What is 4N28 Optocoupler : Pin Configuration & Its Applications

A semiconductor device like an optocoupler or optoisolator allows the electrical signal to supply in between two isolated circuits. An optoisolator includes two parts within a black box by connecting with pins like an LED & a photosensitive device where an LED emits IR light & a photosensitive device senses light from the LED. So this article discusses one of the IC namely the 4N28 Optocoupler.

What is 4N28 Optocoupler?

A 4N28 optocoupler device is an Industry type single channel-based Photo-transistor Coupler. There are different families of optocouplers available rather than 4N28 namely 4N25, 26, 27, etc. Every optocoupler includes Tx & Rx units like GaAs IR LED & a-Si NPN phototransistor which are electrically isolated. The 4N28 optocoupler symbol is shown below.

4N28 Optocoupler
4N28 Optocoupler

So, these two components will provide an optical trigger mechanism among the two primary & secondary circuits. In the optocoupler, LED is used like a Tx whereas an Rx may be changed like a phototransistor, an Opto-triac, a photo-FET, another kind of photo-sensitive semiconductor element.

4N28 Optocoupler Symbol
4N28 Optocoupler Symbol

Equivalent 4N28 optocoupler ICs are 4N25, 4N26, 4N27, 4N33 & MCT2E but electrical parameters like current, frequency & voltage may vary. So before replacing these ICs in place of 4N28IC, one has to check these parameters otherwise they may get damage.

4N28 Optocoupler Pin Configuration

The pin configuration of a 4N28 Optocoupler is shown below. This IC includes 6-pins where we can simply use 4 or 5 pins. Here each pin of this IC and its functionality is described below.

Pin Configuration
Pin Configuration
  • Pin1 (Anode): Internal Infrared LED’s +Ve terminal
  • Pin2 (Cathode): Internal Infrared LEDs -Ve
  • Pin3 (NC): No Connection
  • Pin4 (Emitter): Emitter terminal of inner Photo Transistor
  • Pin5 (Collector): Collector terminal of inner Photo Transistor
  • Pin6 (Base): Base terminal of inner Photo Transistor

Features & Specifications

The features & specifications of the 4N28 Optocoupler include the following.

  • Interfacing can be done through common logic families
  • No extra power is required to make this chip active.
  • Test voltage for Isolation is 5000VRMS
  • Max collector current allowed by a transistor is 100mA
  • I/O coupling capacitance is below 0.5pF
  • Current Transfer Ration/CTR is 10%
  • I/O isolation voltage is 500VRMS
  • Typical Rise & Fall Time: 3us
  • Forward Voltage of an IR LED ranges from 1.2V to 1.5V
  • Max voltage across C&E terminals of a phototransistor is70V
  • The Forward Current of an IR LED ranges from 10mA to 80mA
  • Max Reverse Current of IR LED is 10uA
  • Max Reverse Voltage of IR LED is 6V

Optocoupler Configurations

Optocouplers are available in four configurations but the main disparity is the usage of the photosensitive device. The configurations are phototransistor, photodarlington transistor, photo SCR & photo TRIAC. For controlling AC circuits, Photo-TRIAC & Photo-SCRs are used whereas DC circuits are controlled through Photo-Darlington & Photo-transistor.

In the photo-transistor configuration, the transistor which is used is either NPN/PNP. In the Darlington transistor, one transistor is used to control the base terminal of the transistor. This transistor offers high gain.

Both the optoisolator & optocoupler are used by interchanging frequently, however, there is a small disparity among the two components. Here, the voltage difference is the unique factor that is expected mainly in between the input & the output.

The main function of an optocoupler is to broadcast analog/digital data in between two circuits while keeping electrical isolation on potentials equal to 5,000V. Similarly, an optocoupler is used to broadcast analog/digital data in between two circuits where the potential disparity is >5,000 V.

What is CTR (Current Transfer Ratio) in an Opto-coupler IC?

The term CTR stands for Current Transfer Ratio and it is the gain of the IC. It can be defined as the ratio of the phototransistor’s collector current to the IR diode’s forward current.

CTR = (IC / IF) x 100%

Here, the current transfer ratio of an optocoupler mainly depends on the temperature, the supply voltage toward the transistor, the forward current through the infrared LED & the current gain of the transistor.

How to use/Circuit Diagram of 4N28 Optocoupler

The 4N28 Optocoupler includes two main components namely the Photo Transistor & IR Diode. The connection of this diode can be done in between two terminals like 1&2 whereas the phototransistor is connected to the remaining three terminals like 4, 5 & 6. This optocoupler IC is connected with a few basic components to make an application circuit.

4N28 Optocoupler Circuit
4N28 Optocoupler Circuit

In the above circuit, the microcontroller generates trigger pulses to optocoupler IC & they work like input signals. An electric motor is connected at the collector terminal which works like an output.
Here, the IR diode gets a +3.3V input signal from the controller then the IR diode will be activated.

Once it gets power, it will produce IR signals internally that will drop on the phototransistor to activate it. Once the transistor is switched ON, the current supplies throughout the load circuit & voltage will come into view across the electric motor. Therefore this motor will turn once the microcontroller generates HIGH logic toward the IC at the input.

Once the trigger pulses generated by the microcontroller go low, then the input of the infrared diode goes LOW. So no power is given to the IR DIODE so that it will stop producing emissions. Once emission is not present the phototransistor will be switched OFF and the transistor moves from less resistance condition to high.

Through high resistance, the whole voltage supply will come out across the transistor & the flow of current within the load circuit will be ZERO. Consequently, the motor stops turning whenever the microcontroller gives less logic toward the IC at the input.

In the above circuit, both the motor-transistor works like load circuit whereas controller-infrared diode performs like trigger circuit. Here, the power drawn by the motor can be done from the voltage source used in the circuit.

The secondary circuit will be in complete isolation through the primary circuit. As a result, we have attained the two circuit’s isolation with the help of 4N28 Optocoupler.

Different factors need to consider while selecting an optocoupler like CTR (current transfer ratio), BW (Bandwidth), input current & max voltage.


The advantages of optocoupler include the following.

  • Interfacing of an optocoupler is simple by logic circuits.
  • Small size component
  • Lightweight
  • Wideband signal transmission
  • Because of the unidirectional signal transmission, the noise from output to input doesn’t get integrated
  • Ensures the control circuits’ protection due to the electrical isolation.

Where to use 4N28 Optocoupler/Applications

Generally, an optocoupler is used for isolating two different circuits. If you want to control a tiny DC motor with the help of a microcontroller, but the DC motor cannot connect to the controller directly because it is a responsive device. Thus, for isolating the load circuit & guarding the controller against voltage variations, a 4N28 optocoupler is used.

  • Optocoupler removes noise from electrical signals
  • It isolates the devices with less voltage from circuits with high voltage.
  • It avoids disturbances from voltage surges like power supply spikes, RF transmissions, etc
  • Allow the usage of small digital signals to control larger AC voltages
  • Isolation of Circuit
  • Logic Coupling through High-Frequency Noise Rejection
  • Lighting systems
  • Isolation of Logic Ground
  • Applications of PWM
  • Detection of Telephone Ring
  • Detection of AC Mains
  • SMPS Feedback
  • Driving of Reed relay

Thus, this is all about an overview of a 4N28 Optocoupler datasheet. This is an industry-standard one-channel phototransistor coupler that includes an LED & an NPN phototransistor. In this IC, both the circuits like primary & secondary are electrically isolated. Here is a question for you, what are the disadvantages of optocouplers?