Optocoupler

What is Optocoupler

An optocoupler (also called optoisolator) is a semiconductor device that allows an electrical signal to be transmitted between two isolated circuits. To understand what an optocoupler is and how it works, we’ll evaluate how it functions. Two parts are used in this electrical component: an LED that emits infrared light and a photosensitive device that detects light from the LED. Both parts are contained within a black box with pins for connectivity. The input circuit takes the incoming signal, whether AC or DC, and uses the signal to turn on the LED.

The photosensor is the output circuit that detects the light, and depending on the type of output circuit, the output will be AC or DC. Current is first applied to the optocoupler, making the LED emit an infrared light proportional to the current going through the device. When the light hits the photosensor, a current is conducted and switched on. When the current flowing through the LED is interrupted, the IR beam is cut off, causing the photosensor to stop performing.

Benefits of Optocoupler

Prevent ground loops in equipment that drives a remote load
Most ac-operated switching supplies (e.g., those used in computers, telecommunications, and instrumentation) use optocouplers for the isolated feedback path.

Suppress electrical noise effects
Suppress electrical noise effects. For example, it is difficult to take full advantage of a 16-bit ADC because the digital output signals (and noise on the digital ground to which you connect the converter's output) get back into the analog front end. You can extricate yourself from noise with optical isolation of the digital half.

To get a signal up to a circuit floating at high voltage
Designers of high-voltage power supplies sometimes use optocouplers to get a signal up to a circuit floating at high voltage.

The Functions of an Optocoupler

These components serve several critical functions within electronic systems:

Noise Elimination: They help remove electrical noise from signals, ensuring cleaner data transmission. They create an isolated pathway that blocks noise interference by utilizing an LED and a photosensitive receiver.

Voltage Isolation: They isolate low-voltage devices from high-voltage circuits, safeguarding sensitive components. This isolation is crucial for preventing damage from voltage surges such as those caused by radio frequency transmissions, lightning strikes, and power supply spikes.

Signal Control: These components allow small digital signals to manage larger AC voltages. By providing a bridge between different voltage levels, optocouplers enable precise control over high-power applications without direct electrical contact.

6N137 3.3v
The 6N137, HCPL2601, HCPL2611 single-channel and HCPL2630, HCPL2631 dual-channel optocouplers consist of a 850 nm AlGaAS LED, optically coupled to a very high speed integrated photo-detector logic...
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Reflective Photo Interrupte
The reflective photoelectric switch is a light sensing device that emits and receives the body. The diode emits 940nm itself and receives near-infrared light as the light source.
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In-ear Sensor
The TR2FdTRF4-BS01-RTA Composed of a GaAs VCSEL and a NPN Photo-transistor which is encased side-by-side on converging optical axis in a black thermoplastic housing.
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ITR 8307
The TR2DfTRF4-BS01-RWR is a light reflection switch which includes a GaAs IR-LED Transmitter and a NPN photo-transistor with a high photosensitive receiver for short distance，operating in the...
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SOP4 RP TRIAC
The TDM301X and TDM302X and TDM305X and TDM307X series combine an AlGaAs infrared emitting diode as the emitter which is optically coupled to a monolithic silicon random-phase photo triac in a...
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IGBT Driver Gate
The TDL341 series Photocoupler is ideally suited for driving power IGBTs and MOSFETs used in motor control inverter applications and inverters in power supply system. It contains an AlGaAs LED...
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SOP 4 Optocoupler
TD817 SOP4 Optocoupler Belong to SOP 4 Optocoupler
The TD817 series combine an AlGaAs infrared emitting diode as the emitter which is optically coupled to a silicon planar phototransistor...
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DIP 4 Optocoupler
TD816 belongs to DC Input, Photoelectric Crystal coupler DIP 4 Optocoupler
The TD816X1 series combine an AlGaAs infrared emitting diode as the emitter which is optically coupled to a...
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SOP 4 Transistor Optocoupler
TD356 series provide the most stable isolation belong to SOP4 Transistor Optocoupler
The TD356 series combine an AlGaAs infrared emitting diode as the emitter which is optically coupled to...
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L SOP 4 Optocoupler
1018 LSOP 4 Optocoupler DC Input Photo Transistor Coupler
The TD1018 series combine an AlGaAs infrared emitting diode as the emitter which is optically coupled to a silicon planar...
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S SOP 4 Optocoupler
TD217 DC Input, Photo Transistor Coupler belong to SSOP4 Optocoupler.
The TD217 series combine an AlGaAs infrared emitting diode as the emitter which is optically coupled to a silicon...
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DIP 6 Zero-Cross Optocoupler
TD3061 DIP6, DC Input, Zero-Cross Photo TRIAC Belong to DIP6 Zero-Cross Optocouple
The TD303X, TD304X and TD306X seriescombine an AlGaAs infrared emitting diode as theemitter which is...
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Optocoupler And Opto-Isolator Specifications

