Finally Complete! Enjoy.
Cool stuff today, folks, cool stuff. As was mentioned in the first article about the Burton induction cooktop, we traced out the circuit and found some very interesting tidbits to share. This device is amazingly simple! It’s actually a self-oscillating resonant drive with integrated power control.
That microprocessor that we were speculating about? It just monitors stuff, it isn’t necessary for the resonant heating. The “PWM” output from the micro to the heart of the power stage is actually R-C filtered down to a DC voltage, and then fed to a comparator to do the power control. Replace that with a potentiometer and you’ve got yourself a pretty sweet power control knob with no micro needed!
But we’re getting ahead of ourselves. In the time that’s passed since the last article, we’ve traced the circuit and put it into both LTspice and eagle so we could simulate and make our own test PCB. It works great! You’re not going to get the PCB today (sorry) because it’s big and clunky and developed for 600V IGBT’s, whereas you need 1200V IGBT’s to really run this beast off the AC line. But we will give you the LTspice files so you can simulate it on your computer and play around, adjusting stuff and making your own improvements.
For those that don’t know, LTspice is a circuit simulator developed by Linear Technology. Get LTspice here. It’s free and is quite a good simulator, although the schematic interface is clunky and the waveviewer is damn near awful. It looks like a knockoff of PSpice, where the sim engine was replaced with a turbocharged V8 and the seats and interior were replaced with wooden crates and astroturf. That being said, it’s by far the best spice engine you’ll get for free.
A word of warning – to actually simulate this beast, you will need to lower the spice tolerances to get convergence. More on that later.
ON TO THE CIRCUIT!
Fig 1 – Schematic of the Induction Cooktop Power Stage
Please bear in mind that this is a simplified circuit. To get the full trace, just wait longer or trace it yourself! 
Some of the input filtering has been removed, along with the overcurrent and overvoltage protections. Not a good idea to build a circuit without those (although we did), but what we’re focusing on today is the heart of the self-oscillating resonant control.
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Great post, very informative. Enough that it made me want to start a project! I’m having trouble downloading the LTspice files, the link appears to be broken. Keep up the good work!
Good call brother! The link was shortcutted and shouldn’t have been. It’s repaired now..
Glad to hear you’re interested in starting a project. We’ve been so swamped with other stuff that ours has been sadly neglected, but we’ll be back soon with TWO new induction heating projects. First, our own take on this design. And second, hacking this actual device for manual operation – it’s easy! And dangerous!
The High Voltage power supply inside Panasonic “Inverter Series” Microwave ovens uses the IDENTICAL principle involved except that it uses proprietery IC’s.
That’s great to know – we’ll have to go check out those devices to see how it’s put together. So how did you learn about those ovens?
Thanks for the tip!
I started taking panasonic “inverter” microwaves to dig out the IGBT’s. I’m surprised that a huge amout of them failed due to “transformer hv winding arcover” but most of the time, the IGBT’s survived.
It used a circuit identical to the above with the HV transformer primary in place of the pan heating coil. I knew it because when I fired the unit without the HV transformer primary connected, Gate driver signal only does “click” “click” “click”, Assuming it was the starting signal from the IC to sense transformer “ring”.
Those inverters work on three modes. the start mode, The idle mode (also known as the magnetron presence test mode), (inverter powered, no magnetron connected) and the cruise mode. (magnetron connected and inverter powered for 3 seconds) Interestingly, these inverters will fail if the magnetron is disconnected (or fails) when the inverter is in “cruise mode”. Either the IGBT will short out or, continuous arcover of the HV transformer. (90% of the time this happens, as the magnetron fails in mid-use).
Yes, these inverters used for powering magnetrons in Panasonic ovens LACK overvoltage, and short circuit protection! Yes, they go to the grave with the magnetrons!
Very cool stuff! It sounds like you know quite a bit about these inverter microwaves. If you’re interested in doing a writeup on what you know, and what goodies you can scavenge from them – shoot us an email, we’d be happy to post it up here for you.
I had a look at your induction cooktop teardown, I built a similar ciruit (with two comparators and that…), but the circuit keeps stopping. and my question is, how did the startup mechanism worked?
The startup is achieved by disturbing the first comparator (the one that compares the divided-down SW to the divided-down reservoir cap). When this comparator is disturbed, it fires one pulse which gives one switching cycle. If the power is set high enough, then the SW node will ring up and back down to 0, triggering another cycle.
If your device is running for a while but stopping, then a couple things might be the problem.
