Welcome back, folks.
You may have read our first two articles on investigating and analyzing a low-cost induction cooker, but if not – check ‘em out!
1800W Induction Cooktop Teardown and
Circuit Analysis of the 1.8kW Induction Hotplate
And you’ll see that we’ve left it kind of open. Promising more, but never delivering – oh, snap! Well it took some ego-boosting and a genuine request from Mark in the comments of the last article, but we are now sufficiently shamed to go ahead and spill the beans.
This device is remarkably easy to control manually.
As we were testing different methods, we did a dumb thing and left a wire connected to the comparator right next to the power switches. Needless to say, the wire drooped over accidentally and touched the heatsink (which is also electrically “hot”, BTW) and 1200V blew out the majority of the important silicon on the power board. Doh! So while we wait for our replacement device we’d be glad to share what we know. We believe that it’s enough to get you up and running with manual control, but since the “striking” technique has only been tested in sim and not on the bench, please consider it beta and subject to change.
You need three things to get this board running under manual control.
- To apply +5V to the “K” line in order to enable the device.
- To apply an adjustable voltage (about 0-3V) to the PWM line to control the power output.
- To strike the device into oscillation using the PAN line.
So let’s take a step back and show you this dangerous piece of pwnership from start to finish. All the control can be achieved through the ribbon cable – no splicing into the circuitry is needed! In our case, we just chopped the ribbon cable off at the controller board, and threw the controller board away. You may want to build a nice professional board, so consider this before chopping anything up. Let’s investigate the ribbon cable and it’s functions, starting at the pinout. Here’s a pic of the discarded control board to help us define the pinout. Pin #1 will be “I-AD”, the pin closest to the 7-segment LED display. Pin 12 will be the one closest to the edge of the board.
Fig 1 – Control Board with Ribbon Cable Removed
The pinout, and functional description is as follows:
- I-AD. Voltage output (for an ADC) corresponding to the average current from the AC line
- V-AD. Voltage output corresponding to the AC input voltage
- GND. The Low-voltage ground to run the control circuitry. WE USE THIS.
- +5V. 5 Volt supply for the control circuitry. WE USE THIS.
- INT. Square wave output of the first comparator. Tells the micro the duty cycle. We don’t use it
- PWM. Power control line. Usually takes a PWM from the micro, but we will drive with a DC voltage. WE USE THIS.
- PAN. Used to strike the switching. WE USE THIS.
- K. ON/OFF control. WE USE THIS.
- FAN. Fan control. Probably should use it, but don’t right now.
- BUZ. Buzzer? Not used.
- T_IGBT. A thermistor tied to the IGBT’s for overtemp monitoring. Currently unused.
- T_PAN. Another (optional) thermistor near the pan for overtemp monitoring. Currently unused.
Now here’s your 2-minute overview of manual control for this device.
- Tie K to +5V to enable the device, or to a switch for on/off control.
- Apply an analog voltage (about 0-3V) to the PWM line to adjust the power. We use a potentiometer.
- Strike the device by pulling the PAN line high momentarily.
Simple, eh? So what’s all this “striking” business we keep talking about? Well basically, since the system is self-oscillating, it uses the ringing on SW to decide when to start a new cycle. If SW is not oscillating, it’s happy to just stay still. In order to get the system running, we must disturb it using the PAN line and force it to start a cycle. After that first cycle, the resonant ringing on SW will allow it to self oscillate and Bob’s your uncle – you’ve got an induction heater!
So let’s do it. Please double, and triple check that the AC cord is disconnected and the capacitors have had time to drain before playing around in the circuitry. We’re assuming that you’ve already cut the ribbon cable and identified the 5 lines you’ll need (GND, +5V, PWM, PAN, K).
For simplicity, let’s just tie K to +5V for now. That will enable the device any time the AC line is plugged in. Good enough for our needs.
