WARNING: This circuit analysis deals with high-voltage, high-current induction heater power stages. Touching one of the HV nodes (600V @ 30A) of these types of circuits would probably be instantly lethal. We take no liability for your actions, so if you don’t know what you’re doing – stay away!
Hello Fellow ‘Schemers,
Today we’re looking into the guts of an induction cooktop. For those of you not familiar with induction cooking, the theory is as follows:
By running a big current through a coil, you can induce an incredibly massive current in a ferromagnetic (say, steel) pot sitting on top of that coil. Since the steel is very conductive, there is essentially no voltage built up across the metal but plenty of current still flows. This heats the pot through resistive heating and eddy losses, transforming the pot itself into the heating surface. Cool stuff. Want to heat a pan? Apply power directly to the atoms of the pan through electromagnetic tomfoolery. Go check out Induction Heating at Wikipedia for a more complete description.
Induction heating has become a meandering side interest here at oschemes, due to our desire to build a sweet induction forge for a metalsmithing buddy. The web has a nice selection of induction heating (IH) projects, including
- Neon John’s Open-Source IH. Pretty fantastic, and he has a kit or prebuilt devices for instant gratification
- Richie Burnett’s IH Discussion and Projects. IH 101 and 102 all in one page. Brilliant description, beautiful theory and plenty of sims and scope shots to illustrate the details. Plus killer glowing metal pics to show that he ain’t screwin around.
- Uzzors2k PLL Based IH. A self-tuning masterpiece. Really great stuff for when you’re ready to expand your mind to orthogonal V/I phase control.
All of those pages have additional links to even more information, so enjoy! We’ve built a couple self-biased Royer Oscillators at a hundred or so watts, and have gotten metal hot enough to burn ourselves. Ow! Dammit, it works! But never any orange-glowing slag or anything else deliciously lethal – that’s next.
So as a starting point for our own designs, we always like to see what’s going on in the industry. Sure the Bosch’s and the Westinghouses of the world can build some cool stuff, but what about the minimalistic approach? High power, low cost! For that, we decided to take a look into a low-priced induction cooktop available around the web. Ours was purchased from Amazon for $70, the Max Burton Model 6000 1800W induction cooktop.
Fig 1 – Yum, Looks like lemon and corpse soup!
As an appliance… Well there’s a reason this model’s on the sale rack. The switching noise (high pitched whine) is earsplitting for those of you that can hear it, and is modulated by the position of the pan. In other words, if you were to wiggle your pancakes or stirfry pan around the noise will cycle from inaudible to teeth-grating and back. Have fun seeking the perfect pan position where the noise is tolerable! It seems to get better after warming up, but it is never silent.
There’s also a weird startup noise that sounds like the device is ramping the frequency (phase?) back and forth to try to check the resonance. This seems like a legitimate technique to us and probably a necessity with jerky customers moving their pancake pans around. But please, designers – continually seek resonance, don’t sweep it 4 or 5 times and then have to resweep whenever the pan moves or is lifted. Ugly!
All bitching aside, this piece of crap can certainly deliver power to a load. It will boil a small pool of water almost instantly (err, after the 3 seconds required to find resonance) and can even heat thin copper such as PCB traces if you trick it by having a pan partially on the coil area. That’s interesting because non-ferrous metals are usually hard to heat at the 20kHz frequency this thing seems to run at, but fun nonetheless. Not sure if it could deliver enough power to reflow a PCB but there’s probably risk of blowing up your components from induced currents, so that kooky idea will have to live on the shelf a while
Let’s look a little further, shall we?
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