Fod852 photodarlington optocoupler high temperature 4 pin fairchild semiconductor fod852 the fod852 is a photodarlington optocoupler that provides 5000v of electrical isolation between the input and the output using a gallium arsenide ir emitting diode as the source driving a silicon output in darlington configuration with an integral base emitter resistor the device is enclosed in a dil 4 package and is suitable for a number of applications such as power supply regulators digital logic