Optocoupler 4N28 Pin Details

The 4N28 is a Industry type One Channel Photo-transistor Coupler. This family includes the 4N25, 4N26, 4N27 and 4N28. Each optocoupler consists of Gallium Arsenide infrared LED and a silicon NPN photo transistor. In the chip the primary circuit and secondary circuit are isolated electrically. The pair of diode and photo transistor is there to provide optical trigger mechanism between the primary and secondary circuit.

Optocoupler 4N28 Pin details:
4N28 is a six pin IC TYPE. Basically we use only four pins in the chip but can use up to five pins. The internal connections of diode and photo transistor are described below.

Pin  Pin Name Pin description
1 Anode (A) Positive terminal of internal IR LED
2 Cathode (C) Negative terminal of internal IR LED
3 NC  No Connection
4 Emitter (E) Emitter of internal Photo Transistor
5 Collector (C) Collector of internal Photo Transistor
6 Base (B)  Base of internal Photo Transistor

4N28 Features and Specifications
Isolation voltage 5000V RMS
Input-Output- capacitance < 0.5pF
Input-Output isolation voltage: 500V RMS
IR LED Forward Voltage for turning ON: 1.2V-1.5V
IR LED Forward Current during ON (IF): 10mA – 80mA
IR LED Maximum Reverse Voltage: 6V
Maximum voltage across COLLECTOR and EMITTER of PHOTO TRANSISTOR: 70V
Maximum TRANSISTOR COLLECTOR(IC): 100mA
Typical Rise Time: 3us
Typical Fall Time: 3us
No additional power needed to be applied for chip for making it work.

4N28 Equivalents
4N28 optocoupler IC – 4N26, 4N27, 4N25, 4N33, MCT2E

How to use 4N28 optocouple:
They are IR Diode and Photo Transistor. The IR DIODE is connected between terminals 1 and 2. The Photo Transistor Photo Transistor is connected at terminals 4, 5 and 6. We will connect chip with other few components to form an example circuit. At the INPUT we will get the +3.3V logic signal from a microcontroller. Since the pin represents POSITIVE of IR DIODE, it will get powered. Once powered, the IR DIODE will emit Infrared rays internally. These rays will fall on the Photo Transistor to gets it turned ON. When transistor gets turned ON the current flows through load circuit and a voltage will appear across the motor. Thus the motor rotates when microcontroller provides HIGH logic to the chip at the INPUT.
When the microcontroller trigger goes LOW, the IR DIODE input goes LOW. Since no power is provided the IR DIODE stops emitting radiation. Once radiation is absent the Photo Transistor gets turned OFF.And the transistor jumps from LOW RESISTANCE state to HIGH RESISTANCE state. 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 provides LOW logic to the chip at the INPUT. Motor draws power from the +12V battery source and not from the trigger circuit. And Photo Transistor side secondary circuit here is in complete isolation with the IR DIODE primary circuit. So we have achieved the objective of isolating the two circuits using optocoupler.

Use of Optocoupler:
1. Circuit isolation
2. DC motor speed control
3. Lighting systems
4. PWM applications
5. AC mains detection
6. Reed relay driving
7. Switch mode power supply feedback
8. Logic ground isolation

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