What is Optoisolators ? Properties, Applications - Electronicsinfos

What is an Optoisolator?

Optoisolators, also known as optocouplers. it is electronic components that transfer electrical signals between two isolated circuits using light. They are used to prevent high voltages from affecting the system.

Operation Principle of OptoIsolators 

The operation principle of optoisolators is based on the transfer of signals through optical. Optoisolators consist of two main components: an optical emitter and an optical detector. 

The optical emitter is typically a light-emitting diode (LED), and the optical detector can be a phototransistor, photometric sensor, or other light-sensitive device.

When a voltage is applied across the LED, it emits light. The LED's brightness is proportional to the input current. The emitted light travels across an air gap or transparent material within the optoisolator package. This gap provides electrical isolation between the LED and the phototransistor.

The light emitted by the LED falls on the phototransistor which generates a current in response to the light. The amount of current generated by the phototransistor is proportional to the intensity of the light it receives.

Where

• $C$ is the collector's current 
• $beta$ is the current gain 
• $IB$is the base current 
• $\mathrm{\eta \; is\; the\; phototransistor\text{'}s\; responsivity}$
• $Popt$is the optical power 

Types of Optoisolators

There are different types of Optoisolators including

• Phototransistor Optoisolators
• Photodiode Optoisolators
• Photovoltaic Optoisolators
• Phototriac Optoisolators (Opto-Triacs)
• Photodarlington Optoisolators

Phototransistor Optoisolators

A Phototransistor optoisolator is an optoelectronic device that uses light to transfer electrical signals between two isolated circuits. It consists of an LED on the input side and a phototransistor on the output side.

Types of Phototransistor Optoisolators

• Single-Channel Optoisolators
• Multi-Channel Optoisolators
• Darlington Phototransistor Optoisolators
• High-Speed Phototransistor Optoisolators

Construction of Phototransistor Optoisolators

The LED is mounted in such a way that it directly faces the phototransistor. They ensure that the maximum amount of light from the LED reaches the phototransistor.

The LED and phototransistor are encapsulated in a transparent, light-proof material to shield them from ambient light. This allows only the light from the LED to activate the phototransistor.

The device is built with an insulation layer that physically isolates the input side from the output side. This provides high-voltage isolation.

Working Principle

When a voltage is applied to the input terminals, the LED inside the optoisolator emits infrared light. This light is detected by the phototransistor on the output side. This allows current to flow through its collector-emitter circuit.

The amount of current generated by the phototransistor is proportional to the intensity of the light it receives, which in turn is controlled by the input signal to the LED.

Photodiode Optoisolators

A photodiode optoisolator, also known as an optocoupler. This is an optoelectronic device that transfers electrical signals using light. It typically consists of an LED on the input side and a photodiode on the output side.

Types of Photodiode Optoisolators

• Analog Photodiode Optoisolators
• Digital Photodiode Optoisolators
• High-Speed Photodiode Optoisolators
• Linear Photodiode Optoisolators

Construction of Photodiode Optoisolators

The input side contains an LED that emits light. it is usually in the infrared spectrum, when current is applied. The LED is encapsulated within the optoisolator to direct the light toward the photodiode.

The photodiode is positioned to receive the light emitted by the LED. When the photodiode is illuminated, it generates a photocurrent proportional to the light intensity.

The entire assembly is encapsulated in a plastic or epoxy package to protect the components.

Working Principle

When the input signal drives current through the LED, it emits light. This light is detected by the photodiode on the output side. The photocurrent depending on the design produces an output signal that corresponds to the input signal.

Key Characteristics of photodiode

• Photodiode optoisolators are faster than phototransistor optoisolators.
• Photodiodes provide a more linear response compared to phototransistors.
• Photodiodes generate a lower output current than phototransistors.

Applications

• it is used in applications like fibre optics and high-speed digital interfaces.
• This is Ideal for isolating analogue signals in audio equipment, sensors, and medical devices.
• They are Commonly used in feedback circuits for power supplies and amplifiers.
• They are Employed in the feedback loop to regulate output voltage.

