I have this PCB which is the motherboard of my pellet stove. It is reading a value "T Board" which is the temperature of the board. Now it's reading faulty values (86°C at ambient temperature) and the stove won't work normally.

I disassembled the motherboard, disconnected everything, put it on power and I can still see the temperature on the remote control and make it change using a hair dryer to heat the board. It will not change much however, up to 91°C, may be because of the faulty sensor.

The problem is, I failed to find which component is the sensor. After some research on the subject, I think it might be a thermistor but the one I found at the bottom of the board (blue near the transformer) seems to be just for the power part (it wont change the reading if I heat only this component.)

If it is an SMD thermistor, is there any tips to distinguish it from resistors/capacitors?

Full board picture:

enter image description here

Zoom on the part that seems the most sensitive to the hair dryer:

enter image description here

Update 1:

Following suggestions in comments, here are the things checked:

  • RTC component is MCP79411, it does not have a temperature sensor according to datasheet.
  • 8 pin component near ATmega is labeled 33064, an undervoltage circuit according to what I found on the internet.
  • TO92 component on the top is labeled 7333-A, it's a LDO regulator. Second TO92 (between transformer and radiator) is not visible, but it goes to a connector labeled "Vacuometro" so I guess it's not a temp sensor.
  • I traced ADC pins of the ATmega (ports F) but I couldn't find anything useful. Of course I could have missed something.
  • I also tried moving around the hot tip of my soldering iron, but I failed to trigger any temperature change on the remote control. Very disappointing since hair dryer will still trigger a 5°C variation.
  • \$\begingroup\$ Could be a temp sensor in the RTC chip? Could be one of the TO92 transistor like devices? Trace the circuits on the ADC pins of the mega64. \$\endgroup\$
    – Kartman
    Feb 5 at 12:10
  • \$\begingroup\$ @Kartman Thank you for suggestions, they all made a lot of sense. Sadly I couldn't find anything useful. I will probably get a replacement board next week. \$\endgroup\$
    – Nicolas P.
    Feb 5 at 17:20
  • \$\begingroup\$ Does the atmega64 have an internal temp sensor? \$\endgroup\$
    – Kartman
    Feb 6 at 12:32
  • \$\begingroup\$ @Nicolas SW2 has writing "KTY/NTC" next to it, and it's in the "NTC" position. "NTC" probably stands for "negative temperature coefficient [thermistor]". The thermistor may be nearby, although I can't spot one in the photo. \$\endgroup\$ Feb 22 at 4:08
  • \$\begingroup\$ @Nicolas similar "where's thermistor" question: electronics.stackexchange.com/a/205789/7036 \$\endgroup\$ Feb 22 at 4:12


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Browse other questions tagged or ask your own question.