075582814553
Modern electronic systems often need to exchange signals between circuits that use different voltages, separate grounds, or noisy power sources. Connecting these circuits directly can expose sensitive components to voltage spikes, ground loops, electrical noise, and fault currents. An optoisolator solves this problem by transferring the signal through light while keeping the input and output electrically separated. This article explains what an optoisolator is, how it works, the problems it prevents, and how different output types affect performance.
An optoisolator, also called an optocoupler, is an electronic component that transfers a signal between two circuits using light while keeping the circuits electrically separated. Inside the device, an input component, usually an LED, produces light when current flows through it. A light-sensitive output component, such as a phototransistor, photodiode, or photo-triac, detects the light and creates a corresponding electrical output. Because the input and output sides do not share a direct conductive connection, the signal can pass across an isolation barrier without allowing normal current to flow between the two circuits.
• High-voltage damage: Protects low-voltage control circuits from dangerous voltage on the power side.
• Voltage spikes: Helps prevent sudden electrical surges from reaching sensitive components.
• Ground loops: Breaks direct ground connections that can cause unwanted current flow and signal errors.
• Electrical noise: Reduces interference from motors, relays, switching power supplies, and other noisy equipment.
• Different ground voltages: Allows two circuits with different ground potentials to exchange signals safely.
• Fault current transfer: Limits the chance that a short circuit or failure on one side will damage the other side.
• Unsafe control connections: Lets a microcontroller or logic circuit control high-voltage equipment without a direct electrical connection.
• Signal interference: Improves signal reliability in industrial, power-control, and communication circuits.
An optoisolator transfers an electrical signal from one circuit to another by using light. The input and output sides are electrically separated, so there is no direct conductive path between them. This separation is called galvanic isolation.
On the input side, the transmitter is usually an LED or infrared LED. When input current flows through the LED, it produces light. The amount of light depends mainly on the LED current, so a current-limiting resistor is normally used to protect the LED and control its operating current.
The light travels across the internal isolation barrier to the receiver. The receiver is a photosensitive device, such as a phototransistor, photodiode, photo-triac, or photo-SCR. When it detects the light, it changes its electrical state and produces an output signal.
For example, in a phototransistor optoisolator, the phototransistor turns on when the LED is illuminated. This allows current to flow through the output circuit. When the input LED turns off, the phototransistor also turns off, stopping or reducing the output current.
The input and output circuits can use different voltage levels and separate grounds. Only the signal is transferred through light. This helps protect low-voltage electronics from high voltage, switching noise, voltage spikes, ground loops, and fault currents on the other side.
The diagram shows the correct basic process:
Input signal → LED produces light → light crosses the isolation barrier → photosensor detects light → output signal changes
However, an optoisolator does not directly increase power. It transfers control or information. The output side usually needs its own power supply and supporting components, such as a pull-up resistor, load resistor, transistor, or driver circuit.
This circuit allows an Arduino to detect a 12 V signal without connecting the Arduino directly to the 12 V circuit. When the 12 V supply is active, current flows through the 2 kΩ resistor R2 and the internal LED of the CNY17-2 optoisolator. The LED produces light, which turns on the internal phototransistor. On the Arduino side, R1 acts as a pull-up resistor to +5 V. When the phototransistor turns on, it pulls the GPIO signal toward ground, so the Arduino reads a LOW level. When the 12 V input is removed, the phototransistor turns off and R1 pulls the GPIO back to HIGH. This means the circuit has an active-low output. The 12 V ground and Arduino ground should remain separate to preserve galvanic isolation.
This circuit uses an optoisolator to transfer a PWM control signal to a MOSFET that switches a 9 V DC motor. The PWM signal activates the optoisolator’s internal LED, causing its output transistor to change the voltage applied to the MOSFET gate. The IRFU3708 MOSFET then switches current through the motor, allowing the PWM duty cycle to control the motor’s average voltage and speed. The 1.5 kΩ resistor pulls the MOSFET gate toward +9 V when the optoisolator output is off, while the optoisolator pulls the gate lower when it turns on. Therefore, the switching logic may be inverted, depending on the optoisolator connection. The diode across the motor provides a safe path for inductive current when the MOSFET switches off, protecting the transistor from voltage spikes. For true electrical isolation, the PWM-side ground must not be connected to the motor power ground.
This circuit uses a TLP250 optically isolated gate driver to control a power MOSFET connected to a separate load supply. The input signal VIN passes through resistor R1, which limits current through the TLP250’s internal LED. The optical signal crosses the isolation barrier and activates the driver’s output stage. The TLP250 then charges or discharges the MOSFET gate through the 10 Ω resistor R2. This resistor limits peak gate current and helps reduce ringing and electromagnetic interference.
