I'm asking my question here and hope to find my problem. about a year ago I started to make my soldering iron station. It's based on Hakko 951 series.
I designed a GUI and a PCB for it. about after a month, I gave up the project because I was busy until now. Everything was fine save one part. thermocouple! I don't know why but it doesn't work correctly. the problem is that it doesn't give correct output.
How does this thermocouple work?
It's simple. A solder tip contained a heater in series with a thermocouple. when the tip gets hot, the thermocouple should give us a voltage between 0 to 9 mV due to the heat you apply to it or the built-in heater make(when it's turned off) and I amplify this voltage to measure the temp. Look at this picture for more details:
How do I configured the tip (thermocouple + heater)?
Here is the schematic I used:
I need to add that the circuit was used 1 - 2 years ago by this guy and it had worked successfully.
What's the problem?
At first I have to say it gives me a voltage between 0 -3 mV (although the output changes due to the temp change linearly) instead of 0-9mV(alone problem is that this range is smaller than I expected). And second it doesn't work correctly when I add it to the above circuit. E.G. It gives me about 300mV at the output at room temperature. If I remove the tip from the circuit and measure the output of the tip, it gives me an output due to the temp.
How do I measure the output?
For measuring the output when it is unconnected to the circuit: I connect one lead(wire) of the tip to the red probe of my multimeter and another to the black one. if you apply heat to the tip, you would see that DMM show increasing the voltage between 0 to 3mV.(while I expect to see an increase between 0 to 9mV ).
For measuring the output when it is connected to the circuit:
Like above, each prob to one lead but as I said the output isn't correct.
In your opinion why doesn't the tip give me the correct output?
Please feel free to ask any question