To adjust the power level, you can use a 10k potentiometer between +5V and GND whose center tap (the adjustable node) is tied to PWM. The working range on PWM is about 0.5V minimum to 3v maximum. If you go too low, the device will lose self-oscillation and you’ll have to restrike. If you go too high, then the cycles will self-terminate at about the level they would at 3v. You may also lose oscillation. So keep it around 1/3 to 2/3 and you should be able to sweep the power level from a few hundred watts to the full 1.8kW. We’ve gotten away with pots as high as 100k, but internally there is a 200k resistor to GND which will end up acting as a voltage divider to limit your maximum V(PWM). That might be a feature to prevent over-revving, actually!
The strike requires a momentary pushbutton switch, connected from +5V to the PAN line. When you tap this switch, it whacks the TOPREF and SWREF nodes in such a way as to force the first comparator ON, which tricks the device into starting a cycle. There is already a 22k pulldown resistor from PAN to GND, so it doesn’t matter if you push and hold the strike button or just tap it, it will decay back to 0V pretty quick so all that is needed is a pullup. It’s the sharp rising edge of PAN that does the deed, so a momentary pushbutton switch is the perfect thing to do this.
The timing is not critical – that ugly startup noise we described in the first article is the micro banging this line tens of thousands of times in a row. So for you – it’s probably OK to tap it a couple of times, but you don’t want to sit there whacking on it for too long – you are forcing a cycle every time you tap the switch and the switches don’t want to be forced on when the SW node is at it’s 1200V resonant peak. We’ve never killed any switches with our manual striking, but we can tell you that repeating striking while the device is running is simply not a nice thing to do to your induction heater.
So there you have it: Enable, set power level, and strike. Easy as pie.
It’s certainly possible to unwind the pancake coil and rewind it around a piece of PVC or something in order to get a cylindrical coil for heating rods, etc. Keep in mind that the lacquer used may chip off and give some risk of breaking the insulation. Exposed coil wiring is something you definitely would not want to touch with either your hand, or the piece of metal you are heating.
More details are just around the corner when the new unit arrives – stay tuned, and good luck!
Any idea if this thing could be modified to degauss a hard disk?
I’m wondering as the degaussing plates made for hard disks are quite expensive, and superficially appear to be similar in functionality to an induction cooktop.
Probably not, but you could try it. Perhaps at low power levels it could erase a disk, but at high power it would probably just cook it. You would also have issues with the outer plates (or even the shell of the drive) acting as a magnetic shield and sucking up all the flux. The outside would cook, but the inside may not be affected at all.
Great bit of hacking you did there lads !
May I ask if you tried different coils configurations on this baby ?
Because as I see it, this hotplate design isn’t fit for metal tampering/melting under a frequency this low (Foucault currents would dominate over the skin effect at such a low frequency, am I right ?).
So I wondered if the coil intensity feedback signal was used in some kind of protection mechanism in the original design. Cuz having too small a work inductor (say 5 to 10µH) would cause higher intensity spikes, wouldn’t it ?
Messy question made simple : What is the highest frequency you obtained out of this nice setup ?
We have used a 45uH cylindrical coil to heat rods. In the original application, the micro probably checks the frequency and shuts it down if it’s too high. As you may recall, the cooktop won’t run without a pot or some other significant load.
The highest frequency was about 75kHz. The problem is that without a strong gate drive, your SW flies up before the IGBT is fully off. This will end up cooking and killing your IGBT’s when attempting to run at a higher frequency. We’ve duplicated the cooker circuit on our own PCB and found exactly this problem. And as soon as we finish the Xyron hack, we plan to work up another PCB with a real mosfet driver running the switches.
That one will be limited only by the storage time of the IGBT’s, which is still significant. So could you possibly run it at 200kHz? Probably, but 1MHz would never work without MOSFETs.
However, industrial metal melting does not use high frequencies – more like 10kHz to get some actual penetration into the work. It’s only surface treating and nonmagnetic stuff that needs higher frequencies.