Current transfer ratio, CTR
The current transfer ratio of an optocoupler is one of the key specifications. It is the ratio of the current that flows in the output device divided by the current on the input device.
The CTR will vary according to the type of opto-coupler used in the output, those using photodarlingtons will be much higher than those using ordinary phototransistors. Values may be anywhere between 10% and 2000% - 5000%.It should be noted that the CTR tends to vary with the the input current level. Although it will vary according to the device, for man optocouplers it will peak for input current levels around 10mA falling either side of this value.

Bandwidth
In order to understand the maximum data rates that can be used with an opto-coupler, it is necessary to know the bandwidth.
For many opto-couplers using phototransistors it may only be in the region of 250 kHz, and for those using photodarlingtons it may be a tenth of this figure. Some much faster optocouplers are available. Typically the lower the CTR, the faster the rise and fall times

Input current
This is the current required for the input transmitter device - LED. The value is used to calculate the series resistor used to limit the current.

Output device maximum voltage
For opto-couplers using transistors, the maximum figure will be equal to the VCE(max) for the transistor. For opto-couplers using other devices in the output, the equivalent rating should be used. Also remember that a suitable margin should be retained as it is never advisable to operate devices close tot heir maximum ratings.

Optocoupler Parameters

Isolation voltage

This is the maximum permissible DC potential that can be allowed to exist between the input and output circuits. Typical values vary from 500V to 4kV.

VCE(MAX)

This is the maximum allowable DC voltage that can be applied across the output transistor. Typical values vary from 20V to 80V.

IF(MAX)

This is the maximum permissible DC current that can be allowed to flow in the input LED. Typical values vary from 40mA to 100mA.

Bandwidth

This is the typical maximum signal frequency that can be usefully passed through the optocoupler when the device is operated in its normal mode. Typical values vary from 20kHz to 500kHz, depending on the type of device construction.

Comparison Of Optocouplers And Digital Isolators

Item	Impact	Digital Isolators	Optocoupler
Timing performance	Enables higher throughput and efficiency for end product	Low propagation delay and skew, better part to part matching	High propagation delays and skew, worse part to part matching
Parasitic capacitive coupling	The lower the parasitics, the higher the CMTI	Less than half the parasitic coupling of optocouplers	High parasitic coupling with interdependent parameters
Reliability and high temperature operation	Longer product lifetimes	No wear out mechanisms, 60+ year operating lifetime at 125 °C at maximum VDD	Intrinsic wear-out mechanisms; 10x lower lifetime
Input current	High input current means higher power consumption	CMOS input buffers need very low input current	Requires higher input current to be competitive
Ease-of-use	Minimum external BOM needed to extract full functionality and performance	Fewer second order effects, minimum BOM required for full performance guarantee	Significant first and second order effects, temperature dependencies, imprecise current thresholds, CTR require external BOM to get stable performance
Electro-magnetic immunity and radiation	Immunity provides robustness and low radiation implies low noise generation	Capacitive-coupled devices are comparable to optos while magnetically coupled devices can be noisy and are susceptible to external EM noise	Optos are generally highly immune and have low radiation
Safety compliance	Ensures safety standards are tested and certified	General trend is newgeneration isolators are on par with optos	Optos have traditionally been used for many years and are compliant

PCB Layout Guidelines

Before adding an Optocoupler to your PCB layout, consider these three guidelines:

Keep optocoupler ground connections separate

A standard Optocoupler includes two ground pins, one for the LED and another for the photosensitive device. Connecting these grounds will open your sensitive circuitry to any noise from the external ground. To avoid this, always create two connection points, one for external ground pins and the other for input ground wires.

