Optoisolators: What They Are and How They Work

An optoisolator is an electronic component that transfers electrical signals between two isolated circuits by using light. Optoisolators prevent high voltages from affecting or destroying the system receiving the signal. They can also remove electrical noise from signals and enable communication between incompatible circuits. Optoisolators are also known as optocouplers or optical isolators.

Types of Optoisolators

Optoisolators consist of a light source, a light sensor, and a dielectric barrier that blocks electrical current. The light source is usually a near-infrared light-emitting diode (LED) that converts an electrical input signal into light. The light sensor can be a photoresistor, a photodiode, a phototransistor, a silicon-controlled rectifier (SCR), or a triac. The light sensor detects the incoming light and either generates electric energy directly or modulates the electric current flowing from an external power supply.

The most common type of optoisolator is the LED-phototransistor combination, which has a fast response time and a high current transfer ratio. Other types of optoisolators include LED-photodiode, LED-LASCR, and lamp-photoresistor pairs. Some optoisolators can transfer analog signals as well as digital signals by varying the intensity of the light source.

How Optoisolators Work

The basic working principle of an optoisolator is shown in the figure below.


The input circuit consists of a variable voltage source and an LED. The output circuit consists of a phototransistor and a load resistor. The LED and the phototransistor are enclosed in a light-tight package to prevent external interference.

When the input voltage is applied to the LED, it emits infrared light proportional to the input signal. The light travels across the dielectric barrier and reaches the phototransistor, which is reverse-biased. The phototransistor converts the light into electric current, which flows through the load resistor and produces an output voltage. The output voltage is inversely proportional to the input voltage, as shown in the graph below.

The input and output circuits are electrically isolated by the dielectric barrier, which can withstand high voltages up to 10 kV and voltage transients with speeds up to 25 kV/μs. This means that any surge or noise in the input circuit will not affect or damage the output circuit.

Applications of Optoisolators

Optoisolators are widely used in various fields, such as:

  • Power electronics: Optoisolators can control high-voltage or high-current devices, such as relays, motors, valves, solenoids, lamps, heaters, etc., with low-voltage or low-current signals from microcontrollers, sensors, switches, etc.
  • Communication: Optoisolators can enable data transmission between different circuits or systems that have different voltage levels, ground potentials, or noise characteristics. For example, optoisolators can connect digital devices to analog devices, such as modems, telephone lines, audio equipment, etc.
  • Measurement: Optoisolators can isolate sensitive instruments from noisy or hazardous environments, such as high-voltage lines, industrial machines, medical equipment, etc. For example, optoisolators can protect oscilloscopes, multimeters, thermocouples, etc., from electric shocks or interference.
  • Safety: Optoisolators can provide protection for humans and equipment from electric shocks or fire hazards caused by short circuits, overloads, faults, etc. For example, optoisolators can prevent sparks or arcs from igniting flammable materials or gases.

Advantages and Disadvantages of Optoisolators

Some of the advantages of optoisolators are:

  • They provide electrical isolation between input and output circuits.
  • They prevent high voltages or currents.
  • They prevent high voltages or currents from damaging or interfering with the low-voltage or low-current circuits.
  • They enable communication between circuits that have different voltage levels, ground potentials, or noise characteristics.
  • They can handle high switching speeds and data rates.

Some of the disadvantages of optoisolators are:

  • They have limited bandwidth and linearity compared to other isolation methods, such as transformers or capacitors.
  • They have temperature and aging effects that can degrade their performance over time.
  • They have variations in the current transfer ratio and input-output capacitance that can affect their accuracy and stability.

Optoisolator Parameters and Specifications

Some of the important parameters and specifications of optoisolators are:

  • Current transfer ratio (CTR): This is the ratio of the output current to the input current, expressed as a percentage. It indicates how efficiently the optoisolator transfers the signal from the input to the output. A higher CTR means a higher output current for a given input current. The CTR depends on factors such as the type of optoisolator, the input voltage, the output load, and the temperature. The CTR can vary widely among different optoisolators, even within the same batch. Therefore, it is important to select an optoisolator with a suitable CTR range for the application.
  • Isolation voltage: This is the maximum voltage that can be applied between the input and output circuits without causing damage or breakdown. It indicates how well the optoisolator provides electrical isolation between the two circuits. A higher isolation voltage means a higher level of protection and safety. The isolation voltage depends on factors such as the dielectric material, the package design, and the clearance and creepage distances. The isolation voltage is usually specified for a short duration (such as 1 minute or 1 second) and at a certain frequency (such as 50 Hz or 60 Hz).
  • Input-output capacitance: This is the capacitance between the input and output terminals of the optoisolator, measured with both terminals shorted to ground. It indicates how much electrical coupling exists between the two circuits. A lower input-output capacitance means a lower level of interference and noise. The input-output capacitance depends on factors such as the dielectric material, the package design, and the distance between the terminals. The input-output capacitance is usually specified at a certain frequency (such as 1 kHz or 100 kHz).
  • Switching speed: This is the time required for the optoisolator to switch from one state to another, such as from on to off or from off to on. It indicates how fast the optoisolator can respond to changes in the input signal. A faster switching speed means a higher data rate and bandwidth. The switching speed depends on factors such as the type of optoisolator, the input voltage, the output load, and the temperature. The switching speed is usually specified as rise time (the time required for the output to rise from 10% to 90% of its final value) and fall time (the time required for the output to fall from 90% to 10% of its final value).


Optoisolators are useful devices that can transfer electrical signals between isolated circuits by using light. They have many advantages, such as providing electrical isolation, preventing high voltages, removing electrical noise, and enabling communication between incompatible circuits. They also have some disadvantages, such as limited bandwidth, aging effects, variations in performance, and switching speed. Optoisolators have various parameters and specifications that determine their suitability for different applications. Optoisolators are widely used in power electronics, communication, measurement, safety, and other fields.

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