Obtaining a voltage that is proportional to a current drained by a load can be important in industrial instrumentation. How, however, to obtain a relatively high voltage without causing losses in the load and without dissipating a good power in a resistor in case of higher currents? This can be achieved with the figure circuit, suggested by National Semiconductor. The voltage obtained at the output, depending on the current, depends on the values of R1, R2 and R3 and is given by the formula next to the diagram. The general-purpose field effect transistor supports the equivalent of the BF245. Note that resistors must be accurate, as they determine the correspondence between current and voltage by the formula. Also note that the power supply must be symmetrical, taking advantage of the positive voltage of the fed and monitored load.