Choose the right current limiting resistor value

Selecting a current limiting resistor that operates at an Optocoupler's minimum value will produce erratic behavior. It's also possible to choose a resistor that provides too much current, which will pop the LED. When selecting a value for your resistor, be sure to find the value of the minimum forward current from the Current Transfer Ratio chart in your Optocoupler's datasheet.

Know what kind of optocoupler you need

Not every Optocoupler is created equal, and you'll need to select the right type for your application. For example, an Opto-Triac is used if you need to control an AC load. Opto-Darlington's are only for small input currents. If all you need is a standard input isolation, then a general PC817 Optocoupler will get the job done.

Important Optocoupler Specifications

Isolation voltage (Viso): It is defined as the absolute maximum AC voltage that can exist across the input and output circuit stages of the optocoupler, without causing any harm to the device. The standard values for this parameter may fall between 500 V to 5 kV RMS.

VCE: It may be understood as the maximum DC voltage that could be applied across the device's phototransistor pinouts. Typically this may range between 30 to 70 volts.

If: It is the maximum continuous DC forward current that may flow in the IR LED or the IRED. It is the standard values of current handling capacity specified to a phototransistor output of the optocoupler, which may range between 40 to 100 mA.

Rise/fall time: This parameter defines the logical speed of the optocoupler response across the internal IR LED and the phototransistor. This may be typically from 2 to 5 microseconds for both rise and fall. This also tells us about the bandwidth of the optocoupler device.

Types of Optocoupler

Photo-transistor optocoupler
The Transistor type can be anything whether PNP or NPN.Photo-Transistor can be further of two types depending on the output pin availability.This pin 6 is used to control the sensitivity of the photo-transistor. Often the pin is used to connect with ground or negative using a high value resistor. In this configuration, false triggering due to noise or electrical transients can be controlled effectively.
Also, before using Photo-transistor based optocoupler, the user must know the maximum rating of the transistor. PC816, PC817, LTV817, K847PH are few widely used photo-transistor based optocoupler. Photo – Transistor based opto-coupler is used in DC circuit related isolation.

Photo-darlington transistor optocoupler
Darlington Transistor is two transistor pair, where one transistor controls other transistor base. In this configuration the Darlington Transistor provide high gain ability. As usual the LED emits infrared led and controls the base of the pair transistor.
This type of opto-coupler also used in DC circuit related area for the isolation. The 6th pin which is internally connected to the base of the transistor, used to control the sensitivity of the transistor as discussed previously in photo-transistor description. 4N32, 4N33, H21B1, H21B2, H21B3 are few photo-Darlington based opto-coupler example.

Photo-TRIAC optocoupler
TRIAC is mainly used where AC based control or switching is needed. The led can be controlled using DC, and the TRIAC used to control AC. Opto-coupler provide excellent isolation in this case too. Here is one Triac Application. The photo-TRIAC based opto-coupler examples areIL420, 4N35 etc are example of TRIAC based opto-coupler.

Photo-SCR based optocoupler
SCR stand for Silicon controlled rectifier, SCR also referred as Thyristor. In the upper image a Photo-SCR based opto-coupler’s internal construction is shown. Same as like other opto-coupler the LED emit Infrared. The SCR is controlled by the intensity of the LED. Photo-SCR based Opto-coupler used in AC related circuitry. Learn more about Thyristor here.
Few Examples of photo-SCR based opto-couplers are:- MOC3071, IL400, MOC3072 etc.

Application of Optocoupler

Power supply regulation
Optocouplers are often used in switch-mode power supplies to provide feedback and maintain a constant output voltage. They isolate the primary side from the secondary side, ensuring safety and minimizing noise interference.

Industrial automation
In industrial automation systems, optocouplers help protect sensitive control circuitry from high voltage transients, noise, and ground loops that can occur in harsh environments. They are used for isolating PLC (Programmable Logic Controller) inputs and outputs, as well as for data communication between devices.

Motor control
Optocouplers are employed in motor control circuits to isolate high-power components, such as relays or motor drivers, from low-power control signals. This ensures that the control signals remain unaffected by the high voltages and currents generated by the motor.

Digital logic interfaces
Optocouplers enable communication between different digital logic levels and incompatible voltage levels, allowing them to function as level shifters. They can also be used to isolate and protect microcontrollers and other sensitive digital components from high-voltage signals.

