• After the first attempt to build a Tesla Coil failed 10 years ago I finally fulfilled this childhood fantasy. Remembering the problems of obtaining high voltage capacitors and the like I went for the Solid State (No big capacitors and spark gaps, just transistors) approach this time. Not being the most skilled person when it comes to analog, and especially high frequency, circuits I started searching on the Internet. The design had to meet some requirements: It should run on a DC power supply (i.e. not mains power) and not need any exotic parts.

    Sectional Drawing of the Tesla Coil

    Sectional Drawing of the Tesla Coil

    I finally settled for a circuit from this site (Mini Tesla Coil 3) and made my own PCB layout. This was the first PCB I made using a laser cutter to transfer the layout to the board. A blank copper-clad board was sprayed with two layers of black paint which was then etched away by the laser cutter. To completely burn away the paint the same image was lasered several times on the highest power. Afterwards the board was etched with HCl/H2O2.

    The circuit uses an LC filter which has to be tuned to the resonant frequency of the secondary oscillator circuit.  (If you choose to use it. I found that the circuit works just fine with out it. Apparently it is only necessary to filter any unwanted oscillations.) To measure the resonant frequency, connect a function generator to the lower end of the secondary coil and set it to a square wave output with 50% duty cycle. Then place a piece of wire parallel to the coil and hook it up to an oscilloscope (Also connect the grounds of function generator and oscilloscope.) Now increase the frequency of the square wave until you see a sine wave on the oscilloscope. When the sine wave has the biggest amplitude, you have found the resonant frequency. With a 9 Vpp square wave I measured a sine wave of about 90 Vpp. You have to do this procedure twice. Once with the topload and once without.

    With the frequency you can now calculate the necessary values for the inductor and capacitor via the formula

    Resonant Frequency

    Use available values to approximate your frequency as closely as possible.

    The secondary coil has 1200 windings and was wound with 0.15mm enamelled copper wire on a Ø75mm PVC drain pipe. At each end the thin copper wire is routed to the inside of the pipe through small holes and soldered to a thicker wire. The holes are then sealed with hot glue. The pipe is mounted to a wooden base which also holds the posts that hold the primary coil. The Topload Capacity is a stainless steel ball which was sold as a home decoration item.

    The primary coil is wound from Ø1mm copper wire. In my case it has about 8 windings of which a section can be selected with wire clamps. This makes quick adjustments possible. The driver circuit has four connections to the coils. One leads to the bottom of the secondary coil and is used as a feedback to measure the oscillations and drive the primary coil accordingly. This is done with the remaining wires. Voltage is applied alternately between wires 1-2 and 2-3 thus doubling the effective amplitude over the primary coil.

    Tesla Coil and Driver Board

    Tesla Coil and Driver Board

    Here are some photos of sparks I made with my coil. You can generate quite interesting effects with light bulbs and other things filled with thin gas.

    Tesla Coil sparking to my Finger

    Sparking to my Finger

    Discharges in Lightbulb

    Discharges in Lightbulb

    Discharges in Lightbulb

    Discharges in Lightbulb

    Discharges in Lightbulb

    Discharges in Lightbulb

    approx. 10cm Sparks

    approx. 10cm Sparks

    Tags: , ,

  • After building a flyback high voltage supply, which I will describe in a later post, I was experimenting with putting the electrodes into thin glass tubes. This produced much longer arcs due to the concentration of the ionized air.

    To get an even better result, I tried evacuating the tubes with a syringe.  To seal one end of the tube I placed a thick wire in one end and put a drop of hot glue around it.  Over the other end I put a piece of PVC hose and pierced the wire through it into the tube. The entryhole was also sealed with hot glue. To ensure a tight seal some air is sucked out of the tube while the hot glue is still liquid.  This way the glue fills the holes.

    Tube with sealed Electrodes

    Tube with sealed-in Electrodes

    As the glass tubes only have a volume of about 5ml and the syringe 65ml, one pull reduces the pressure inside the tube from 1bar to about 0.05bar. (p*V = const.)

    When the voltage (between 20kV and 30kV) is applied to the electrodes, a continuous channel of violet (the photo does not reproduce the color correctly)  nitrogen plasma is formed inside the tube. The plasma quickly heats the glass and the hot glue, so don’t apply the voltage too long or the seal will break. Magnetic fields will deflect the plasma channel.

    Plasma Channel deflected by magnetic Field

    Plasma Channel deflected by magnetic Field

    These small tubes quickly got boring, so I decided to make a bigger version with a Ø30mm L=100mm plexiglas tube. To seal of the ends, I turned caps on the lathe. The caps were then fitted with a hole for the electrodes and a hose connection. The holes around the electrodes are again sealed with hot glue. To get the caps airtight, o-rings were used.

    To get a low enough pressure inside the tube, the syringe has to be pulled several times. Between each pull, the tube has to be closed. This is achieved with an electric valve.

    Cap with O-Ring

    Cap with O-Ring

    Tube with Cap and Hose

    Tube with Cap and Hose

    When the pressure inside the tube is low enough and the voltage high enough, a glowing channel of plasma will form. Due to the heat it generates in the residual air, it will bulge upwards. This will heat the tube and slowly melt the plexiglas. The plasma stream can be deflected with strong magnets or by influencing the electrostatic field around the tube, i.e. by placing your fingers near or on the tube.

    Plasma Channel in Discharge Tube

    Plasma Channel in Discharge Tube

    Plasma Channel in Discharge Tube

    Plasma Channel in Discharge Tube

    With one of the smaller tubes I put a drop of ethanol (C2H5OH) inside the syringe before pulling the air out. This produced a thin ethanol vapor inside the tube and caused the plasma to glow blueish-white instead of violet.

    Blueish Glow from Ethanol Atmosphere inside the Tube

    Blueish Glow from Ethanol Atmosphere inside the Tube

    Tags: , , ,