• While experimenting with flyback transformer circuits I found a simple induction heater circuit and decided to give it a try.

    Induction heating uses a high frequency electromagnetic field to induce eddy currents in a piece of conductive material. The current and the resistance of the material lead to ohmic heating. To generate the high-frequency field a self resonant oscillator is used. The circuit mainly consists of two MOSFETs, a tank capacitor and a center tapped coil.

    Circuit Schematic

    Circuit Schematic

    For a detailed description of the circuit operation see the source page.

    I did not make a PCB for this circuit but instead soldered it on a piece of perfboard. This is not optimal in regard to trace resistance because the solder has about 7.5 times the specific resistance of copper, but it works ok. An improvement would be to solder some thick copper wires between the components.

    Induction Heater

    Induction Heater

    My circuit resonates at approx. f=115 kHz (no load inside the coil). With a capacitance of C=440 nF the inductance L of the coil can be calculated by the following formula

    Induction Formula

    to be approx. 4.3 µH.

    With an input of 15 V at 660 mA the voltage over the coil is 92 Vpp.

    Voltage across the Coil

    Voltage across the Coil

    Heating a bolt or nail to a red glow takes about 5 minutes with an increased current draw of approx. 3 A.

    Nail heated by Induction

    Nail heated by Induction

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  • 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

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