@Nick Gammon
I just finish a simulation using Circuit Wizard. I re-do the circuit of your "Dummy Load" circuit, And the circuit can be control be Arduino using a D/A for the 5 V section ( Vin ) and the current monitor, the voltage at the Power resistor can be scale down ( to 5 V ) and going into an analog pin.
The analog circuit follow this formua.
Vout = 9/5 * Vin + 3
First Op-Amp section : Vout-a = - ( Vin * 1.8 )
Second Op-Amp section : Vout = -( Vout-a + -3 V )
Therefore : Vout = - ( - ( Vin * 1.8 ) + - 3 )
Here the schematic.
