Broken Sous Vide Supreme

Sous-Vide is a cooking method involving precise temperature control. A temperature controller of some kind (usually a PID controller) is usually required. The Souse Vide Supreme is one such controller integrated with a water bath.

I have in my posession a broken sous vide supreme. I knew it was broken when I bought it, but I figured that worst case, I can use the water bath with my existing PID controller (I’m currently using it with a cheap deep fat fryer, but some of the clips are rusting). The symptoms are that the temperature is permanently reading 200 degrees C. Otherwise the unit seems to be working correctly (I’ve not yet proved that the heating circuit working as it never kicks in, believing the temperature to be always higher than the set point).

Step 1. Take it apart – that I’ve managed, but I didn’t take enough pictures. From memory the hardest part was reaching some of the nuts. There were no obviously visible faults, all cables look fine and no components are noticably burnt out. It’s interesting to note that most of the components are through-hole which should make debugging a bit easier.

Step 2. Check the thermistor – It’s plugged into the board with a socket, making it very ewasy to remove. Sadly (as it’s would have been an easy fix) it seems to be working fine. At room temperature a voltmeter reads the resistance at close to 100k Ohms, this drops noticeable when I warm it between my fingers. It appears to be a glass encapsulated 100k Ohm NTC thermistor in working order.

LM258AP Dual Op Amp in souse vide supreme circuit
LM258AP Dual Op Amp in souse vide supreme circuit


The thermistor is wired up to an LM258AP op amp. I’ve done a bit of reading and I assume the op amp is there to amplify a signal from the thermistor in preperation for passing to an ADC. Step three will be to analyse the circuit to see if I can confirm that this is how the op amp is being used, then attempt to test the op amp. Feel free to give me any advice in the comments, circuit debugging is not my area of expertise!