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Induction heating II.

´      The principle of induction heating is simple. The coil creates high-frequency electromagnetic field and metal objects in the coil or nearby induces eddy currents that are heating it. The hysteresis losses also contribute to the heating. Even for a small coil like this a current about 100A is needed, therefore, in parallel with the coil there's a resonant capacitance, which compensates its inductive character. Coil-capacitor circuit must be driven at its resonant frequency. The drive current is much smaller than the current through the coil. The power supply is a simple MOSFET Half Bridge controlled by circuit IR2153. The MOSFETs have a small heatsink. Operating frequency is tuned to resonance by potentiometer. The resonance is indicated by a LED diode. The frequency can be tuned in the range of about 22 to 90 kHz. The control circuit needs some auxiliary voltage of 14-15V AC adapter, but due to only few mA current you can use a simple supply with a power resistor or a capacitor. Because the output driver can not be connected directly, there's a matching choke in series. It has 14 turns wound using four wires of 1mm diameter. It has s ferrite core with an effective cross-section 12 x 39 mm. By adjusting the air gap the power can be set. The induction heating is powered directly from mains. It is using full-wave rectified voltage without filtering electrolytic capacitor. A light bulb cca 1000W is connected in series to limit the current and save the circuit in fault condition, overload or bad tuning. The bulb acts like a variator. MOSFETs are located on heatsinks originally intended for older processors.
     Work coil is made of wire diameter 2mm. Coil glows red after prolonged operation. Better would be a copper tube that allows the water cooling. Coil has 11 turns, diameter 23 mm and height 30 mm. Capacity is due to the large current made of many parallel capacitors (I am currently using 37 psc in parallel, the total capacity is 8u6). The resonant frequency is 44kHz.
     I also recommend:
Induction heating III. with IGBT

     Warning! The induction heating circuit is electrically connected to the mains and contains lethal voltage! Use a potentiometer with plastic shaft. High-frequency electromagnetic field can be harmful and can damage to electronic devices and storage media. The circuit is causing significant electromagnetic interference. It may cause electric shock, burns or fire. Everything you do at your own risk. I do not take responsibility for any of your harm.

Schéma indukčního ohřevu II.
Induction heating II. schematic

Thieves dare you touch it? :-P

... what are the Chinese screwdrivers good for :)

screws and rods can be easily bent after heated.

The voltage at the halfbridge output, sine wave at the work coil.

Napětí na tlumivce při buzení pod rezonancí, nad rezonancí a v rezonanci

matching inductor current at resonance off resonance
waveforms on this scope

Aluminium subjects bend and deform under its own weight after heated.

Working coils, inductor and capacity (before adding more capacitors)

the entire induction heating

The halfbridge

dismantled matching inductor. Core is made of four E pieces with 12x19mm effective cross section.

Video (Staccato added :))