Optocoupler, Phototransistor Output, with Base Connection.



The 4N25 family is an industry standard single channel phototransistor coupler. This family includes the 4N25, 4N26, 4N27, 4N28. Each optocoupler consists of gallium arsenide infrared LED and a silicon NPN phototransistor.


4N25 Features and Specifications

  • IR LED Forward Voltage for turning ON: 1.25V-1.5V (Typically 1.3V, 1.5V being absolute  maximum forward voltage)
  • IR LED Forward Current during ON: 10mA - 60mA (Typically 10mA, 60mA being absolute maximum forward current)
  • IR LED Reverse Voltage Maximum: 5V
  • IR LED Reverse Current Maximum: 100uA
  • Maximum voltage across COLLECTOR and EMITTER of TRANSISTOR: 70V
  • Maximum current allowed trough TRANSISTOR COLLECTOR: 100mA
  • Typical Rise Time: 2us
  • Typical Fall Time: 2us
  • No additional power needed to be applied for chip for making it work.



4N25 OPTOCOUPLER IC has two components integrated in it. One is INFRARED DIODE and another of INFRARED PHOTOTRANSISTOR. The IR DIODE is connected between terminals 1 and 2, the PHOTOTRANSISTOR is connected at terminals 4, 5 and 6.The internal setup of two components can be seen below. The IR radiation emitted by IR LED will not be visible outside the chip. The whole issue of radiation will be working under background.

For understanding the OPTOCOUPLER we will consider a circuit explained below.

We will get the +3.3Voltage pulses from a microcontroller and these are driven to the POSITIVE of IR DIODE. When IR DIODE gets powered it will emit Infrared rays internally, when these rays fell on the PHOTOTRANSISTOR the transistor gets turned ON. When transistor gets tuned ON the current flows through load circuit and the voltage will be seen across the motor. Thus the motor rotates when microcontroller circuit provides HIGH logic to the 4N25 chip.

When the controller output goes LOW, the IR DIODE input goes LOW. With that the IR DIODE stops emitting radiation internally.  Since radiation is cutoff the PHOTOTRANSISTOR gets turned OFF since the radiation acts as BASE TRIGGER. So its LOW RESISTANCE state to HIGH RESISTANCE states. With HIGH RESISTANCE the complete supply voltage appears across the transistor and current flow in the load circuit gets ZERO. So the motor stops rotating. Thus the motor stops rotating when the microcontroller input to 4N25 gets LOW.

In this circuit you can see the motor draws power from the +12V battery source and not from the controller circuit. The PHOTOTRANSISTOR side secondary circuit here is in complete isolation with the CONTROLLER-PHTODIODE primary circuit. The resistors here are placed for limiting the currents in the circuit. Make a note the resistance values