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?
Continued on Next Page… Jump to Page 2
A bunch of the images on page 4 and 5 seem to be broken, please fix.
Otherwise excellent article, thanks
Thx for the heads up!
[...] dumperoslash on SPMP8k FRMorp – USB IMG dumperfredoagain on FRMorp v1.2 – bugfix!openschemes on 1800W Induction Cooktop TeardownMost [...]
hello,
i am just interested in doing some PEMF tests on myself, the induction cooking device is a ready made PEMF device that usually retails for thousands of dollars and is much the same as far as i can tell.
but how would one safely make this device work without the metal on top to trigger the coil to work at maximum all the time?
thank you,
gene
Try our article about Manual Control of the 1.8kW Induction Cooktop. That’s exactly what we’re doing there
i read your beautiful article.
but although i am technically minded i am not familiar with electronics; if the coil is tricked to be “ON” when there is no metal on it would it heat up to a self damaging degree?
thank you,
gene
You will want to run at a low power if there is no load. If there is no load, then the voltage ringing will be higher than if there is a load present because there is no place for the energy to go. If it gets too high, it will pop the power switches. The coil is at no risk of overheating – frankly, if any wiring were to burn out it’d be the PCB but that’s still no risk.
If you are going to do loaded/unloaded operation, please consider a foot pedal to increase the power setting once you have the object to be heated at the coil.
Great stuff lads, very interesting (and entertaining!)
I came across this article whilst trying to figure out if an induction cooker with enough oomph can be modified into a small heater for silver/gold melting. The idea I had is to swap out the L~C circuit for something a bit more industrial that I can drop a small crucible into. Temps required would be around 600 to 1500 degrees C, variable as required.
The unit I have in mind is the Buffalo CE208 (3kW) which sounds like it has enough juice, but I need to figure out (with my limited understanding of electronics) if a water-cooled, tuned tubular coil/cap setup can be used as the output stage. Typical coil inductance is a few uH in normal heaters if I’m not mistaken? Not sure about frequency though (65kHz??)
Do you think this is possible without nuking the IGBTs? Or myself? =:O lol
Your input greatly appreciated!
Hmm – maybe. Metals such as silver and gold are very conductive, meaning that you have to put a TON of power into them in order for them to heat up. Also, the lower frequencies of these stoves may not be great for heating the metal directly. BUT…
If you get yourself a graphite crucible then the graphite can be easily heated by even these stoves, and you probably won’t need 3kW to melt a modest (1oz-ish) amount of silver if the crucible is sucking up the power so easily.
As for the coil design – this coil is about 50 uH whereas a few spirals of copper tubing is barely 1uH. So they are pretty much incompatible. You could use many spirals of copper tubing or find some way to insulate/cool the litz wire coil from this beast. Rewound into a cylinder shape, of course.
The best idea may be to leave the coil as a pancake, use a flat graphite crucible/ingot mold, and place said mold on top of a thin ceramic tile that is NOT touching the work coil. Then direct a fan upwards to try to keep the tile’s heat from melting the coil’s insulation. Or something like that.
Thanks for the reply, that’s given me some great ideas & potentially made the job a whole lot cheaper!
I’ve heard of indirect heating but hadn’t really given it much thought until now & that definitely sounds like the way to go. Do you think the work coil could be re-wound as a cylinder with a heat shield between it & the crucible or will this radically change the inductive characteristics of the coil?
I would like eventually to get a small system up & running with a remotely operated ‘hatch’ at the base of the crucible, & the whole lot enclosed in a mini vacuum chamber. Moulding & general metal quality (esp with silver) is a lot better this way as it’s prone to porosity & nasty oxidation bubbles.
Thanks for opening up some very cool possibilities!!
Glad to help! You can certainly re-wind the work coil as a cylinder. In fact, ripping apart the pancake and winding a coil on PVC was one of our earliest experiments on the Burton stove. Wear gloves, the damn lacquer used to hold the original coil in place is as sharp as a razor blade when you break it up by unwinding the pancake.
The vacuum chamber is a great idea, as you can heat right through the sidewalls and you don’t have to worry about vacuum sealing a bunch of hot power terminals. Are you thinking about melting and casting all in the vacuum? Just trying to imagine how to flip that hot tube upside down to get the melt into the mold. Not our area of expertise, of course.
Keep us posted on your project – we’d be happy to post your writeup here if you end up with a system you’d like to share with others.
Once again, thanks for the info & reply! I was wondering about the resin holding the coil together, thought that might be an issue & will try a hair drier to soften it up first. As for the vacuum chamber, I’m planning on incorporating the coil, crucible, trapdoor actuator for pouring/vacuum release timing electronics yada yada inside the chamber. I’m currently looking for a suitable weldable steel box section over 300mm round or square to house it all. It’s a pretty mammoth job, but the results will be well worth the effort.