1) Most likely is that the power is too low and it can’t regenerate a cycle because SW doesn’t fly back down. Either the inductor is too big, or perhaps you are using a smaller input voltage? We run a lot of tests at 12V, which requires much less inductance than 120V. How big is your coil? If you know the inductance, that’s great. Otherwise, you’ll have to calculate it from the diameter and # of turns.
Another thing that could cause very low power is the wrong values on the INT/VRAMP section of the first comparator. For example, if the RC is so small that the pulse width is tiny, then there will not be enough current built up in the inductor to fly down and retrigger.
One more thing would be if you’re trying to run the device too fast (say, 100kHz). This device is good for a maximum of 40-50kHz at the moment. It’s typical frequency is 20kHz and lower. We are trying to develop a modified device that can be used for aluminum, etc – but these IGBT’s are VERY SLOW (like any IGBT) and getting it all running fast is not trivial.
Have you simulated your circuit in LTSpice? It’s free, and you can use our schematic as a starting point. If you’d like to email us your schematic, or a picture of your board, we’d be happy to advise. IH at openschemes dott com.
Yes, I knew the startup circuit of the IHcooker is done, I did realized that the circuit stops only if I used unfiltered AC (like the above), probably because the dc rail goes to zero every half ac cycle. But when I put a total of 6800uf filtering, the circuit operates continuosly, (albeit with the worst power factor I’ve ever seen
)
Hooray!!!!
I bought a counter-top induction cooker to perform quick heating of steel rings for a project for my company. I tried it and of course, because it didn’t see enough load or something, because the part is not a complete surface like a pan, it didn’t work. So I casually looked on the internet and saw that there were several chip development sites that displayed proposed circuits and I figured that I could use those to decipher mine and make it do what I wanted. Then I put in the chip number from my device and Voila!!(Wa Laa) you guys pop up and not only have the exact circuit that I purchased but have a hack besides.
God is so good to me!! Really!! I am Irish.
I mean, first, two months ago He had you pull apart and hack this device, then He had me purchase the right device and lastly He suggested that I search based on the chip #.
I don’t hear Him or anything, but I can’t discount the many fortunate coincidences that have happened in my life. So enough about that.
What I really wanted to do was thank you for this article and plead for any further work you have done with it.
I have only just read the article and so now plan to try the potentiometer power control. I will let you know what I find out about the protection circuitry as I can. This is a definite R&D side side project for me though so I don’t get much time for it.
However, you people ROCK!!!
Mark
Gentlemen,
Please followup with the manual control of this system as I really want to use it for a production machine I have envisioned.
If the circuit design already exists please direct me to it.
Thanks,
Mark
Hi Mark,
Glad you’re getting some use out of the article – it’s always great to find exactly the hack you need! Thanks, interweb!
The reason that we’ve not put up our manual control stuff is that there was an “event” where a flying wire used for the startup (strike) drooped over and touched the 1000V heat sink. Pow – dead circuitry! And we’ve been MEANING to order another Burton but just never have. However, another is on it’s way so you should expect the full article soon.
For now, we will generate a placeholder article that will be expanded later. Go look at the main page to find it.
Gentlemen,
I really need help understanding the circuit which I traced that the enable input (K) is concerned with.
I have a pdf of my sketch (not pretty but accurate) which I would send you if you would give me an address or if you already have the enable circuit and can explain it to me I sure would appreciate the help. It looks to me like “K” needs to be grounded to enable the system.
Thanks,
Mark
Are there two bridges for rectifcation of AC-DC? The IN4007, may not be able to handle high current?
The 1N4007 is only for simulation. You can find the actual component by looking back at page 5 of the first article.
So, it’s actually a 35A bridge.
Hi, this article has been a great help. I am working on a science project investigating the heating of nano-sized iron oxide particles at various concentrations (I have used them in the past to make ferro-fluid). I am hoping to use a induction cooker (same as that in the article) to raise the temperature of my samples a few degrees but of course they are too small for the cooker to detect and operate. While the manual control sounds like a good way to go, I was wondering if there was an approach that would allow the control panel to still be operational? My thought was to override the appropriate input signals to the micro-controller to make it think it was detecting a suitable load. Any suggestions would be appreciated.
You should consider making a significant batch – say 3-4oz, and then just placing that entire batch in a glass container right on the hotplate. Since the ferrofluid is magnetic, the cooker will detect that it is a suitable load (as long as the batch size is significant) and will go ahead and cook it. Just like iron powder toroids, magnetic flux can jump between particles even if those particles are not connected together physically. And since the concentration (at least in ferrofluid) is quite dense, the cooker should have no problem detecting and cooking the particles.
It would sure be interesting to see what happens when ferrofluid is induction cooked. The field is much faster (kHz) than it can respond (spiking, etc) but it sure seems like something neat would happen. Maybe an oily blast onto the ceiling – anyone care to try?