Thank you for dissipating the fog in this hazy brain of mine. Skin effect…surface tampering…makes sense here.
Tweaking the original circuit by using mosfet would be great (I endorse it, thumbs up), but is there any reference which could withstand such current and voltage ?
As I read it, the here used IGBTs are rated 20A (60A peak) @ 1.2KV. It will be difficult to find mosfets rated more than 18A under 800V with the same package. We’ll soon hit the SOT-227 family and such modules.
The affordable STW20N95K5 can endure 15.5A (62A peak) @ 950V with a huge heatsink, but that’s it.
Don’t you fear the project can quicky turn into a bottomless pit ?
Man, oh man, I’m thrilled about your next experiment.
This article is a real cliffhanger.
Yup, finding a FET that will drive that much power may be impossible or at least impractical – as you mentioned, bottomless pit.
The beauty of this design is it’s simplicity, and ease of low-side driving. The drawbacks are seeing the huge ringing voltage on the switching device, necessitating something like an IGBT to take the abuse. To get increased power and increased frequency while keeping your voltage ratings low, you’re going to want to go to a half bridge configuration. Uzzors2k, Tim Williams, and others have build extremely powerful devices that can use MOSFETS due to the fact that the switches only see one VIN’s worth of voltage. Still 200-300V, but it ain’t 1200V! The drawback of those is that the high side drive is not trivial.
We may try one of those alternate topologies in the future, but the goal of a 30kHz, 1.5kW device seems reasonable for this one here.
Is there any progress on this. Did you get a chance to obtain another device? Also, what would be the simplest means of getting more power at 10 – 20 kHz?
Thank You
Mark
Funny story – another device was purchased a long time ago but was delivered somewhere else. Never found out where. It was such a pain to get the money back that no second replacement was ever purchased. But that should be corrected – the replacement will be purchased tonight.
Some calculations would need to be done to figure out the total max power that this topology could do. Probably the limiting item when using a modified stove is the power switch current, so in that case more switches could simply be added in parallel. Second limit would be the 15A breaker on a typical household plug, you’d want 20A or 30A and then an upgrade to the bridge rectifier would be required. Next, the tank and resonant caps would need to be upgraded and finally, switch to 240V to obtain even more input power.
Why, what’cha trying to melt?
Great article! I’m looking into driving my hotplate with a PID to create a hotplate for a water bath that will be able to hold a “constant” temp… I have an Aroma Induction cooktop that I got at Costco for $49, and really want to give it a try… I have a little electronics knowledge, arduino, some temp control & PID, and a little programming but usually stay away from AC, especially 2500V!
How hard would it be to hack my induction cooktop to just turn on high if there is AC power and a capable pot was on the burner??
Sounds like I need 5v and 3v, I’m still trying to figure out this Strike thing… It’s outside my normal knowledge set!
Great post and I will check back to see how it progresses after your replacement burner is in.
-Jason
Hmmm.. $49 at Costco, eh? Sounds like something we should pick up.
The burton can already hold at a constant temp. Are you saying that yours cannot? To venture a guess, adding PID would probably be a matter of reading a thermocouple or thermistor with your micro, and outputting a PWM signal that you RC filter to a DC voltage in order to control the cooker. The replacement burton cooker is in, but has not been hacked due to an overwhelming pile of other projects. We will kick some stuff off the top and see if it floats up to a higher priority for ya..
In case you need extra Max Burton units to play with ($50, shipped) : http://www.toolup.com/max-burton_6000_induction-cooktop-by-max-burton-6000-series.aspx?&utm_source=CAfroogle&utm_medium=CA&CAWELAID=631497450
I’m really hoping you pick this project back up. I’d really love to hack a Auber PID temp. controller into the unit for more precise temp control, but I don’t quite have the know how to pull it all together … basically just enough knowledge to be dangerous (mostly to myself). Ideas?