Optocoupler: Basics, working principle, applications

Owing to the increasing degree of automation, the separation of potential between control circuits (control side/field side) is becoming increasingly significant. Optocouplers have been the unchallenged signal isolation solution for more than four decades. They are a good alternative to the traditional, mechanical relay interfaces.

What is an optocoupler?

An optocoupler is a relatively simple device consisting of a light source (usually an LED) and a light sensor, both embedded in one package. It is used primarily for isolation rather than to switch a high current.

Sometimes known as an optoelectronic coupler, optoisolator, photocoupler, or optical isolator.

What does an optocoupler do?

An optocoupler allows one section of a circuit to be electrically isolated from another. It protects sensitive components, such as logic chips or a microcontroller, from voltage spikes or incompatible voltages in other sections of a circuit.

Optocouplers are also used in medical devices where a patient has to be protected from any risk of electric shock, and are used in devices that conform with the MIDI standard for digital control of music components.

Applications of an optocoupler

The output from a logic chip passes through an optocoupler to an inductive load such as a relay coil, which may create voltage spikes that would be hazardous to the chip.

The noisy signal from an electromagnetic switch passes through an optocoupler to the input of a logic chip.

The low-voltage output from a sensing device on a human patient passes through an optocoupler to some medical equipment, such as an EEG machine, where higher voltages are used.

Internally, an optocoupler works on the same principle as a solid-state relay. An LED is embedded on the input side, shining light through an interior channel or transparent window to a sensing component that is embedded on the output side. Because the only internal connection is a light beam, the input and output of the optocoupler are isolated from each other.

Isolation transformers were traditionally used for this purpose prior to the 1970s when optocouplers became competitive. In addition to being smaller and cheaper, an optocoupler can also pass slow-changing signals or on-off DC states that a transformer would ignore.

More recently, inductive and capacitive coupling components have become available in surface-mount packages that are competitive with optocouplers for high-speed data transfer. They also claim to be more durable. Because of the gradual reduction in output from an LED, the performance of an optocoupler degrades over time and is typically rated for up to 10 years.

How does an optocoupler work?

The LED in an optocoupler almost always emits light in the near-infrared part of the spectrum and is matched to the sensitivity of a phototransistor, or a photodiode, or (less often) a photoresistor that provides the output. Photosensitive triacs and SCRs are also sometimes used.

The most common type of optocoupler uses a bipolar phototransistor with an open-collector output. Schematic symbols for this type are shown below.

The most common generic form.

Two diodes on the input side allow the use of alternating current.

An additional terminal allows the addition of bias to the photosensitive base of the output transistor, to reduce its sensitivity.

An Enable signal can be used as the input to the NAND, suppressing or enabling the output.

A photodarlington allows a higher emitter current.

Relatively uncommon, and is also used for a solid-state relay.

In each symbol, the diode is an LED, and the zigzag arrow indicates the light that is emitted from it. A pair of straight arrows, or wavy arrows, may alternatively be used.

An optical switch can be thought of as a form of the optocoupler, as it contains an LED opposite a sensor. However, the LED and the sensor are separated by an open slot, to allow a thin moving object to pass through, interrupting the light beam as a means of detecting the event.

Features of optocoupler

An optocoupler provides the necessary degree of safety and features other technical properties such as:

• Low power consumption on the controller side
• High switching frequency
• No contact bounce
• Wear-free switching
• Vibration resistance
• Use independently of positioning
• No need for mechanical parts
• Long service life
• High insulation voltage

In a datasheet, the characteristics of primary importance in an optocoupler are:

• CTR is the Current Transfer Ratio, the ratio of maximum output current to input current, expressed as a percentage. With a bipolar phototransistor output, 20% is a typical minimum CTR. With a photodarlington output, the CTR maybe 1,000% but the bandwidth is much lower—the response time may be measured in microseconds rather than nanoseconds. Optocouplers with a photodiode output have a very low CTR, and their output is in microamps. However, they provide the most linear response.
• VCE(MAX) is the maximum collector-emitter voltage difference (in an optocoupler with a bipolar phototransistor output). Values from 20 to 80 volts are common.
• VISO is the maximum potential difference, in VDC, between the two sides of the optocoupler.
• IMAX is the maximum current the transistor can handle, generally in mA.
• Bandwidth is the maximum transmittable signal frequency, often in the range of 20kHz to 500kHz.

The LED in an optocoupler typically requires 5mA at a forward voltage of 1.5V to 1.6V. The maximum collector current on the output side of an optocoupler is unlikely to be higher than 200mA. For higher output currents, a solid-state relay should be considered. It provides photo isolation on the same basis as an optocoupler, but high-current versions tend to be considerably more expensive.