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Mar 2 2018

How Optocoupler Works and the Optocoupler Basics


Optocoupler (OC), also called optoelectronic isolator or optocoupler, is a device that transmits electrical signals on the medium of light. It usually encapsulates the luminator (infrared LED) and the light receiver (photosensitive semiconductor tube) in the same shell. When the input signal is powered on, the luminator emits light, and the receiver receives the light and generates photocurrent, which flows out from the output end, thus realizing the "electric-optical-electric" conversion. The optocoupler, which uses light as the medium to couple the input signal to the output end, has the advantages of small size, long life, and no contact point. It is widely used in digital circuits because of its strong anti-jamming ability, insulation between output and input, one-way transmission of signal and so on.




I How Does the Optocoupler Work? 

II Advantages of Optocoupler

III Types of Optocoupler

IV Structure 

V Performance 

VI Technical Parameters

VII Octocoupler Application Example

Book Recommendation

I How Does the Optocoupler Work? 

Couplers transmit electrical signals on the medium of light. They have good isolation for input and output signals, so they are widely used in various circuits. At present, it has become one of the most widely used optoelectronic devices. Optocoupler generally consists of three parts: the light emission, light reception and signal amplification. The input electric signal drives LED to emit light of a certain wavelength , which will received by the photodetector and then generate photocurrent, and then is output after further amplification. The conversion of electro-optical-electric then is completed, which plays the role of input, output and isolation. Due to the interaction between the input and output of the optocoupler and the transmission of electric signals is unidirectional, optocoupler has good electrical insulation ability and anti-jamming ability. Also, because the input end of the optocoupler is a low resistance component of the current type work, it has a strong common mode suppression ability. Therefore, it can greatly improve the signal-to-noise ratio (SNR) when it is used as a terminal isolation element in long line transmission information, and is used as an interface device for signal isolation in computer digital communication and real-time control. The reliability of computer work can be greatly improved.

This video demonstrates how an optocoupler/optoisolator works.

II Advantages of Optocoupler

The main advantages of the optocoupler are: one-way signal transmission, complete electrical isolation between the input and output terminals, no effect of the output signal on the input, strong anti-jamming ability, stable work, no contact, long service life and high transmission efficiency. Optocoupler is a new device developed in 70s and now it is widely used in electrical insulation, level switch, interstage coupling, drive circuit, switching circuit, chopper, multivibrator,  signal isolation, inter-stage isolation, pulse amplifiers circuit, digital instruments, remote signal transmissions, pulse amplifiers, solid state relays(SSR), instrumentation, communication equipment and microcomputer interfaces. In monolithic switching power supply, the optical coupling feedback circuit can be constructed by using the linear optocoupler, and the duty cycle can be changed by adjusting the control current to achieve the purpose of precise voltage stabilization.



III Types of Optocoupler

There are two kinds of optocoupler: one is nonlinear optical coupling and the other is linear optical coupling.

The current transmission characteristic curve of nonlinear optocoupler is nonlinear. Such optocoupler is suitable for switching signal transmission, and is not suitable for analog transmission. The commonly used 4N series optocoupler is a nonlinear optocoupler.

The current transmission characteristic curve of the linear optocoupler is close to straight line, and the performance of the linear optocoupler is better. It can be isolated and controlled by linear characteristics. The commonly used linear optocoupler is PC817A-C series.

Linear optocoupler is commonly used in switching power supply. If nonlinear optocoupler is used, it may make the oscillation waveform bad, and parasitic oscillation may occur in severe cases, so that the oscillation frequency of thousands to hundreds of Hz will be modulated in turn by the low frequency oscillation of several tens to hundreds of Hz. The consequence is that it will interfere with the image of the color TV, color display, VCD, DCD and so on. At the same time, the load capacity of the power supply will decrease. In color TV, display and other switching power supply maintenance, if the optocoupler is damage, it must be replaced by linear optocoupler.

The commonly used 4-pin linear optocouplers: PC817A, PC111, TLP521 and so on. Commonly used 6-pin linear optocouplers include: LP632, TLP532, PC614, PC714, PS2031 and so on. And the commonly used 4N25, 4N26, 4N35, 4N36 are not suitable for being used in switching power supply because these four kinds of optocouplers are nonlinear optocouplers.

Linear optocoupler circuitLinear optocoupler circuit

There are so many kinds and types of optocouplers. In the optoelectronic DATA manual, there are more than a thousand types, which can usually be classified according to the following methods:

  • Classified according to the optical path

Divided into external optical optocoupler (also called optoelectronic interrupter detector) and inner optical optocoupler. External optical optocoupler can be divided into transmissible and reflective optocouplers.

  • Classified according to output form 

A. Photosensitive device output, including photodiode, photistor , photocell, optical thyristor , etc.

B. NPN Triode output, including AC input, DC input, complementary output,etc.

C. Darlington Triode output, including AC input, DC input.

D. Logic gate output, including gate output, Schmidt trigger output, three-state gate output, etc. 

E. Low conduction output (Output low level)

F. Optical switch output (Ron<10Ω)

G. Power output (IGBT/MOSFET)

  • Classified according to package

Divided into coaxial, UPRND, TO, flat package, patch package, and fiber optic transmission, etc.

