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madworm avatar madworm commented on August 28, 2024

You are 100% spot on. As I did not trace out the schematic for the thermo-couple circuit, I left that cal-potentiometer alone. I don't know what it does exactly and how it might affect the temperature reading.

"Never change a running system".

from youyue-858d-plus.

fiechr avatar fiechr commented on August 28, 2024

Ok! Thanks for clearing that up.

I tried to make a schematic of that part of the board. I'm really not an electronics expert, so I can't guarantee everything is correct, especially not about the part before the OpAmp (this diode stuff seems odd to me).
But nevertheless the part after it seems more clear: The resistance of the negative feedback loop of the OpAmp is changed by the potentiometer. This changes the gain of the amp, which means the voltage on the OUT pin changes also slightly.
So if the thermocouple measures a too low (high) temperature, the amplified voltage can be raised (lowered) by this mechanism.

So I guess it should be possible to use this trimmer without negative side effects, because the value is corrected before the software control loop. I would prefer to set it this way, because the value is device-dependent anyway.

But I could be wrong about that...

youyue858d_cal sch
(Updated schematic, changed direction of D10)

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madworm avatar madworm commented on August 28, 2024

The double diodes clamp the input voltage to about +- 0.7V or so. No cold-junction compensation.

D10 is very odd...

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fiechr avatar fiechr commented on August 28, 2024

Well, D10 is actually facing in the other direction on the board. ;-)

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