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Induction heater (featuring my new PLL driver :-O)

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uzzors2k

Tue Oct 21 2008, 12:23PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Air core would work, but it would spread the field out everywhere and be rather large in order to keep the amount of turns down. In a pinch it will work fine, but it's not something that's easy to build into a compact setup.

Update news on this project: I've got a piece of copper tubing, and a soon to be obso1337 water cooling pump. Oh, and two-hundred 22nF capacitors. I'll wind a tighter coil with only a few turns this time, and see if I can't melt some metal.

Hazmatt_(The Underdog)

Wed Oct 22 2008, 08:25AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I think that's awsome! I'm trying to push RF power right now too and its really difficult sometimes, my case I need clean sine, but I think induction is really cool too.

Keep it up! Hope to see an RF induction foundry soon! ^^

Raka

Tue Dec 02 2008, 06:53PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
Location:
Posts: 38

Uzzors I just build your circuit, and Im very impressed with it. Id like to ask a couple of questions about it. I noticed that you supply the 4046 with 12V, according to the datasheet the maximum supply voltage should be 7V. Have you encountered any problems with that? Secondly, the resonant freq of my circuit seems to increase as I increase the voltage on the supply rail with my variac. Is that normal? The frequency goes to high for the LM393 to keep up. Lastly, Im wondering what can be done to make the work coil a bit saver. It seems rather dangerous to have a coil with several hundreds of volts bare

uzzors2k

Tue Dec 02 2008, 10:21PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Welcome to the Forum, Raka! You must have used a 74HC chip, they're only good for 5V or 7V max. The standard type works up to 15 or 20V. I can't remember how the frequency varies with power on mine, but the change in resonant frequency sounds normal. Does it keep locked, or is it regulating when the frequency increases? What's the resonant frequency of your tank circuit? When locked the waveform will be a clean sinewave. For shielding the coil from an accident simply placing the it in a ceramic flower pot would work.

Raka

Wed Dec 03 2008, 06:02PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
Location:
Posts: 38

Thanks for the speedy reply. My biggest handicap is that I donâ€™t have an oscilloscope, so I canâ€™t say for sure if the circuit is locked. Ive measured for flyback current through the protection diodes, across my FETâ€™s and there is practically no current, so I think it must be locked to resonance. Also, the frequency is very stable and reacts to me placing a load in the coil, which makes me trust that the circuit is locked. I calculated the resonant frequency to be 187KHz. So I made that the free running frequency of the 4046. When I increase the supply rail voltage, the frequency starts approximately at 187 KHz and gradually increases to 191 KHz as the supply rail goes from 0 to about 80V. Any more power and the frequency drop rapidly and become unstable. If Im not mistaken that is due to the LM393 that cant keep up. If there is a sizable load in the coil this does not happen and the power can be increased all the way to 320V. Mabye the current limiter needs tuning? As for the ceramic pot to isolate the coil: I was also thinking in that direction, but Ive never seen any photos of people isolating the work coil in that manner and that made me wonder if there is some other reason for not covering the coil.

uzzors2k

Wed Dec 03 2008, 10:07PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Sounds more like the tank voltage limiter kicking in. It's a pity you don't have a scope to watch the circuit work, but a load "loads" down the tank circuit, reducing the voltage across it. Counterintuitively, a load also reduces the current drawn from the inverter. Do you know how large your PLL range is? It should go up to 250kHz or so, just to give enough regulation margin. You are right about the LM393 of course, and unless you want to lower the tank's resonant frequency finding a faster op-amp might be a good idea. The LM393 was sort of usable at 100kHz, but at twice that the delays starting making up 25% of the waveform.

GeordieBoy

Thu Dec 04 2008, 04:57PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> It seems rather dangerous to have a coil with several hundreds of volts bare,

The induction heating voltages and currents themselves are relatively safe due to the high-frequency, at least for small systems. However the work-coil is not safe to touch if it is connected through the inverter to the mains line with no isolation. Such a connection imposes low frequency mains voltage on top of the HF induction heating voltage and presents a significant shock hazard.

The easiest ways to achieve galvanic isolation between the mains line and the work coil are:

1. Use a 50Hz mains transformer to supply the inverter.

2. Use an HF ferrite transformer between the output of the inverter and the input of the matching network.

In the later case the transformer will be smaller and lighter. It also has the additional benefits that it's Secondary-side leakage inductance partially contributes to the L-match inductance, and multiple taps can be employed to adjust impedance matching.

In the case of the HF transformer placed as stated, the huge circulating current in the work-coil is kept out of the ferrite transformer. This eases the transformer design because water cooled windings are not necessary.

-Richie,

Raka

Thu Dec 04 2008, 07:23PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
Location:
Posts: 38

Thanks for all the advice. I recon the best thing to do would be to wind more windings on the work coil, there is room for plenty since I only have four windings. At the moment I canâ€™t do much testing since Im busy fitting the circuit into a box, and I need to have the circuit at my workshop. Ill try to post some pics during the weekend to add some colour to the topic.

Richie, how suitable is the ferrite core found on a cathode ray tube of a television for isolating the work coil from mains. I have access to loads of those.

Have a lovely weekend

GeordieBoy

Thu Dec 04 2008, 07:54PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

It is core-cross section and material grade that is important for power handling. That found in the line output stage of a TV is likely not capable of handling the required power. It will also likely overheat if run much above its design frequency of 16kHz.

Something like an ETD44 or ETD49 core set in 3C90 or 3F3 material should be good for more than a kilowatt when wound with litz.

-Richie,

Raka

Sat Dec 06 2008, 02:12PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
Location:
Posts: 38

Hereâ€™s the promised pics

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