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Thanks to your very helpful suggestions (davekni and Mikkelson), I have revised the induction heater circuit in such a way that it works in a predictable way and can handle both low and high power situations. By getting rid of the 2nd 555 startup oscillator and using the diode clamp and hex inverter as startup oscillator the circuit was simplified. This also got rid of the startup jitter and improved the frequency range for startup signal. Here are some pictures where I tested it at high power around 5 to 6 kW bringing about 1 lb of scrap steel to curie point. In order to avoid heating of the 1W clamp diodes (1N5819) as well as the opposing 1W 9.1 V Zeners, additional series resistance was added (470 ohms), which kept these diodes cool even at high power levels and in a hot garage. At 100 Ohm series resistance, these diodes still heated up to around 40 to 50°. With no resistance, they heated up to greater than 90°C. I was able to determine this using a thermal camera while the unit was running. With 20T on the ferrite transformer, large ferrous loads would not quench the feedback signal.
Oh Btw sorry for the oversized pictures.
Modified circuit:
new current control ver 3.pdfOn a PCB (let me know if you want the Gerber to experiment with the set up)
Heating up 1 lb of scrap Fe
More heating scrap Fe
Melting copper in a graphite crucible
Current draw during scrap iron melt:
One thing I found is that the startup frequency does not matter as long as it is higher than the resonant frequency of the tank circuit. I could get resonance to start at 50 kHz with a preset startup freq of 100 kHz. So basically as long as it’s at a certain frequency and left that way then there’s no issue. There’s no need to ever touch it again (startup freq pot) unless you plan to go with different work coil frequencies that are higher than the initially set startup frequency which just involves adjusting the 200k pot.
Will keep you posted if I discover anything new with this project.
Cheers
Brian
