Optical current isolator circuit

Abstract

A current isolation circuit converts a pulse width modulated input signal to an isolated DC output current whose magnitude is a function of the duty cycle of the input signal. The isolation circuit includes an optical isolator which receives the pulse width modulated input signal and has an output which changes between a conductive and a non-conductive state in response to the input signal. A voltage regulating diode, such as a Zener diode, is connected to the output of the optical isolator and to an isolated power supply so that the voltage across the Zener diode varies between two known voltage values in response to the switching of the optical isolator output between conductive and non-conductive states. A signal derived from the Zener diode is filtered to produce a reference signal with a magnitude which is a function of the duty cycle of the input signal. An output current path is connected between the power supply and an output current terminal, and includes a current control device which controls the output current as a function of a control signal. A comparison signal derived from the output current path is compared to the reference signal, and the control signal is produced as a function of the comparison. As a result, the output current is controlled as a function of the input signal duty cycle.