The 100 ohms resistors in series with the coil increase the voltage required to operate the relay.

100 ohms + 40 ohm max for the coil makes the relay close at about 8V for certain units, instead of 2.3V max for the lone coil. This is good because the resolution of the measurement is increased, the digital PSU has 10mV steps so the virtual resolution is around 2.5V. I intend to almost double this resolution with another 100 ohms resistor in series, just to be safe :-P

The max voltage might increase to 15 or 16V for certain outliers.

The max. current will be set to 50mA because that's the maximum that the 1/4W resistor can handle:
P=R×I² => I²=P/R => Imax = sqrt(P/R), and here we have P=1/4W and R=100.

This also means that my C code must handle input and output of fixed point numbers in ASCII, while the internal resolution is an integer number of mV. I get a "fixed point 2.3" format (2 digits for Volts and 3 digits for millivolts) that fits in an unsigned short int (16 bits => 65.536V, while the PSU is limited to 61V) and this is useful for the current as well (in mA).

I have added another C source code file that handles the conversion to/from ASCII format and integer millivolts. What else should I code now ?