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!
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!