Re-building a Crock-pot controller - how are the heating elements powered?

Question

After a few good years, my CrockPot gave up. It looks to me that there may be something wrong with electronics, and being and Arduino enthusiast, I thought I could re-build it. The symptoms are that the whole device switches on but whenever the actual program is set, it just switches itself off again (goes dark).

From what I can see there is one heating element and a temperature probe attached to the main body (white wire):

.. another view at the connections:

The main circuit board features connection of the temperature probe (NTC1) and a transistor to switch the heating element on/off, as well as all of the buttons, display and power processing components:

The markings on the main driver are can seen on a closeup here: (probably this one: http://www.nxp.com/documents/data_sheet/BTA208X-600E.pdf)

Rear view of the circuit board:

Is my assumption correct that there is probably something wrong with the controller and the heating elements are most likely fine & can be reused?

My main hurdle is to understand exactly what the circuit may look like in this case and how much of it do I need to re-create for basic control. I am happy to design the control/arduino part but for exact understanding what are the requirements of the heating elements, that is where I need help!

Update

I've now also measured resistance between the red and black wire (heating element) - 250Ω. The thermistor seems to be clocking at 13.8kΩ.

Answer 1

Before you go trying to duplicate the controller, at least take a basic stab at fixing what you've got. You've measured the heating element and the thermistor, and those resistances seem reasonable, so they are probably fine.

One thing that jumps out at me is the single-sided circuit board. That's the one with the orange color and copper traces only on the bottom. Those things are notoriously unreliable. There is no plating in the holes, so all the leads that stick thru the board are held only by the solder meniscus on the other side. On "real" boards, they are held by a thin layer of solder making a good connection inside the cylinder formed by the plated hole, then again by the menisus on the other side. That's much more solid than what your single-sided board can do.

Take a soldering iron and re-flow all (yes, every last one of them) the solder connections on the bottom side of that single-sided board. I have seen such boards where over time the solder meniscus cracked, but it still looked fine unless you examined it with a microscope. Even then it can be hard to tell. Heat each of the joints until the solder visibly flows. Add a little solder if you can do that without making a blob.

After reflowing the solder joints, reassemble the unit and see if it works now. The fault could well be somewhere else, but a failed solder joint on a single-sided board that has been subjected to regular temperature cycles is likely enough to be worth the attempt.

Answer 2

I can think of a number of ways to answer your question. One source of information might be a service manual (if one is available), but perhaps an even easier source of information is the US Patent Office. For example, US patent 6573483 is a patent for a slow-cooker accessory that shows both a thermistor and a triac for controlling the heating element. That particular patent is rather light on details, but you might look at the referenced patents for more information if you need it.

As for details, triac manufacturer application notes are likely a good source of information for redesigning this circuit. In particular, you may find Power Control with Thyristors and Triacs from NXP useful for this purpose since it describes not only the basics of triac operations, but also a specific section titled "Design of a Time Proportional Temperature Controller" describes a control algorithm that is likely to be of interest.

Finer control of the temperature, including temperature ramp and tight regulation, might be an application worthy of throwing a microprocessor at it!

Update

I would be skeptical of the version 1 module mentioned in the comments, but it appears that some of the problems are address in version 2, including making the traces on the PCB wide enough for the expected current. Before proceeding too much farther, it would make sense to measure the AC current used by the heating element and/or seeing if there are markings on it indicating the current. Since the original triac was rated for 8A, I would be cautious about substituting one with a significantly lower current rating (as with the BT136S that comes on that module.)

Answer 3

This looks like fun :-).

This will be a rolling answer - or hopefully others will join the fray - I'm about to have pieces hacked out of me early next week * and will hopefully be back online some days later. Hopefully ... .

Where are you?
Looks like your mains is 230 VAC.
Power = V^2/R.
For 230 VAC power = 230^2/250 = 212 Watt ~= 200 Watt you mentioned.

You can check TRIAC is OK by finding trigger cct .
The diagram on page 8 here shows the genera form it will take. Controlling the trigger "switch" however it is done should turn TRIAC on.

WARNING: 230 VAC

Units like this are often not isolated between controller and processor.
Odds are the controller is all effectively at mains potential.
Try not to die.

The easiest safest way to control a mains TRIAC is to use an optically coupled zero-crossing TRIAC driver. Thse are often small TRIACS themselves. (Zero crossing means it turns on when mains voltage passes through zero so minimises EMI (electromagnetic interference) amd load on TRIAC for resistive load. For an inductive load where V & I are out of phase TRIC may be less happy, but thems the breaks. Here the element is almost certainly close to pure resistive.

The TRIAC driver I referred to above is a Liteon MOC3020-MOC3023 series driver. t is 400V rated which is more marginal than I'd like for 230 VAC.
230 RMS x 1.414 = 325V peak. Add some noise spikes and ... .
Better to use a 600V driver.

Another 400V rated part - TLP160G

Liteon MOC3063 - 600 V min, zero crossing, 5 kV isolation, $US0.63/1 in stock Digikey - a bargain. This needs 5Ma LED drive max for guaranteed switching - some need 15 mA - something to check in every case.

You'll find a zillion application circuits here.
Some samples that look OKish include

A question but looks useful

In Russian? circuit self explanatory. How hard can it be.Looks like he is having fun.

Russian again but looks good and Google translates well enough (right click - "Translate to English").
Yielding this circuit.

And similar here

http://optoelectronics.liteon.com/upload/download/DS-70-99-0019/S_110_MOC3020%20thru%20MOC3023%20SERIES.pdf

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Temperature control.
Thermistor will drop in resistance with temperature.
To get a reasonable idea of performance, connect ohm-meter to thermistor with clips.
Orient unit as usual. Fill crockpot with boiling water and insert thermometer.
Record temperature and resistance as water cools.
Later on when it is heating you can do this with cooking oil (or motor oil :-) ) to record higher temperature results BUT there are more analytical ways that should suffice when you get that far.

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• C4-5 discectomy & cage fusion. What fun.