Optocouplers Overview

An optocoupler, also referred to as an optoisolator, is a device that is primarily used to isolate sensitive circuits from electrical transients. In the simplest description, the device consists of a photodiode that is packaged with either a visible light emitting diode or an infrared light emitting diode. Between these two components is a transparent barrier, usually a simple gap.

An optocoupler component usually looks quite a bit like any other integrated circuit. It will consist of a square body with a variable number of leads projecting from the body, which are integrated into the circuit in which the optocoupler is to be included.

Most optocouplers are digital devices, only able to transmit an on or off signal. Circuits can be engineered, however, that allow optocouplers to be used as analogue devices in some circumstances.

 

Construction

There are several different types of optocouplers on the market, some of which are older designs and not in particularly common usage anymore.

The first designs for optocouplers were what were called resistive optoisolators. These devices utilized incandescent light bulbs as the light source used to control the transmission of electric signal. The photo resistors were typically made out of cadmium selenide or cadmium sulphide.

Incandescent bulbs require a certain amount of current to run and, in cases where a circuit did not have adequate current, a neon lamp might be utilized as a substitute for the incandescent light bulb.

One of the limitations of these early designs comes from the fact that incandescent light bulbs have a long lag time before they turn on or off. With the advent of LED components, this problem was greatly reduced, as LED components can be activated very quickly compared to the speed of an incandescent bulb.

These early devices were utilized heavily in telecom, with the properties of the designs making them particularly useful. Optocouplers are able to exhibit a great amount of variability in resistance, which made them ideal for use as compressors and other common components.

These devices also found use in musical instruments, and are still utilized today.

 

LED Optocouplers

Because of the relatively low lag time, LEDs are natural choices for use in optocouplers. These types of optocouplers are referred to as diode optocouplers. They utilize LED components to produce the light – oftentimes near infrared – and silicon photodiodes as the sensor element.

PIN diodes are employed in very fast optocouplers, drastically lowering the response time of the device. Some of these devices are constructed with their own LED drivers and with their own output amplifiers. This can help increase the speed of the devices considerably and devices made to these specifications are referred to as full logic optoisolators. Within one of these devices, the entire circuit is a self-contained unit, with the LEDs and sensors being completely enclosed.

 

Speed and Types

The response speed of an optocoupler is a major consideration in choosing the right component. Different designs perform at different speeds and, depending upon the application, some may be far more suitable than others are.

The slowest types of optocouplers include the resistive optocoupler designs. These utilize a neon lamp, and incandescent light bulb or an infrared LED as their light source. These have very slow response times, but are still used in a variety of different applications.

Silicon controlled rectifier and triac optocouplers are also slower devices, though they may be a bit faster than resistive optocouplers, in some cases. These utilize LED light sources and either a silicon controlled rectifier design or a triac as the sensor.

The next highest incremental speed utilizes infrared LEDs as light sources and either bipolar silicon phototransistor components or Darlington phototransistor components as sensors. These are generally in the mid-range where speed is concerned.

The fastest optocouplers employ infrared LEDs as their light sources and silicon photodiodes as their sensors. These are referred to as diode optocouplers. These are fast enough to be used in digital applications and offer quick response times.

A solid-state relay provides another option for optocouplers. These devices can function at low or high speeds and have a current transfer ratio that is essentially unlimited. These utilize a stack of infrared LEDs as their light sources. The sensor is made up of a stack of photodiodes, which are connected to either an IGBT or two MOSFETs.

 

Current Transfer Ratio

The current transfer ratio is an important consideration in the selection of an optocoupler. In operation, the current transfer ratio – typically abbreviated CTR – is functionally similar to the DC current amplification ratio specification in a transformer.

This specification will be given as a percentage, which refers to the ratio of the output current to the input current. It’s important to keep in mind the fact that the temperature of the room and the age of the component can affect the CTR. Different types of optocouplers have different CTR percentages.

Of the aforementioned types, resistive optoisolators have a variable CTR, which will be less than 100%. SCR optocouplers have a current transfer ratio that will be greater than 100%. Triac designs have a very high CTR.

The fastest optocouplers, diode optocouplers, have a current transfer ratio of 0.1% to 0.2%. Transistor optocouplers, which include Darlington and bipolar silicon designs have a CTR of from 2 to 120% in the former case and from 100 to 600% in the latter.

Solid-state relays have the least limitation on their current transfer ratio.

 

Phototransistor Types

A phototransistor optoisolator is a relatively early design for these components and one that is quite a bit slower than the photodiode option. These require that the phototransistor be biased correctly and that it has the proper load to compensate for the typically low speeds.

Phototransistor optocouplers have some significant variability in their performance, which can be problematic for certain applications.

 

Bi-Directional Optocouplers

Bidirectional optocouplers take advantage of the fact that not only can LEDs emit light, but also they can detect light. Because of this, bidirectional optocouplers can be constructed by including two LEDs in the design. One of the LEDs takes the place of the photo detecting element.

