Optocouplers or optoisolators are devices that use light to transfer signals and data from one system to another within a piece of electronic equipment without a direct electrical connection. They are both housed in a single package with a light-conducting medium between them. This allows the total electrical isolation of electronic circuits while transmitting information from one voltage potential to another. This information can be either an analog or digital signal and the circuits can be isolated up to 100,000V, depending on the type of device and the application. The devices may be packaged in a variety of confi gurations such as P-Dips, mini fl at packs, SOIC’s, cylinders, cubes, and other specialty package designs. In all optocoupler or optoisolator devices, input signals are converted to a pulse of light from a LED. This pulse of light is transmitted to a silicon photosensor.

The photosensor can be either an analog or digital device depending on the type of input signal to be transferred across the device. When an analog signal is required for the application, such as for 4 to 20mA circuit applications, the photosensor can be either a photodiode or phototransistor. Both of these devices provide an analog output signal that can be used for a variety of analog
applications. The most critical parameters for analog devices are: isolation voltage, linearity and current transfer ratio. The current transfer ratio is defined by

A photodiode provides the best linearity with fast transition times that are typically in the nanosecond range and a very low CTR. The largest concern using a photodiode is that the CTR is very low typically in the micro amp current levels.

The phototransistor is a much easier analog device to use with CTR levels typically between 10% and 200%. As the CTR increases, the transition times increase from typically nanoseconds to as much as microseconds. Also with the increase in CTR, the linear range of the device is reduced while still meeting most typical applications. An analog product group with a higher CTR is the photodarlington series. These devices have a very narrow linear range with long transition times, typically from microseconds to milliseconds depending on the load conditions. Photodarlington optocouplers provide the best CTR range, typically 200% to 1,000% and are used in applications requiring minimal drive currents.

For those optocoupler and optoisolator applications that operate using logical states, the industry provides Photologic sensor confi gurations. Typically photologic devices are available with six different configurations; Totem-Pole (push-pull), Internal 10K pull-up, Open-collector and the inverted output versions of these three types. The photologic device is the most accurate in the fact that as long as suffi cient light is irradiated on the photologic sensor, the output switches to a high and when the light signal removed, the device switches to a low logical state.

The basic optocoupler/optoisolator is designed with a single LED and photosensor while some versions may include back-to-back LEDs and a variety of photosensor confi gurations.

The optocoupler/optoisolator market includes switch mode power supplies, telecommunications equipment, and programmable logic controllers. Some optoisolators are called Triac Drivers, for controls in appliances, lighting ballast, etc. In the industrial and medical markets, communication applications are used to control input/output (I/O) systems, sensors, temperature control systems and regulation systems, electric motor and drive systems as well as instrumentation isolation.

Source: Technology First