Amateur design report by Johan Liljencrants

The Q1 heating current is converted into a proportional voltage by R2. This also raises the potential of the amplifier inputs enough that no special trick is necessary to hold down the amplifier negative supply rail. Further the diagram suggests a convenient arrangement of zero and calibration trimmer potentiometers, should you want to display results directly with a milliampere meter.



The four calibration curves pertain to different implementations of the transistor probes, all run in identically the same circuit except for the bias resistor R3. Two use classical style TO18 metal cased transistors (type BC107B), the other two use miniature surface mount SOT23 (type BC847B). The big difference between the red (copper) and black (iron) traces is a change in thermal conductivity in the connecting wires used. Not until late stages in my experimentation I realized the extreme importance of this feature. Theory and experiments on this sub topic of probe design follow below.



The optional R3 controls the Q1 base current at rest and thereby indirectly governs its temperature elevation. Alas, it seems you have to adjust this to match whatever current amplification factor that device happens to have. A goal may be to set output voltage U at still air to come in the 1-2 V range.