Photovoltaic Optoisolators

Photovoltaic Optoisolators, also known as optical isolators. They are electronic components that provide electrical isolation between different circuits while allowing signal transmission through light. 

They are used in various applications where isolation between high-voltage and low-voltage circuits is required.

Operating Principle

When an electrical signal is applied to the LED it emits light. The emitted light travels across an insulating gap to reach the photovoltaic cell. The photovoltaic cell detects the light and generates a corresponding electrical signal. 

The gap between the LED and the photovoltaic cell provides electrical isolation. it protects the low-voltage side from high-voltage transients or noise.

Advantages of Photovoltaic Optoisolators

• They Provide high isolation voltage between input and output.
• It typically consumes less power compared to traditional optoisolators.
• They often have high reliability and long operational life due to the absence of moving parts.
• They are capable of fast switching speeds.

Disadvantages of Photovoltaic Optoisolators

• The output current of photovoltaic optoisolators is typically limited.
• The Performance can be affected by temperature variations.
• They can be larger and more expensive than traditional optoisolators.

Applications 

• it is used in industrial control systems, power supplies, and data acquisition systems.
• They Provide isolation between the high-voltage input and the low-voltage control circuitry.
• it is Used to isolate communication signals in systems where electrical isolation is crucial.

Phototriac Optoisolators (Opto-Triacs)

Phototriac Optoisolators are commonly known as Opto-Triacs. They are electronic components that combine an optoisolator with a TRIAC to provide electrical isolation and control of AC power. 

They are widely used in applications where isolation between the control signal and the AC power circuit is required, such as in dimmers, motor controllers, and solid-state relays.

Operating Principle

A low-voltage control signal is applied to the LED to emit light. The light from the LED is transmitted across an insulating barrier to the photodiode or phototransistor, which is optically coupled to the LED.

The photodiode or phototransistor receives the light and generates a small electrical signal that triggers the gate of the TRIAC. Once triggered, the TRIAC switches the AC load on or off. The TRIAC remains on until the AC current naturally goes to zero.

Advantages of Phototriac Optoisolators

• They Provide excellent isolation between the low-voltage control circuit and the high-voltage AC load.
• It combines optical isolation and TRIAC switching in a single package.
• They are capable of fast switching.
• They have no moving parts.
• It requires very little power to operate the LED.

Disadvantages of Phototriac Optoisolators

• The current carrying capability of the TRIAC may be limited.
• TRIACs can generate significant heat.
• Opto-Triacs are specifically designed for AC loads and cannot be used for DC load switching.
• May be sensitive to electrical noise.

Applications

• These are used in SSRs to switch AC loads on and off without mechanical wear.
• triac used in dimmer circuits for controlling the brightness of lamps.
• They control the speed of AC motors by regulating the power delivered to the motor.
• They are found in devices like washing machines, dishwashers, and HVAC systems to control various AC-powered components.

Photodarlington Optoisolators

Photodarlington Optoisolators, also known as Opto-Darlingtons, are a type of optoisolator that uses a photodarlington transistor as the output component. These devices provide electrical isolation between input and output circuits while amplifying the signal significantly, making them ideal for applications requiring high current gain and signal amplification.

Operating Principle

An electrical input signal is applied to the LED, causing it to emit light. The light emitted by the LED is transmitted across an insulating barrier to the photodarlington transistor. 

The first transistor in the Darlington pair detects the light and generates a small current. This small current is then amplified by the second transistor in the pair, resulting in a much larger output current.

The optical coupling provides electrical isolation between the input and output circuits, protecting sensitive components from high-voltage spikes or interference.

Advantages of Photodarlington Optoisolators

• The Darlington configuration provides a very high current gain.

• They offer excellent electrical isolation between input and output circuits.

• They Combine optical isolation and signal amplification in a single package.

Disadvantages of Photodarlington Optoisolators

• The Darlington configuration has slower response times compared to standard optoisolators.

• Photodarlington transistors typically have a higher saturation voltage.