Please beware that ferrofluid can be cooked to death – it has a curie temperature which is probably below 100C considering it’s tiny size.
As far as tricking the micro into running without a load – hmm, it can probably be done but the suspicion is that the micro checks the resonance every cycle. So some kind of circuit would have to continually retrigger in order to keep it tricked. Not that it’s impossible, just that it might not be simple.
Thank you for the quick response. I plan to wire up the manual control today and try heating a sample. I’m not sure I can mix up such a large batch of fluid with what I have on hand, but as a test I filled a 4″ diameter flask 1/8″ – 1/4″ deep with a fine black magnetite sand (same stuff I believe, just much larger particles) and put that on the cooker. It wasn’t detected (but did heat up slowly when I placed a metal pot to the side to trigger the cooker). Since magnetite is a ferromagnetic iron oxide, I suspect it doesn’t interact with the field to the same degree that the same mass of iron would. So, it looks like the manual control will be necessary to activate the cooker.
Another issue I am concerned with is whether the frequency will be high enough. From papers I have read 20kHz might be a bit too low to see much, though 40kHz might be enough.
I wired up the control circuit you provided in the other article and was able to try it this morning. One issue that came up was that using a momentary switch to +5 wasn’t causing the circuit to oscillate. I checked with a voltmeter and found that it the PAN line was already high. However, switching the button to ground instead of +5 is working great. I also connected the FAN line to +5 and it is working nicely as well.
Your manual control circuit looks like it will be perfect for my experiment. I already tested it with a small sample of the magnetite sand and a sample of the nanoparticles in water – both showed a small amount of heating (a 5-10 degrees) over the period of a minute. Just what I was hoping for.
Thanks for your help.
Nice work debugging the momentary switch problem – we’ll check your technique here and add it to the article if it works here!
Glad the manual control is working for you, and it sounds like its just the right thing for your nanoparticles. Good luck, and let us know how your experiments turn out!
Thanks.
I do have a couple more questions I would like to ask. One is about the frequency – you mention above that this unit might be good to 40kHz-50kHz (which I would like). Would that just be modifying (lowering) the LC of the coil and associated capacitor or is it something else? I noticed that the frequency ranged from about 31kHz when the power was turned down but was about 20kHz at the high-power end just before the circuit began to “buzz”.
The other question is about the ferrite concentrators that are located below the coil. It appears that they might be heating up a little (the center of the cook top is warm to the touch after a few minutes). I’d like to remove this as a heat source. Would removing them be detrimental to the neighboring circuitry (since the magnetic field won’t be as localized)?
Sure, you can remove the ferrite concentrators.
The frequency is determined by the on pulse time (made by R C in the comparator circuit) as well as the L C tank of the power stage. That’s why the frequency droops as the power is cranked up – same LC ring but longer on time.
To easily increase the frequency, you may try winding a coil with fewer turns. That mega litz wire is probably not needed for shorter, lower power runs. Some if the web projects get away with hollow copper coil or even AWG 10 and get bolts glowing hot! Beware that this coil is driven to 1200V and would be instantly lethal to touch.
Alternatives to a new coil would be lowering the R or C in the timing stage or lowering the tank cap in the power stage. It might not be good to just halves the tank cap, but would probably work ok for light loads – you will be putting twice the ripple current thru the remaining cap.
I am trying to build the circuit for my own domestic cooker ambitions. Is there anything on the overvoltage and overcurrent protection circuits? (apart from the 3000:1 current transformer and usage of the two remaining comparators on the LM-339 of course). Looking forward for replies.
Are you asking if the circuits are available? Not really – they were sketched out at one time, but the sketches are not in the file cabinet so who knows where they went to. If you’re asking if any other circuitry is required to build one, then nope – nothing else is required. We have a couple little ones built that only use an LM319 dual comparator and they work just fine.
Hi;
Thanks for the prompt reply. I did not expect one so soon so I did not check back.
I was wandering about the general scheme of the overvoltage or overcurrent protection used. For example what parts were protected? The work coil? The IGBTs? And did it use something active such that the voltage or current get regulated or did it go into shutdown mode and needed a restart? If you had any bits that you can share on these, I will much appreciate it. If not, it is all right. It has been one of the greatest circuit articles I have ever seen. Thanks.
No prob, thanks for the kind comments. The work coil is durable, its the igbt that need protection. As well as limiting the input current to avoid tripping breakers
If memory serves, the ovp/ocp comparators also pull down on the gate driver ( to stop the cycling) if they are tripped. This would probably interrupt the oscillation and require a restrike to get going again.