Audio equipment
In audio equipment, optocouplers are used to isolate signal paths, preventing ground loops and other noise sources from interfering with audio quality. They can also be employed in volume control circuits and compressors.

Telecommunications
Optocouplers play a crucial role in providing signal isolation in telecommunications equipment, such as modems and telephone exchanges. They prevent ground loops and voltage surges from damaging sensitive circuitry, ensuring reliable data transmission.

Medical devices
In medical devices, optocouplers are used to maintain electrical isolation between the patient and the device, ensuring safety and preventing electrical interference from affecting the device’s operation.
In summary, optocouplers serve a wide range of applications across various industries, offering electrical isolation, noise immunity, and protection for sensitive circuits. Their versatility makes them an essential component in many electronic systems.

How to Use an Optocoupler to Pass Signals Between Controllers at Different voltages

Gather materials
For this project, you will need:
-Devices for communication (Raspberry Pi, Arduino, PLC, etc.) and their associated power sources
-Prototyping breadboard
-A variety of resistors (we used 2x56 Ohm and 2x1K Ohm)
-A 4N25 optocoupler, or similar chip

Understand 4N25 pinout
The 4N25 optocoupler datasheet shows the pinout, and the short version is this: the signal you pass through the left-side circuit (pins 1 and 2) will power an LED. This LED will emit photons that interact with the transistor connecting pins 4 and 5, allowing current to flow between the pins (from 5 to 4).

Choose an appropriate resistor
The 4N25 LED, according to the datasheet, typically drops 1.3 V, and can handle a maximum current of 60 mA.
Using Ohm's Law, choose a resistor value to place between your signal source and pin 2. The voltage drop across this resistor will equal the voltage of your source signal minus the 1.3 V of the LED, and you can set the current equal to 40 mA (or any value you like, as long as it is under 60 mA).
One additional consideration is the power through the resistor. Most conventional resistors are rated for 1/4 Watt, so be careful that your power (voltage times current through resistor) does not exceed .25-- otherwise, your resistor will get very hot, and may potentially fail.

Build the circuit
To build the circuit, you'll want your signal source to lead to the resistor, resistor to pin 1, then pin 2 to signal ground.
On the other side of the chip, you'll want your receiving source to lead to pin 5, then receiving circuit connected to pin 4. This circuit should eventually connect to ground; the chip acts as a switch between pins 5 and 4.

Test your circuit
You should now be able to pass an electrically isolated signal from the left side of the chip to the circuit on the right side.
Some final considerations: there is a collector current limitation with the optocoupler, in general, not allowing more than 50 mA to flow. If your receiving circuit requires more current/power, you may want to consider a different solution. I use this circuit to pass signals between my PLC and Raspberry Pi, but cannot use it to power components of my project (like solenoids).

How to test an optocoupler

Step 1
First the anode and cathode of the LED ( in this case pins 1 and 2 ), and then using an ohmmeter set on the ‘X1 Ohm' domain, measure between pins 1 and 2, and you should get one reading measuring one way and no reading the opposite way (just like you check a diode). If you get a value either way or no value at all, then certainly there is a problem with the LED, and you should find another optocoupler.

Step 2
If the LED is good then we should check the phototransistor, you could measure it with the ohmmeter just like the LED between pins 3 and 4 ( the emitter and collector ), and you should get a high resistance value both ways if the phototransistor is good. If you'll get no reading at all, is probably because most phototransistors have such high resistance between emitter and collector that the ohmmeter can't measure; if this is the case you could connect two ohmmeters in series thus increasing the measuring domain; …although i think most don't have two meters so i recommend the 'empirical' method, presuming you have a variable DC regulated power supply.

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Asked Questions

Q: What are the limitations of optocoupler?

A: Optocoupler needs external biasing voltage for its operation. The High-frequency response is poor. Optocouplers using phototransistors do not have such as good a linear relationship between the changes in light input and output current as photodiode types.

Q: What are the primary factors used when assessing an optocoupler?

A: While there are many important factors, there are several which are noteworthy: isolation, current-transfer ratio (CTR), linearity (for analog use), and speed.

Q: When should I use an optocoupler?

A: Optocouplers manage to send signals between circuits with separate grounds, providing an isolated galvanic barrier between them. Therefore, an optocoupler is a solution for circuits that need to be isolated from each other for safety or regularity reasons and need to have interaction in between.

