Description 
The 4N28 is an Industry Standard Single Channel Phototransistor Coupler. This family includes the 4N25, 4N26, 4N27 and 4N28. Each OPTOCOUPLER consists of gallium arsenide infrared LED and a silicon NPN phototransistor. In this chip the primary circuits and secondary circuits are isolated electrically. The pair of diode and phototransistor is there to provide optical trigger mechanism between the primary and secondary circuits.
Specifications
Attribute
Value
Mounting Type
Through Hole
Output Device
Phototransistor
Maximum Forward Voltage
1.5V
Number of Channels
1
Number of Pins
6
Package Type
DIP
Input Current Type
DC
Typical Rise Time
2µs
Maximum Input Current
60 mA
Isolation Voltage
5 kVrms
Maximum Current Transfer Ratio
30%
Typical Fall Time
2µs