The 0.02 ohm resistor is in series with the Inductor being measures so affects the Q measurement. I don't understand why the inductance of the thermocouple is meaningful, considering it's in series with the inductance of the connecting cable and wiring, switch contacts, and the oscillator coil secondary winding?
John
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On 9/15/2022 4:10 PM, Mikek wrote:
That brings up the question, "What is the inductance of the precision resistor and the nichrome wire?
My thinking is they had to have a lot of heat to develop the current required to drive the meter.
See diagram.
We have opamps to give us all the gain we need. I did some figuring, a 10*C increase in temp will cause a 400mv change in a type K thermocouple.
A 0.125" long by 11.6 mill trace of 1 oz copper will rise 10*C from ambient with 1 amp flowing.
A 1" by 1", 1oz copper pcb will have about 7nH (I think that's high compared to the inductance of the precision resistor and the nichrome wire, but I don't know?)
If you etch it to have a 0.125 x 11.6 mill trace in the middle, the inductance will increase. the concept would be to put the thermocouple in the middle of that 0.125" trace,
spot weld as in the prc68 webpage in my last post.
?Here is a drawing of my thinking, ( I drew most of it so I would understand what the circuit is)
John Kolb