an i / o circuit for use with an industrial controller provides a photovoltaic optical isolator communicating between a controller and a field - side of the i / o circuit , the photovoltaic optical isolator eliminating the need for high wattage power dropping circuits for powering the field - side of the i / o circuit from high - voltage field signals . the field effect transistor type transistors permit use of the low - power photovoltaic optical isolator while allowing flexible connections to various field circuit types .