Optocoupler: LED and  phototransistor

The Optocoupler is a device that consists of a LED (light-emitting diode) and a phototransistor. When the LED emits light, it illuminates the phototransistor causing a current to flow through it.

These two elements are coupled in the most efficient possible way. The Ic output current of the optocoupler (phototransistor’s collector current) is proportional to the input current (LED’s current).

The relation between these two currents is called Current Transfer Rate (CTR), and depends on the environmental temperature. The higher the environmental temperature, the greater the phototransistor’s collector current for the same current (current on the LED) is.

The input (LED circuit) and the output (phototransistor circuit) are 100% isolated and the input impedance is very large (10¹³ ohms, typical) The optocoupler is a frequency sensitive device and if the frequency increases the CTR decreases.

The optocoupler can replace electromechanical components such as relays, switches. This will eliminate the clicking sound that the switches make, improving the switching speed, so there is almost no need for maintenance.

Other types of optocouplers

Photodiode Optocouplers	Darlington Optocouplers
SCR Optocouplers	TRIAC Optocouplers