Use of an optical isolator as an analogue controller

An application is discussed in which the variable photodiode current characteristic of a conventional opto-isolator is used to provide an analogue control signal for a zero-voltage switch thyristor power controller. The device was used to provide 4 kV isolation between the 230 V thyristor circuit and a microcomputer-controlled datalogging and control system in the event of failure of the power control circuitry. It has been extremely reliable in performance and has now been operating for over three years with no signs of deterioration in the photodiode characteristics.