Induction heating simplified
- A few years ago we had a number of people in the group either interested, or actively experimenting with induction heating. The subject was tossed around quite a bit and one member did a considerable amount of work in the area. We got into discussing various aspects of induction heating systems, including tank circuits, capacitors, inductors, inductive reactance, frequency as related to heating depth, curie point, and so forth............ Leaving many of us with that glazed over expression you get when you're beyond your depth. Myself being of those. For Tim, and a few others, it was a piece of cake, and I believe I still have the schematic he put out for his induction heating system........... I never built one, as electronics was very much outside my normal area of endeavor.........That's an inertia that is difficult to overcome.
In the mean time, other forces have pushed me into playing with electronics at a very rudimentary level... I have an assortment of resistors and capacitors, mosfets, bipolar transistors, 555 timer chips, op amps, comparators, triacs, etc.......... all of which I acquired cheaply over a period of about a year. Last year at this time, I had some very basic and limited knowledge of using resistors and capacitors, and that was about it. Now I don't have any hesitation about soldering timer chips capacitors and mosfets into a useful circuit. My forklift now has it's rear steering operated by proximity sensors through mosfets and a solenoid valve..... a rudimentary circuit......but a great leap for me. I have a number of other devices I've built using the same principles including a device to test Caterpillar electronic diesel injectors......... something that pretty much doesn't exist outside the rebuilder's shops.
I recently got interested in induction heating again, and discovered this extremely simple schematic that depends entirely on the tuned frequency of a tank circuit to control the input power. Two mosfets a couple of schottke diodes, a pair of resistors. There are a number of different schematics available based on this principle. The oscillation of the circuit pulls down the gate voltage alternately of one mosfet, and then the other, causing the circuit to oscillate at whatever circuit the tank (capacitor inductor circuit) resonates at. As you can see it is so incredibly simple, that just about anybody could put one together. The diagram here does not give any values.......but there are others that do. The coil on the right with the center tap is the induction coil (L1), and the one to the right of it (L2) is a choke to protect the power supply. There is a Utube video of the construction and operation of this... which does in fact give values.
While this for practical purposes is a toy with the components used here, and not capable of doing much more than superheating screwdrivers and such, it is scalable to a larger size and lower frequency needed for actual melting. The design shown on the Utube video runs at 200+khz.......... about 20 times the ideal frequency for metal melting. This is a matter of heating depth. Higher frequency produces more of a surface heating and is useful for induction hardening more than for melting.
Here's the Utube Video Link
The design in question is of interest to me in an entirely different application...... heating water in a metal pipe, but with the same basic design and a larger working coil and capacitor bank could be used for melting metal. It would take some fairly high capacity semiconductor devices to handle say 10KW at the low voltages used here.......a useful power level. I believe SCRs which are available in some very large capacities and often cheaply, could be set up in more or less the same fashion. In fact I have a welder that does exactly this.
This schematic as it is, is a very simple build any of us could do.......... a place to start without investing much money at all.