Resistor R3 pulls the gate to the source when the driver is inactive, preventing the MOSFET from turning on accidentally. Capacitors C1 and C2 decouple the TLP250 supply and provide short bursts of current during fast gate switching. However, the drawing appears to connect the signal ground and power ground along the same bottom line. If they are physically connected, the optical isolation is defeated. The input-side ground and driver-side power ground must remain separate when galvanic isolation is required.
A phototransistor optoisolator contains an LED on the input side and a light-sensitive transistor on the output side. When current flows through the LED, it produces light that turns on the phototransistor without creating a direct electrical connection between the two circuits. The 4-pin version provides collector and emitter terminals, while the 6-pin version also includes a base connection that can help adjust switching performance. This type is commonly used for logic isolation, microcontroller inputs, switch detection, and power-supply feedback because it is simple and affordable, although it is generally slower than high-speed optoisolators and its current transfer ratio can vary between devices.
Photodiode optoisolator uses an infrared LED on the input side and a photodiode on the output side to transfer a signal through light while keeping the two circuits electrically isolated. When current flows from the anode (A) to the cathode (K), the IR LED emits light toward the photodiode, which then produces a small output current between its positive and negative terminals. Photodiode optoisolators respond faster than phototransistor types, making them suitable for high-speed signal transfer, communication circuits, pulse detection, and precision sensing. However, because the photodiode output current is very small, an external amplifier or signal-conditioning circuit is usually required.
Photo-Darlington optoisolator uses an LED on the input side and a Darlington transistor pair on the output side to transfer a signal through light while keeping the two circuits electrically isolated. When current flows through the LED between the anode (A) and cathode (K), the emitted light activates the two connected transistors, providing much higher current gain than a standard phototransistor optoisolator.
The collector (C) and emitter (E) form the main output terminals, while the base (B) connection may be used to adjust switching behavior, and the NC pin has no internal connection. This type is useful for low-level signal detection, alarm circuits, industrial controls, and other applications that need high sensitivity, but its higher gain also makes it slower and gives it a higher saturation voltage than a basic phototransistor type.
Photo-triac optoisolator uses an LED on the input side and a light-activated triac on the output side to switch AC signals while keeping the control circuit electrically isolated from the load circuit. When current flows through the LED between the anode (A) and cathode (K), the emitted light triggers the internal triac, allowing current to flow between terminals MT1 and MT2 in either direction. The NC pins have no internal connection. Photo-triac optoisolators are commonly used to control mains-powered lamps, heaters, fans, and AC motors, usually by triggering an external power triac rather than carrying the full load current themselves. They are suitable for AC switching but are generally not used for normal DC loads because a triac remains on until the current falls below its holding current.
As shown in the figure, a photo-SCR optoisolator uses an LED on the input side and a light-activated silicon-controlled rectifier on the output side to transfer a switching signal while keeping the two circuits electrically isolated. When current flows through the input LED between the anode (A) and cathode (K), the emitted light triggers the internal SCR, allowing current to flow from its anode to cathode. The gate terminal may be available for additional control, while the NC pin has no internal connection. Once triggered, the SCR normally remains on until the output current falls below its holding current, so this type is commonly used in latching circuits, alarms, protection systems, AC control, and high-voltage switching rather than ordinary fast on-and-off signal transfer.
Linear optoisolator transfers a changing analog signal through light while keeping the input and output circuits electrically isolated. The input signal passes through capacitor C1 and controls transistor Q1, which regulates the current flowing through the internal LED of the MOC5010. The emitted light is detected inside the optoisolator and converted into a corresponding electrical output, while capacitor C2 removes the DC component and passes the isolated AC signal to the output. Unlike a standard optoisolator that mainly transfers ON and OFF states, a linear optoisolator is designed to reproduce variations in signal amplitude, making it useful for isolated audio, sensor signals, measurement systems, and analog control circuits. However, this simple circuit may have limited accuracy because LED response, device gain, temperature, and component tolerances can affect signal linearity and distortion.
A high-speed logic optoisolator includes internal signal-conditioning circuitry to provide faster switching and cleaner digital outputs. It is used for isolated communication, PWM signals, data buses, inverter control, and industrial interfaces. Compared with basic phototransistor devices, it offers shorter propagation delay and better timing performance, but it usually costs more and may require a separate output-side power supply.
