I have a hotplate with a TS port meant for an external temperature control unit. The manufacturer's customer service told me that this port is not connected to anything internally in the model I own. Upon opening the appliance I found that it is in fact connected to an opto-TRIAC on the heating element control board.

Using the values listed in the opto-TRIAC's datasheet, I cobbled together my own controller with a black-box temperature hysteresis module, a 12 V power supply, and a current limiting resistor.

It works initially, but if it's left on for an extended period of time one (or both?) of the TRIACs stick on and will not shut off until the entire appliance is turned off for about a minute.

I am not savvy enough to diagnose this alone, but I thought I might find help here. I attached pictures of the heating element control board, back and front, as well as diagrams of the control board circuit and my controller. I did my best to transcribe the circuit into a diagram but it's possible that there are errors.

Here are links to the full resolution images:

Heating control board picture

Heating control board diagram

Hysteresis controller diagram

Opening the thumbnails in a new tab will also display the full resolution source.

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    \$\begingroup\$ Could be that the triac is getting too hot and latching on, but that would also probably happen without the external control, if the temperature control were set to maximum. What happens if you remove the external controller? \$\endgroup\$
    – PStechPaul
    Feb 2, 2023 at 3:09
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    \$\begingroup\$ Actually, I think you need to rewire the external control so the relay contacts are isolated from the battery and power supply of the black box. Also provide details on that device. \$\endgroup\$
    – PStechPaul
    Feb 2, 2023 at 3:12
  • \$\begingroup\$ It doesn't latch on when using the built in potentiometer control alone which leads me to believe either the opto-TRIAC is latching or there's something else that I haven't considered. Overheating does make sense as this unit gets quite hot. \$\endgroup\$ Feb 2, 2023 at 3:15
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    \$\begingroup\$ It looks like the controller is designed for a 12 VDC load. But, in any case, the relay contacts should not be connected to any part of the 12V circuitry. It is possible that the relay contacts could be tack welding and sticking closed. \$\endgroup\$
    – PStechPaul
    Feb 2, 2023 at 4:00
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    \$\begingroup\$ Never mind - I see that the controller is only handling 12V and a few mA to the opto-triac. However, being a 20A relay, it might not switch reliably at low current, \$\endgroup\$
    – PStechPaul
    Feb 2, 2023 at 4:08

1 Answer 1


I have solved the issue, here is my solution for reference.

Initially I believed that the TRIAC was only latching on when I used my external controller. After testing the hotplate a second time without my controller I found that the TRIAC latched on even when using the manual potentiometer control if it was left on long enough.

My first step after testing it again was to replace the BTA06 TRIAC, this did not resolve the issue. However, mounting a small heatsink to the new TRIAC fixed the issue immediately.


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