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Step 2The secondary coil
The secondary coil in a Tesla coil is generally a long coil of fine wire. It is not connected to the primary coil, but picks up current from the primary by induction.
For this tabletop model I used a heavy duty cardboard cylinder formerly used in a roll of commercial paper towels. It's about 3/8ths of an inch thick, 1.75 inches in diameter, and 12 inches long. You can also use PVC tubing, or any other type of insulating plastic. Avoid colored tubing. Sometimes the dye is conductive, which will cause your project to fail. Avoid black tubing. It often has carbon in it, and carbon conducts electricity.
If you use cardboard as I did, coat it well inside and out with several coats of polyurethane varnish or shellac. These help stabilize the cardboard and greatly improve its insulating properties.
Once your secondary form is hard and dry, apply two strips of double-sided carpet tape to the outside, 180 degrees apart. This is very sticky tape used to hold carpets down. On a 12 inch tube, I used two strips of tape, 11 inches long each. This will help hold your secondary wire in place as you wind it.
The wire I used is 24 gauge magnet wire. It is solid copper with a coat of enamel insulation. Start winding half an inch from the end of the tube by taping down the wire (temporarily) and winding slowly and carefully, keeping the wire smooth and snug. Do not cross the wire at any time, or your work will come to naught. It's boring, but I wound the whole secondary by hand in about 80 minutes while watching TV. I wouldn't bother with winding jigs, etc. unless you plan to make several coils.
You can use other fine gauges of wire--22, 26, 28, 30, etc.--and the wire you choose will affect the resonance of your coil. I had 24 gauge on hand, so that's what I used. There are 515.9 turns on this coil, for a total of 249.7 feet of wire.
Once your secondary is wound, paint it several times with clear polyurethane or shellac. Let it dry thoroughly between coats. Some coil makers put on many, many coats of varnish. This both insulates and glues the wire to the form. I put on four coats of shellac. That seemed like enough.
Carefully remove the anchoring strips of tape from each end. I used a bead of hot glue to fix the wire to the form. Leave about eight inches of wire free beyond the glue. These will connect to your terminals.
(The secondary form is supported by PVC plumbing caps inserted inside. I spent quite some time at the home center trying out different bits of plumbing connectors and caps to find ones that fit the cardboard tube nicely, but I eventually found perfect pieces. )
Feed the wires through the holes bored in the upright supports. Scrape off the enamel insulation (I used fine sandpaper for this) from the last three inches of the wire. Push the tubing and PVC cap up, and attach the bare wire ends to each terminal. Solder if you want. I used aluminum HVAC tape, which is strong and conductive. Push the supports back into top T joints. The ends of the terminals will be supported by the cork or other stopper previously inserted into the frame.
Before you push the PVC caps in place, you may want some friction pads to keep the terminals from swinging around two easily. I cut a rubber stopper in half lengthwise and put one piece in each upright. With the caps in place, the terminal rods are jammed against the rubber, which allows fairly good adjustment of the terminals.
See the Terminals page for details of making them.
For this tabletop model I used a heavy duty cardboard cylinder formerly used in a roll of commercial paper towels. It's about 3/8ths of an inch thick, 1.75 inches in diameter, and 12 inches long. You can also use PVC tubing, or any other type of insulating plastic. Avoid colored tubing. Sometimes the dye is conductive, which will cause your project to fail. Avoid black tubing. It often has carbon in it, and carbon conducts electricity.
If you use cardboard as I did, coat it well inside and out with several coats of polyurethane varnish or shellac. These help stabilize the cardboard and greatly improve its insulating properties.
Once your secondary form is hard and dry, apply two strips of double-sided carpet tape to the outside, 180 degrees apart. This is very sticky tape used to hold carpets down. On a 12 inch tube, I used two strips of tape, 11 inches long each. This will help hold your secondary wire in place as you wind it.
The wire I used is 24 gauge magnet wire. It is solid copper with a coat of enamel insulation. Start winding half an inch from the end of the tube by taping down the wire (temporarily) and winding slowly and carefully, keeping the wire smooth and snug. Do not cross the wire at any time, or your work will come to naught. It's boring, but I wound the whole secondary by hand in about 80 minutes while watching TV. I wouldn't bother with winding jigs, etc. unless you plan to make several coils.
You can use other fine gauges of wire--22, 26, 28, 30, etc.--and the wire you choose will affect the resonance of your coil. I had 24 gauge on hand, so that's what I used. There are 515.9 turns on this coil, for a total of 249.7 feet of wire.
Once your secondary is wound, paint it several times with clear polyurethane or shellac. Let it dry thoroughly between coats. Some coil makers put on many, many coats of varnish. This both insulates and glues the wire to the form. I put on four coats of shellac. That seemed like enough.
Carefully remove the anchoring strips of tape from each end. I used a bead of hot glue to fix the wire to the form. Leave about eight inches of wire free beyond the glue. These will connect to your terminals.
(The secondary form is supported by PVC plumbing caps inserted inside. I spent quite some time at the home center trying out different bits of plumbing connectors and caps to find ones that fit the cardboard tube nicely, but I eventually found perfect pieces. )
Feed the wires through the holes bored in the upright supports. Scrape off the enamel insulation (I used fine sandpaper for this) from the last three inches of the wire. Push the tubing and PVC cap up, and attach the bare wire ends to each terminal. Solder if you want. I used aluminum HVAC tape, which is strong and conductive. Push the supports back into top T joints. The ends of the terminals will be supported by the cork or other stopper previously inserted into the frame.
Before you push the PVC caps in place, you may want some friction pads to keep the terminals from swinging around two easily. I cut a rubber stopper in half lengthwise and put one piece in each upright. With the caps in place, the terminal rods are jammed against the rubber, which allows fairly good adjustment of the terminals.
See the Terminals page for details of making them.
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