The proposed induction kadai circuit design presented here is just for experimental purpose and may not serve like the conventional units. It may be used for making a cup of tea or cooking an omelet quickly and nothing more should be expected.
The referred circuit was originally designed for heating iron rod like objects such as a bolt head. a screwdriver metal etc, however with some modification the same circuit can be applied for heating metal pans or vessels with convex base like a "kadai".
For implementing the above, the original circuit wouldn't need any modification, except the main working coil which will need to be tweaked a bit to form a flat spiral instead of the spring like arrangement.
As an example, in order to convert the design into an induction cookware so that it supports vessels having a convex bottom such as a kadai, the coil must be fabricated into a spherical-helical shape as given in the figure below :
The schematic would be the same as explained in my previous article, which is basically a Royer based design, as shown here:
L1 is made by using 5 to 6 turns of 8mm copper tube into a spherical-helical shape as shown above in order to accommodate a small steel bowl in the middle.
The coil may be also compressed flat into a spiral form if a small steel pan is intended to be used as the cookware as shown below:
L2 may be built by winding a 3mm thick super enameled copper wire over a thick ferrite rod, the number of turns must be experimented until a 2mH value is achieved across its terminals.
TR1 could be a 20V 30amp transformer or an SMPS power supply.
The actual induction heater circuit is quite basic with its design and does not need much of an explanation, the few things that needs to be taken care of are as follows:
The resonance capacitor must be relatively closer to the main working coil L1 and should be made by connecting around 10nos of 0.22uF/400V in parallel. The capacitors must be strictly non-polar and metalized polyester type.
Although the design may look quite straightforward, finding the center tap within the spirally wound design could pose some headache because a spiral coil would have an unsymmetrical layout making it difficult to locate the exact center tap for the circuit.
It could be done by some trial and error or by using an LC meter.
A wrongly located center tap could force the circuit to function abnormally or producing unequal heating of the mosfets, or the entire circuit may just fail to oscillate under a worst situation.