Optocouplers and Optoisolators Keep Energy Contained During Sensitive Transmissions

Optocouplers and optoisolators provide isolation between mixed signal circuit blocks to ensure transmission integrity by protecting sensitive circuit blocks. Isolation prevents high-energy voltage or current transients from broadcast into sensitive downstream circuits. Voltage or current transients disrupt data transmission and can damage components. Light is used to effectively transmit intended communication information through air while protecting parts.

An optical interface is created with use of an LED and phototransistor enclosed in a light-tight container. Current in the driving block activates the LED to produce light on the phototransistor. The phototransistor delivers current to downstream blocks for effective communication. There is a proportional relationship, LED to phototransistor, for transmission across the isolation barrier. Sometimes, amplification is included on the receiving end to boost the signal for continuation to the next circuit block.

Use of isolation in mixed signal designs is also used to meet safety standards. Worldwide safety standards exist to guide developers in creating devices to protect users from electrical shocks, mechanical hazards, fire, or electromagnetic interference. Vendors continue to evolve isolation materials for robust protection in both commercial and industrial applications.

Defining Characteristics of Optocouplers / Optoisolators

Optocouplers and optoisolators perform the same function. Optocouplers typically protect from transient energies up to 5000V, while optoisolators protect in the upper energy ranges up to 50,000V. The devices create a material barrier that prevents electrical energy from crossing through the physical medium. Instead, electrical signals are transmitted through air.

The most common parts making the optocoupler / optoisolator combination are an LED and a phototransistor. Other types of combinations include use of LEDs together with either photodiodes or with light-activated silicon controlled rectifiers. Sometimes a lamp is used together with a photoresistor. Whatever the combination, the combinations employ the use of light to transmit photons through air to the receiving half of the opto-device for transmission of data to downstream circuit blocks.

Isolating intended data communication by transmitting through air allows containment of high-energy voltages or currents within one area of the device. Energy isolation addresses safety requirements for devices by preventing large amounts of energy delivery to either humans or machines that may be damaged. Worldwide standards specify energies for protection against electrical shock, mechanical hazard, or electromagnetic interference.

Optocouplers or optoisolators bridge isolation regions onn PCBAs

Parts to Consider When Selecting an Optocoupler/Optoisolator

Take a look at your system to determine the type of optocoupler or optoisolator you’ll need for your design. Understanding the amount of energy your upstream circuit produces will guide selection. The opto-devices produce output current that is a ratio of input current, so look for the current transfer ratio (CTR) on opto datasheets to find one that fits your design intent.

Light-emitting diodes are one part of an opto-device

The following are three opto-devices for consideration.

Broadcom, HCPL-0931-000E

This part is intended for with Fieldbus protocols in industrial applications. It is an optoisolator approved to safety standards described in UL1577 and IEC 61010-1 providing 2500 VRMS isolation. It is a bidirectional communication device and may be paired with either +3.3V CMOS or +5V TTL logic devices.

The HCPL-0931 CMOS digital isolator features high speed performance and excellent transient immunity specifications. The symmetric magnetic coupling barrier gives the device a typical pulse width distortion of 2ns, a typical propagation delay skew of 4ns, and 100Mbaud data rate, making it one of the industry’s fastest digital isolators.

Found on page 1 of HCPL-0931 High Speed Digital Isolator datasheet

This part is available in a dual 8-pin, small outline SO-8 package.

On-Semiconductor/Fairchild Semiconductor, HCPL062N

This part is an optocoupler with common-mode rejection up to 1,000V intended for very high speed Fieldbus applications. It boasts superior common-mode rejection of 25kV/µs and is UL recognized. It’s logic gate output handles either CMOS or TTL allowing wired-OR collector configuration.

The HCPL062N optocouplers consist of an AlGaAs LED, optically coupled to a very high speed integrated photodetector logic gate consisting of bipolar transistors on a CMOS process for reduced power consumption. The output features an open collector, thereby permitting wired OR outputs. The devices are housed in a compact small-outline package. The coupled parameters are guaranteed over the temperature range of -40℃ to +85℃. An internal noise shield provides superior common mode rejection.

Found on page 9 of HCPL062N 3.3V Dual Channel High Speed-10MBit/s Logic Gate Optocouplers datasheet

This part is available in a dual 8-pin, small outline SO-8 package.

Toshiba, TLP2161

This is an optocoupler IC containing two circuits within an 8-pin package. It is rated to isolate up to 2500 VRMS and boasts a common-mode transient immunity of +20kV/µs. It is designed with a +5V totem pole output making it compatible with TTL. This part is approved to several worldwide standards and a packaging option approved to EN60747-5-5 is available by request.

The Toshiba TLP2161 consists of a high-output GaAlAs light-emitting diode coupled with a high-speed photo-diode-transistor chip. It has two circuits built into a single SO8 package, which can reduce the mounting area. This photocoupler guarantees operation at up to 125℃ and on supplies from 2.7V to 5.5V.

Found on page 2 of TLP2161 GaAlAs Infrared LED & Photo IC datasheet

This part is available in a dual 8-pin, small outline SO-8 package.

Optocouplers/optoisolators are great devices for isolating high energy transients from broadcasting into your sensitive low-energy circuit blocks. Most mixed signal designs have high-energy and low-energy blocks that share transmission signals, so isolation must be provided to ensure both signal integrity and safety. Optocouplers or optoisolators are elegant parts to provide the isolation you require. Our website lists all possible opto devices for your needs. Use our part selector to find the one you need.

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