The principle of induction heating is simple. The coil creates
high-frequency magnetic field and metal objects in the the coil 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 neon lamp.
The frequency can be controlled in the range of about 20 to 200 kHz.
The control circuit requires auxiliary voltage of 12-15Vdc. I am using a small wall outlet adapter, but
because only few mA is needed, you can use precipitating resistor or capacitor.
Because the output driver can not be connected directly, there's a matching choke in series.
It has about 20 turns 1.5 mm diameter on a 8x10 mm ferrite core and the power can be set by adjusting the
air gap. The induction heating is powered directly from mains.
It is using full-wave rectified voltage without filtering electrolytic capacitor.
A light bulb is connected in series to limit the current and save the circuit in fault condition,
overload or bad tuning.
Work coil of induction heating must be very strong copper wire or better a copper tube and has about 12 - 30 turns on a 3 - 10 cm diameter. Resonance apacitor is due to the large current made of many (at least 6) capacitors. In my case, the coil has 26 turns and capacity is 6 x 330n 250V~. Both are gettinh very hot after longer operation. The resonance frequency is about 29 kHz. My resonant circuit is assembled quicky, with a little more experimentation you can achieve much better results. This is my first attempt at induction heating.
I also recommend:
Induction heating II.
Induction heating III. with IGBT