Q: What are the 4 types of optocoupler?

A: Optocouplers are available in four general types, each one having an infra-red LED source but with different photo-sensitive devices. The four optocouplers are called the: Photo-transistor, Photo-darlington, Photo-SCR and Photo-triac as shown below.

Q: Why do optocouplers fail?

A: In comparison, the forward voltage drop of the LED in the optocoupler does not change significantly, but there is a phenomenon of slow decline. Therefore, it can be determined that the failure of the optocoupler is caused by the leakage current of the LED and the change in the breakdown voltage.

Q: Why use optocoupler instead of transistor?

A: The voltage difference by itself could be worked out with resistor dividers, or a simple transistor circuit if a boost in voltage is needed. The reason optocouplers are widely used regardless of voltage differences is that they isolate grounds as well for galvanic isolation.

Q: Is optocoupler active or passive?

A: The organic optocouplers (also called “organic optical isolators”) are polymer-based electronic passive optical components able to combine or split transmission data (optical power) from polymeric optical fibers.

Q: Can I use an optocoupler as a relay?

A: Yes, as long as the relay coil current is less than the maximum collector current of the optocoupler then this should work.

Q: Does an optocoupler need a resistor?

A: The input current to the optocoupler LED must be limited via a series-connected external resistor which, as shown in Figure 10, can be connected on either the anode or the cathode side of the LED.

Q: Can I use optocoupler instead of relay?

A: Optocouplers are cheaper than a relay, longer lasting than a relay, use a lot less power than a relay, and handle less power than a relay. So for input it makes no sense to use a relay because the inputs don't use significant power. For output it might, since the relay can switch high power devices.

Q: What are the practical uses of optocoupler?

A: Optocouplers are often used to reject back EMF, noise, and electrical surges from entering an MCU circuit. Optocouplers create a safe connection between high voltage equipment and microcontrollers with a means of complete electrical insulation.

Q: Which optocoupler is best?

A: PC817 is one of the most popular optocouplers available on the market. It is a standard optocoupler circuit consisting of a photo-transistor and an infrared LED diode that isolates two parts of a circuit and prevents the transmission of unwanted noise and ground loops.

Q: What is the principle of optocoupler?

A: The optocoupler is the portal between the trigger and the switching circuit. The input trigger is applied to the LED of the opto which illuminates and makes the photo-transistor conduct. The voltage from the photo-transistor passes across the collector to the emitter and finally reaches the triac's gate to operate it.

Q: Do optocouplers fail open or closed?

A: It's a frequent fallacy that optocouplers always fail with an "open" circuit when exposed to high voltages. Although optocouplers can fail in a variety of ways, depending on the distinct failure modes in high voltage systems, they can also fail in "shorted" circuits.

Q: What is the minimum voltage for optocoupler?

A: Therefore, the optocoupler should turn on at sensor Voltages above 3.3V. The maximum sensor Voltage is 24V. The LED in the optocoupler has an absolute maximum forward current rating of 60mA. 2mA is enough to reliably turn it on.

Q: What is the difference between digital isolator and optocoupler?

A: Yet, isolation imposes constraints such as delays, power consumption, cost, and size. A digital isolator's goal is to meet safety requirements while minimizing incurred penalties. Optocouplers, a traditional isolator, incur the greatest penalties, consuming high levels of power and limiting data rates to below 1 Mbps.

Q: Is an optocoupler analog or digital?

A: The optocoupler is used to transmit analog or digital information between circuits while maintaining electrical isolation at potentials up to 5,000 volts. An optoisolator is used to transmit analog or digital information between circuits where the potential difference is above 5,000 volts.

Q: Why is it called optocoupler?

A: An optoisolator (also known as an optical coupler, photocoupler, optocoupler) is a semiconductor device that transfers an electrical signal between isolated circuits using light.

Q: How much current can optocoupler handle?

A: Typical optocouplers can handle input and output currents from a few microamps to tens of milliamps.

Q: What are the terminals of optocoupler?

A: Two of the terminals connect to and drive, an infrared (IR) LED, while the other two are the output of a phototransistor (housed in the same package) which senses the light given off by the LED. (A special optical conduit is built into the optocoupler between the light emitting LED and the phototransistor receiver).

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SMD 5930 Matel Element, Transistor Optocoupler, Far Infrared LED