So I have an old thermocouple based temperature probe set. I murdered a probe and in my curiosity took it apart and did not find what I expected. The way it works is I have a black box in one hand w/ a 7 seg display, connected by handset cord (original cord was missing when I got it) to one of a selection of probes. the handset cable uses 3 wires. I opened up my dead probe as well as my good one and took some measurments. This is the circuit I saw attached. J1 is the thermocouple, perm connected to the board, and J2 is the plug that goes to the main reading box, which is sealed up too well to open w/o destroying it.
I saw 1.7V on the lead connected to the trimpots, the collecter I declared ground (had yet to figure out the schematic when I did the measurments) and saw 0 on the TC at room temp, and 3mv at 71C.
Really curious about the theory behind this circuit, bit of an electronics idiot, but not really a n00b, should know more than I do.
thermocouple.xcf (60.9 KB)