  • Classified according to transmission signal

Divided into digital optocoupler (OC gate output, Totem Pole output, three-stage gate output, etc.) and linear optocoupler (low drift, high linear type, broadband type, single supply, duplicate supply, etc.)

  • Classified according to speed

Divided into low speed optocoupler (photistor, photocell, etc) and high speed optocoupler (photosensitive diode, photosensitive integrated circuit, etc.)

  • Classified according to channel

Divided into single channel, dual channel and multi-channel optocoupler.

  • Classified according to isolation characteristic

Divided into general isolated optocoupler (generally OCA potting< 5000V) and high voltage isolated optocoupler (10kV, 20kV, 30kV, etc.)

  • Classified according to working voltage

Divided into low supply voltage optocoupler (generally 5-15V) and high supply voltage optocoupler (generally >30V).

IV Structure 

The main characteristics of optoelectronic coupling are following:

The internal structure of optocoupler

The internal structure of optocoupler

  • The insulation resistance between the input and output terminals is generally greater than 10000MΩ, and the voltage resistance can generally exceed 1kV, and some can even reach 10kV.

  • As the optical receiver can only accept the information of the light source, otherwise it can not, there is no feedback phenomenon when the signal is transmitted from the light source to the optical receiver, and the output signal will not affect the input.

  • Because the light-emitting device (GaAs infrared diode) is an impedance current-driven device and the noise is a high internal resistance micro-current voltage signal, the common-mode rejection ratio of the optocoupler is very large. Therefore, optocoupler can suppress interference and eliminate noise well.

  • Easy to match with logical circuit.

  • Fast response speed. The time constant of optocoupler is usually in microsecond or even nanosecond level.

  • Contactless, long life, small size, impact resistance.

V Performance 

The main advantage of the optocoupler is one-way signal transmission, the input and output end completely realize electrical isolation, strong anti-jamming ability, long service life and high transmission efficiency. It is widely used in level conversion, signal isolation, inter-stage isolation, switching circuit, remote signal transmission, pulse amplification, solid state relay, instrument, communication equipment and microcomputer interface. Because the input impedance of optocoupler is smaller than that of general interference source, the interference voltage at the input end of the optocoupler is small, the current it can provide is not large, which is not easy to make the semiconductor diode emitting light. Since the shell of the optocoupler is sealed, it is not affected by external light. The isolation resistance of the optocoupler is very large (about 1012Ω) and the isolation capacitance is very small (about a few pFs), so it can prevent the electromagnetic interference caused by the circuit coupling. The linear optocoupler works by adding the control voltage to the input end of the optocoupler. At the output end, a voltage is produced proportionally to further control the next stage of the circuit. The linear optocoupler consists of a light-emitting diode and a photistor. The optocoupler is current-driven and requires a large enough current to enable the LED to be switched on. If the input signal is too small, the LED will not be switched on and its output signal will be distorted. In the switching power supply, especially in the digital switching power supply, the linear optocoupler can be used to form the optocoupler feedback circuit. The duty cycle can be changed by adjusting the control current to achieve the purpose of precise voltage stabilization.

Hollow core photonic crystal fiber coupler

Hollow core photonic crystal fiber coupler

VI Technical Parameters

The main technical parameters of the optocoupler are LED forward voltage drop VF, forward current IF, current transfer ratio CTR, insulation resistance between input stage and output stage, collector-emitter reverse breakdown voltage V(BR)CEO, collector-emitter saturation voltage drop VCE(sat). Besides, the parameters such as rise time, descent time, delay time and storage time should be taken into account in the transmission of digital signals.

The current-transfer ratio is an important parameter of the optocoupler, which is usually expressed by the DC current-transfer ratio. The ratio is equal to the percentage of the DC output current IC and the DC input current IF when the output voltage remains constant.

Most of the optocouplers use a photistor, which the CTR range from 20% to 300% (such as 4N35), and PC817 is 80% to 160%). Darlington optocouplers (such as 4N30) can reach 100% to 5000%. This indicates that for the same output current, the latter requires only a smaller input current. So there is some similarity between the hFE parameters of the transistor and CTR parameters. The typical CTR-IF characteristic curve of the linear optocoupler and the ordinary optocoupler is obtained.

The CTR-IF characteristic curve of ordinary optocoupler is nonlinear, and the nonlinear distortion is especially serious when IF is small, so it is not suitable for transmitting analog signal. The CTR-IF characteristic curve of linear optocoupler has good linearity, especially when the signal is small, the AC current transfer ratio (ΔCTR=ΔIC/ΔIF) is very close to the DC current transmission ratio (CTR). Therefore, it is suitable for transmitting analog voltage or current signal and can make the output and input linear relationship. This is its important characteristic.

