A power supply unit for use in a measuring instrument at a plant. An output from a current transformer mounted on a power supply line is controlled so that the range of a primary current producing a stable output is increased. The invention includes a current transformer mounted on a cable for providing power to an electric motor, and a power supply unit. The power supply unit includes a rectifier circuit, a short circuit, a reverse current inhibit element, a condenser, a voltage watching circuit, and a constant voltage element. The voltage watching circuit outputs a command signal when the voltage of the condenser exceeds a first level and connects the short circuit 32.
