Yesterday I posted a voltage-versus-current curve for a 1N5817 Schottky diode, to confirm the theoretical formula , where IS is the saturation current of the diode, but I wasn’t really satisfied with the results, either in terms of dynamic range or the quality of the fit.
One problem is that the serial variable resistor I used did not all really low currents. I rewired it today so that I had a potentiometer providing the voltage, rather than a series variable resistor:
I also wrote a little Python program to merge different data files, so that I could combine files in which the resistance of R2 (for measuring the current) differed.
The resulting data fits the model well for over six decades (> 120dB):
The measurements at the high-current end had to be redone with an external power supply for the Leonardo Arduino board (not just USB power), as the reference voltage for the A-to-D converter dipped as the load increased. There is a tiny effect still when using an external power supply, but only at the very highest current level, and it is buried in the noise.
At the low-current end, we can see the flattening of the curve from the “1+” term that is often omitted from the model. The resolution in the voltage is poor there, but the current knee can be fairly accurately set by using a large value for R2.
I should probably characterize the base-emitter junction of a PNP and an NPN transistor this way also, for setting the appropriate resistances for the log amplifier in the loudness circuit.