The main purpose of using optocoupler is to provide isolation between input circuit and output circuit. In designing the circuit, the following principles must be followed: The selected optocoupler must conform to the domestic and international standards for isolated breakdown voltage. The 4N series of optocouplers (such as 4N25, 4N26, 4N35) produced by the British company Isocoman and Motorola of the United States are widely used in China. In view of the switching characteristics of such optocoupler, its linearity is poor, and it is suitable for transmitting digital signals (high and low level), which can be used for the output isolation of singlechip. The optocoupler devices used must have high coupling coefficient.

VII Octocoupler Application Example

  • TLP641J

Let’s take 6-pin optocoupler TLP641J as an example to explain its principle. 



An optically controlled thyristor is coupled to a gallium arsenide infrared light-emitting diode. Pin 1 and 2 are light-emitting diodes. When a voltage is applied, a light emitting diode is driven to send out a certain wavelength of light to trigger a light controlled thyristor. The characteristic of the optical thyristor is that a photodiode is integrated in the gate region, and the trigger signal source is insulated from the main circuit. The key point is that the trigger sensitivity is high. The trigger current of the optically controlled thyristor controller is lifted by the photogenerated carrier in the device. The light-controlled thyristor is transferred from off state to on state. In order to improve the trigger sensitivity of the optical thyristor, the gate structure or the double amplification gate structure are often used in the gate region. The cathode emitter short-circuit structure is often used to satisfy the high rise rate of reloading voltage. Low power optical thyristors are often used in electrical isolation to provide control pole trigger for larger thyristors, and also for relay, automatic control and so on. High-power optical thyristors are mainly used for HVDC transmission.



When pin 1 and 2 are added 5V or more power supply, the luminotron can be luminous and drive the light controlled thyristor into on state. At the same time, pin 5 and 4 form a resistance, whose resistance value is about 10KV.. When 1 and 2 do not add voltage, then 4 and 5 can be regarded as an infinite resistance.



  • PC817 

PC817 is a commonly used linear optocoupler. It is often used as coupler in various functional circuits which require more precision. It has complete isolation function of upper and lower level circuit, which does not have the influence mutually.

When the input signal is powered on, the light emitting device emits light, which is illuminated on the light receiver. After receiving the light, the light receiver leads to the output photocurrent from the output end, thus realizing the "electric-optical-electric" conversion.

PC817 optocoupler circuit diagram

PC817 optocoupler circuit diagram

Ordinary optocouplers can only transmit digital signals (switching signals) and is not suitable for transmitting analog signals. Linear photocouplers are a new type of optoelectronic isolator, which can transmit continuously varying analog voltage or current signals. In this way, with the change of input signal, the corresponding optical signal will be produced, which makes the optical transistor have different conductance and output voltage or current.

PC817 photocoupler can not only play a feedback role but also can play an isolation role.

LED forward current

LED forward current

  • Application circuit

Optical coupling control relay circuit

Optical coupling control relay circuit

Book Recommendation

  • Optocouplers Databook Paperback – 2004

--by Inc Vishay Intertechnology (Author)

  • Advance Elements of Optoisolation Circuits: Nonlinearity Applications in Engineering 1st ed. 2017 Edition

This book on advanced optoisolation circuits for nonlinearity applications in engineering addresses two separate engineering and scientific areas, and presents advanced analysis methods for optoisolation circuits that cover a broad range of engineering applications. The book analyzes optoisolation circuits as linear and nonlinear dynamical systems and their limit cycles, bifurcation, and limit cycle stability by using Floquet theory. Further, it discusses a broad range of bifurcations related to optoisolation systems: cusp-catastrophe, Bautin bifurcation, Andronov-Hopf bifurcation, Bogdanov-Takens (BT) bifurcation, fold Hopf bifurcation, Hopf-Hopf bifurcation, Torus bifurcation (Neimark-Sacker bifurcation), and Saddle-loop or Homoclinic bifurcation. Floquet theory helps as to analyze advance optoisolation systems. Floquet theory is the study of the stability of linear periodic systems in continuous time. Another way to describe Floquet theory, it is the study of linear systems of differential equations with periodic coefficients. The optoisolation system displays a rich variety of dynamical behaviors including simple oscillations, quasi-periodicity, bi-stability between periodic states, complex periodic oscillations (including the mixed-mode type), and chaos. The route to chaos in this optoisolation system involves a torus attractor which becomes destabilized and breaks up into a fractal object, a strange attractor.The book is unique in its emphasis on practical and innovative engineering applications. These include optocouplers in a variety of topological structures, passive components, conservative elements, dissipative elements, active devices, etc. In each chapter, the concept is developed from the basic assumptions up to the final engineering outcomes. The scientific background is explained at basic and advanced levels and closely integrated with mathematical theory. The book is primarily intended for newcomers to linear and nonlinear dynamics and advanced optoisolation circuits, as well as electrical and electronic engineers, students and researchers in physics who read the first book “Optoisolation Circuits Nonlinearity Applications in Engineering”. It is ideally suited for engineers who have had no formal instruction in nonlinear dynamics, but who now desire to bridge the gap between innovative optoisolation circuits and advanced mathematical analysis methods.

--Ofer Aluf (Author)

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