Optically isolated gate driver transfers a control signal across an electrical isolation barrier and then provides enough current to switch the gate of a power MOSFET or IGBT quickly and safely. The HFBR optical device separates the low-voltage control side from the high-power switching side, while the IXYS push-pull output stage uses the +20 V and −5 V supplies to charge and discharge the transistor gate. The separate turn-on and turn-off paths, formed by the diode and gate resistors, help control switching speed, ringing, and electromagnetic interference, while the nearby capacitors stabilize the driver supply during fast current pulses. This type of driver is commonly used in inverters, motor drives, power converters, and industrial switching systems.
| Specification and Symbol | Typical Range and Unit |
| LED forward voltage (VF) | 1.0–1.5 V |
| LED forward current (IF) | 1–20 mA |
| Maximum LED forward current (IF(max)) | 20–60 mA |
| LED reverse voltage (VR) | 3–6 V |
| Current transfer ratio (CTR) | 20–1,000% or higher |
| Collector-emitter voltage (VCEO) | 20–80 V |
| Maximum collector current (IC) | 10–100 mA |
| Collector-emitter saturation voltage (VCE(sat)) | 0.1–0.4 V |
| Output leakage current (ICEO) | Several nA to several µA |
| Output supply voltage (Vcc) | 3.0–30 V, depending on type |
| Propagation delay (tPLH, tPHL) | 10 ns to several µs |
| Rise time (tr) | 10 ns to several µs |
| Fall time (tf) | 10 ns to several µs |
| Data rate | Several kbit/s to 50 Mbit/s or higher |
| Frequency bandwidth (BW) | Several kHz to tens of MHz |
| Common-mode transient immunity (CMTI) | 5–200 kV/µs |
| Isolation test voltage (VISO) | 2,500–5,300 V RMS |
| Continuous working voltage (VIOWM) | Approximately 100–1,000 V RMS or peak |
| Surge isolation voltage (VIOSM) | Approximately 4–12 kV peak |
| Insulation resistance (RIO) | (10^{11})–(10^{12}\ \Omega) or higher |
| Input-to-output capacitance (CIO) | 0.2–2 pF |
| Creepage distance/ Clearance distance | Approximately 3–10 mm |
| Operating temperature (TA) | −40 to +85°C; industrial parts may reach +125°C |
| Storage temperature (Tstg) | −55 to +150°C |
| Maximum junction temperature (TJ) | 125–150°C |
| Total power dissipation (Ptot) | Approximately 100–300 mW |
| Photo-triac off-state voltage (VDRM) | 400–800 V peak |
| Photo-triac trigger current (IFT) | 3–20 mA |
| Photo-triac on-state current (IT) | Approximately 50–150 mA |
| Gate-driver peak output current (IO(peak)) | Approximately 0.4–5 A |
| Linear optoisolator linearity error | Approximately 0.01–5% |
| Package configuration | 4-pin DIP, 6-pin DIP, 8-pin DIP, SOIC, SOP, SSOP, wide-body or stretched SMD |
| Mounting type | Through-hole or surface-mount |
| Channel configuration | Single, dual, quad, or multi-channel |
| Safety certification | UL, VDE, CSA, IEC/EN or CQC, depending on device |
Current transfer ratio, or CTR, shows how effectively a phototransistor optoisolator transfers current from its input to its output. It is calculated by dividing the output collector current by the input LED current and multiplying by 100%. For example, a 100% CTR means that 5 mA of LED current can produce about 5 mA of collector current under the specified test conditions.
CTR is important because it determines whether the optoisolator can provide enough output current to switch the next circuit reliably. However, CTR changes with LED current, temperature, device aging, and manufacturing variation. For reliable design, use the minimum CTR listed in the datasheet rather than the typical value.
An optoisolator uses an LED and photodetector to transfer a signal through light, while a digital isolator uses capacitive or magnetic coupling between internal transmitter and receiver circuits. As shown in the image, the optoisolator has a light path across the isolation barrier, whereas the digital isolator sends encoded electrical pulses across a dielectric barrier. Optoisolators are generally slower and can be affected by LED aging, temperature, and current transfer ratio variation. Digital isolators usually provide faster switching, lower propagation delay, more consistent timing, and better resistance to rapid common-mode voltage changes.
An optoisolator transfers a signal through light using an LED and photodetector, while a relay uses an electromagnetic coil to move physical contacts. As shown in the image, the optoisolator has no moving parts, so it switches faster, operates silently, and does not suffer from contact wear, while the relay is slower and can produce clicking, contact bounce, and electrical arcing. A relay can usually switch higher AC or DC voltages and currents directly, whereas an optoisolator normally handles only low-power signals and often needs an external transistor, MOSFET, or driver for larger loads.
An optoisolator is a practical way to pass control or measurement signals between electrically separated circuits. Its internal LED and photosensitive output protect low-voltage electronics from high voltage, switching noise, ground differences, and fault currents, but the device must be selected according to the signal type and required speed. Reliable design also depends on checking the minimum current transfer ratio, input LED current, propagation delay, working isolation voltage, package spacing, and temperature limits. When these values are matched correctly to the circuit, an optoisolator can improve safety, signal reliability, and protection without creating a direct electrical connection between the two sides.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other