By placing two LEDs facing one another, it’s possible to create an optocoupler that has bidirectional capability. Most of the devices that utilize LEDs in this fashion have near infrared LED components, owing to the fact that near infrared offers better efficiency ratings than visible light.

 

Schematic Symbol

The schematic symbol for an optocoupler includes the lead numbers and the internal components contained within a square, representing the enclosure in which the entire device is contained. A dotted line separates the diagram midway, indicating the dielectric barrier.

 

 

Electrical Characteristics

Optoisolator components are able to provide excellent isolation, protecting one portion of a circuit from being damaged by voltage surges in another portion of the circuit.

Optocouplers have the notable advantage of providing what is called reinforced protection. This term refers to the fact that the optoisolator protects both the equipment into which it is integrated and the operator of that equipment by insulating against voltage surges.

The optoisolator utilizes light to transmit an electric signal. Using a unidirectional model as an example, the LED side of the device receives an electrical signal and transforms it into light via the action of the LED. This light passes through the dielectric barrier and is received by the sensor, turning the light back into an electric signal. These devices do not actually transmit any power.

Another common component utilized for the same purpose is a transformer. Unlike a transformer, however, an optocoupler is capable of working with DC current. These devices are also particularly useful for breaking ground loops. This makes them very useful in industrial applications, where such loops can manifest as common problems.

Optocouplers also have an advantage in the fact that they do not need the resistance between the input and output sides of the circuit to match. This gives them added flexibility.

 

Options for Builders

Optocouplers are very versatile devices and can be selected on the basis of a number of different specifications. Some of the specifications that are provided for these devices include the maximum forward voltage, the maximum input current, the isolation voltage and the output device.

Output devices include Darlington, IGBT gate drives, infrared LEDs and CMOS options, among many others. Transistors and triac outputs, photovoltaic outputs and other options are also available.

Other variables that need to be taken into consideration include the mounting type, which can include through hole, PCB mounting options, DIN rail mounting options and more. Pin numbers generally vary between four and 16 pins. DIP, LSOP, DIP-LF and many other package types are available for these components.

 

Applications

Because of their electrical characteristics, the primary usage for optocouplers is in isolation applications. They are very well suited to protecting circuits from surges and, as an additional advantage, the fact that they do provide reinforced protection makes them very useful for ensuring that the operators using the equipment are also protected from the hazards of voltage surges.

This, of course, gives them obvious applications in digital electronics. Digital electronics are very sensitive to voltage surges and, with proper isolation, they can be protected from hazards such as lightning strikes to power supplies, variability in mains power and other potential hazards that can ruin equipment and, in some cases, go as far as to present a significant danger to the operator of that equipment, as well.

Because they don’t have the limitation of transformers in requiring AC current to operate, they are a go to solution for isolation in DC circuits.

Not all isolation applications are designed to protect against unexpected electrical conditions. Some circuits have one section that will utilize a very high voltage and another section that will utilize lower voltages. Under these predictable conditions, the isolation features of optocouplers are exceptionally useful, providing a very safe and reliable way to ensure that the various voltages utilized in a circuit remain separate and components are not damaged and, of course, that the operators of that equipment are not injured.

Until recently, the performance of these components made them too slow to be used in most digital applications. In recent years, however, advances have been made in their designs that make them entirely suitable for these applications, allowing them to handle higher data transfer speeds and to be made in highly scalable designs, further increasing their flexibility and usefulness.

 

Purchasing

Optocouplers are relatively inexpensive components, quite often coming in packages of more than one unit, allowing for plenty of components to be available for experimentation, testing builds or replacing worn components.

They are produced by most of the major component manufacturers, including Panasonic, Fairchild Semiconductor, Grayhill, International Rectifier, Sharp, Siemens, Toshiba and many other brands. The older, slower designs are still widely available and utilized in applications where there characteristics lend desirable qualities to the end results, such as in audio amplification.

These components can handle large voltages, with the maximum forward voltage specifications reaching into the 200 V range. Isolation voltages are also very high, with some of these devices capable of handling thousands of volts in this role.

These devices are also available with logic output options, which are very common. They come in 1-4 channel designs and a variety of different mounting types and pin numbers. This makes them versatile enough for most any application and supply houses are generally able to accommodate most any need.

Optocouplers are among the best components available for isolation purposes. Most of them are enclosed devices, but there are also devices that have gaps in them which are utilized for specific purposes. These components are very durable and provide very long lifespans, while providing reliable performance throughout their entire life span. In applications where equipment and operators need to be protected from unpredictable voltage surges, whether fast or slow response times are required, optocouplers provide among the most workable and flexible options on the market and have remained consistently popular since they were introduced in the 1960s, having been continuously improved upon since.