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

pcbthermistor

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):

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.. another view at the connections:

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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:

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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)
Closeup on the transistor

Rear view of the circuit board:

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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.

Best Answer

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.)