage Leyden batteries are very inexpensive. If you put them together with care, they should last a long time and give good service.
One warning: None of my coils runs at higher than 30 mA. I have never experienced any heating effects with my Leyden jars, but at higher amperages this could happen. Polypropylene is not that resistant to heat. If you are building a fire-breathing coil, this might not be the setup for you.
I would like very much to hear from other experimenters who build and use a battery like this.