Most of the DIY induction heating projects I have seen tackle the entire power supply from scratch (including voltage and current regulation). My goal is to take advantage of my Harbor Freight 80A inverter constant-current DC stick welder as the power source. This means that the hard part will be inverting the high-current DC to high-current high-frequency AC useful for magnetic-induction heating.
I will use the IRS2453DPBF-ND N-MOSFET H-bridge driver chip from IR to drive the high-current IRFP4310ZPBF-ND N-MOSFETs. My initial approach was to use a LM555CNFS-ND timer chip, a center-tapped work coil, a 497-3110-ND MOSFET for inverting logic, and only two power MOSFETs for the inverter. However, the work coil will not have to be center-tapped if I use the H-bridge driver chip, which should result in better heating and the smallest possible induction coil. If used motor oil turns out to be non-conductive, I will use it in a heat exchanger that pumps oil through the work coil. The work coil will be insulated from the crucible by refractory cement. Otherwise, I will use deionized water.
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After a two-year break, I just found the H-bridge driver and MOSFETs I ordered back in 2009 when I started working on the design for this. Between that and a recent email from someone who came across this page, I want to start working on this project again, but with my Lincoln AC-225 as the constant-current power source. The AC-225 has 100% duty cycle at the 75A setting; I assume they put those windings on the transformer last for better cooling. The only problem is that this is not a nice smooth DC welder. I need a full-wave bridge rectifier capable of 75A continuous. After I calculated the size of a smoothing capacitor and inductor for this current level, I quickly abandoned the idea of filtering the rectified AC-225 output into smooth DC power. After much consideration, I think the idea will still work with the large unfiltered ripple voltage. If the H-bridge inverter runs somewhere in the neighborhood of 30kHz, the 120Hz voltage ripple should not matter because the inverter frequency is about 250x higher. Also, the IRS2453 data sheet lists an allowable offset voltage slew rate of 50V/ns, which is plenty fast. Here is how the high-frequency inversion would progress from the welder output to the induction coil at open-circuit voltage:
60Hz AC Welder Output
120Hz Rectified Welder Output
30kHz Inverted Output 1/60s
30kHz Inverted Output 1/120s
30kHz Inverted Output 1/240s
30kHz Inverted Output 1/480s
The 30kHz high-frequency AC is what allows power to be transferred from the induction coil to the metal you are trying to heat. Plain 60Hz AC cannot transfer power through the air to relatively small parts efficiently. If the induction coil has 5 turns, it forms a 5:1 transformer with the heated metal. Ignoring magnetic hysteresis heating effects, 75A RMS input current ideally becomes 75*5=375A in the part as eddy current heating.
Test Circuit With 15V DC Input and Resistor Load
It took some tweaking, but I got a test circuit working and inverting 15V DC to 15V AC @ 35kHz. Here is the circuit I prototyped:
Here is the whole mess.
Here is a close-up of the breadboard.
Here is a close-up of the oscilloscope showing the high frequency AC.
Test Result With Simple Coil and Small Capacitor
I created a small coil out of some 12AWG speaker wire, attached a small capacitor, and tried it out. I managed to heat up a screwdriver until it was hot to the touch, but I noticed my alligator clips were heating up from the 5-8 A current. So, I used thicker wire and started playing with the frequency by changing the resistor and capacitor in the timing circuit. I managed to increase the power transferred into the screwdriver quite a bit and the circuit was drawing about 10A. I replaced the screwdriver with a large nail and got it hot enough to melt plumbing solder. There was no magic red-hot glow. The capacitor and coil I used were lame and I did not carefully adjust the operating frequency to maximize the power transfer. I was also pushing my luck with my non-current-limited power supply. It seems that a self-oscillating design like this simple ZVS Royer induction heater is a better idea than the one I initially came up with. Here is another self-oscillating induction heater project.