Hey, how cool is this?…. I managed to get the entire guts of the 3kW Buffalo cooker without the casing etc for £56 here:
http://www.nisbets.co.uk/products/productdetail.asp?productCode=CE208 (under the Spares & Accessories tab) seems to be only available within the UK though?
One issue that I’m perplexed by is how to incorporate a thermistor in direct contact with the outside surface of the coil while avoiding a)vaporizing it by inductive heating! b)shielding it from magnetic interference & getting a false voltage from it. Any ideas? I just wanted a way to monitor the coil for overheats, but if this is as challenging as I think in this kind of environment, I may just run it & pray. At some point I want accurate control of the heat for things like annealing & tempering, so will be incorporating a laser thermometer directed into the crucible area since a pyrometer inside the coil will definitely vanish in short order! lol.
Delivery of the parts Monday, next step is the crucible & insulation. That will probably be a graphite tube with two spaced layers of ceramic tubing to support & insulate it plus fiberglass cloth shielding for the coil turns. etc etc. Will take some pics of the setup as I go.
As far as the resin – it was just broken up by forcefully unwinding the coil while wearing gloves. Heat may soften it, but maybe not as it’s intended to endure heat all the time. Watch out that the outer steel box you plan to hold this project is not too close – even with a cylindrical coil it may pick up some field and steal power away from your melt (while heating itself, of course).
That deal on the 3kW heater really is great! Keep in mind that the Buffalo device may be very different from the Burton we opened up here, but it should be no problem to mod that one for similar manual control. Looking forward to seeing pics!
The Burton heater has a thermistor stuck right in the small central hole in the pancake coil to check overheating of the hob surface. That probably picks up as much stray field as your external thermistor would, meaning that your idea will probably work fine without shielding or anything. The saving grace might be that the turns ratio between the coil and your thermistor is low (thermistor = 1 turn) as well as the fact that the thermistor being outside the coil is not well coupled so the actual current induced in the thermistor is probably nil. If your coil wiring is horizontal, then run the thermistor wiring vertically in order to cut down on the coupling.
Hey you guys, it’s me again.
Eventually got all the parts & find that the 3kW model is almost identical to the 1.8kW version, with a few components shuffled around on the mainboard. The only real difference is the control board, which has a completely different user interface system – no logic buttons on this model, according to pictures of the complete unit it’s operated with a single rotary control knob (& nothing else) which I don’t have & a 4 pin socket on the control board to attach *something*. Tracing the pinouts on the socket, to the best of my ability probably incorrectly, I arrive at what’s shown in the pic (see link at end)
As simple as that sounds, it leaves me completely stumped over how to fire up the mainboard correctly. What do you reckon that would be? A simple rotary pot, or say a 10 position selector with resistors/something else? As you can probably tell, I’m at the edge of my understanding of electronics here lol, & with hindsight I think I should have just bought a complete machine! Tried contacting Better Co for info but no response.
Lastly, the 3 pin socket on the mainboard (red circle next to the ribbon connector) – that’s for the thermistor right? Do you have a value for that component?
Thanks for your help! (and this noob apologises for pestering)
mainboard: http://i571.photobucket.com/albums/ss155/sirius_gem/MainBoard.jpg
control: http://i571.photobucket.com/albums/ss155/sirius_gem/Control.jpg
Wow, you lucked the f*ck out in that it is so similar – almost identical, just like you said! Great pics, and you’re definitely not pestering by the way – spawing new misuse of electronic gadgets is why this site is here!
The best suggestion would be to control the device manually using a potentiometer as described in our article on modding the 1.8kW cooktop, but since this mod may take a little more discussion than what we can fit here it might be a good time for us to introduce The Forum!
Why don’t you go over to the forum and start a topic about this 3kW mod. You can either place it in the Site Projects area under a topic like “1.8kW mod for 3kW hob”, or under the Your Projects area with whatever name you choose. We’ll keep an eye out for your post (or you can msg us here) and then we’ll get into the gory details of how to control this beast. Looking forward to it!
AHA! Manual control… AWESOME! I bookmarked that at the beginning & for some reason never revisited. So far it sounds remarkably simple, heading over to the forum after I’ve read through again. >:D
Any clue as to where to snip to disable the beeper?
Ok, found it, next to the ribbon cable that connects from the controls to the main board. It’s a black round cylindrical thing with a little hole in the top. There is nothing to clip, so I stuff some glue into the hole and it’s almost silent now. Now if I can find a quieter fan…