• Photodarlington Optoisolators can generate more heat.

• The power handling capacity of these devices may be limited.

Applications

• They are used in applications where a weak signal needs to be isolated and amplified.

• They are Ideal for interfacing microcontrollers with higher-power circuits.

• They are Commonly used in industrial control systems to isolate relays, motors, or other actuators.

• They are used where analogue signals need to be isolated and amplified, such as in analogue-to-digital conversion systems.

• They are used in telecommunication equipment to isolate and amplify signals between different parts of the system.

Properties of Optoisolators

Isolation Voltage

The isolation voltage of an optoisolator is the maximum voltage that can be applied between its input and output terminals without causing an electrical breakdown. This is often rated in kilovolts (kV).

500 V to 1,000 V for standard optoisolators.

1,500 V to 2,500 V for common industrial-grade optoisolators.

3,750 V to 5,000 V for reinforced isolation optoisolators.

> 7500V For Extreme isolation voltages

Current Transfer Ratio (CTR)

The ratio of output current to input current is called CTR. it is expressed as a percentage. It indicates how efficiently the input signal is transferred to the output.

20% to 50% for low-efficiency optoisolators.

50% to 200% for standard optoisolators.

200% to 600% or more for high-efficiency optoisolators.

For example, a common value for CTR in many standard optoisolators might be around 100% to 200%. This means that for every 1 mA of input current, the output current can range from 1 mA to 2 mA.

Response Time

The time it takes for the optoisolator to respond to changes in the input signal. It consists of two main components:

1. Rise Time ($t_r$): The time it takes for the output to transition from a low state to a high state (typically from 10% to 90% of the final value).
2. Fall Time ($t_f$): The time it takes for the output to transition from a high state to a low state (typically from 90% to 10% of the initial value).

The typical response time varies between a few microseconds to nanoseconds.

Input Characteristics 

The input characteristics of an optoisolator describe how it behaves on the input (LED side). There are different Input characteristics including

Forward Voltage

The voltage required to forward bias the LED within the optoisolator. Its values range from 1.0V to 1.4V.

Forward Current

The current flowing through the LED when forward-biased. Its operating ranges are 5 mA to 20 mA.

Reverse Voltage

The maximum voltage that can be applied in reverse to the LED without causing damage. it  values are 5V to 6V.

Output Characteristics

The output characteristics of an optoisolator describe how it behaves on the output side. There are different output characteristics including

Collector-Emitter Voltlage 

The maximum voltage that can be applied between the collector and emitter of the phototransistor without causing breakdown. it Typical values range from 20V to 80V.

Collector Current  

The current flows through the phototransistor when it is activated by the LED. Its values can vary from 1 mA to 50 mA.

Saturation Voltage 

The voltage across the collector-emitter junction when the phototransistor is fully turned on (saturated). it Typically around 0.1V to 0.3V.

Temperature Range

The temperature range of an optoisolator is the device can operate reliably without degradation of performance. 

• Commercial Grade:-40°C to +85°C
• Industrial Grade:-40°C to +100°C or -40°C to +125°C
• Military/Aerospace Grade:-55°C to +125°C or -55°C to +150°C

Frequently Asked Questions – FAQs

What is the Current Transfer Ratio (CTR)?

CTR is the ratio of the output current to the input current. It is usually expressed as a percentage.

How does an optoisolator provide isolation?

Optoisolators provide isolation by using light as the communication medium between the LED and the phototransistor.

What is the typical isolation voltage provided by an optoisolator?

Optoisolators typically offer isolation voltages ranging from a few hundred volts to several thousand volts.

Can optoisolators be used in both AC and DC circuits?

Yes, optoisolators can be used in both AC and DC circuits.

What is the difference between an optoisolator and a transformer?

Both provide isolation between circuits, but an optoisolator uses light to transfer the signal, while a transformer uses electromagnetic induction.

What is the role of an optoisolator in communication systems?

Optoisolators can be used in communication systems to isolate different parts of the system, such as isolating the control circuitry from noisy or high-voltage sections.

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