Optocoupler: LED (Light-emitting diode) and Fototransistor
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 trought 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 IF (LED`s current).
The relation between these two currents is called Current Transfer Rate (CTR), and depends on the enviromental temperature.
 The higher the enviromental temperature, the greater the phototransistor´s collector current for the same current IF (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 frecuency sensitive device and if the frecuency 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
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Photodiode Optocouplers |
Darlington Optocouplers |
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SCR Optocouplers |
TRIAC Optocouplers |
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