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Optocouplers become specifically useful where an electrical signal is required to be sent across two circuit stages, but with an extreme degree of electrical isolation across the stages. Optocoupling devices work as logic level changeovers between two circuits, It has the ability to block noise transfer across the integrated circuits, for isolating logic levels from high voltage AC line, and for eliminating ground loops.
Optocouplers become an effective replacement for relays , and for transformers for interfacing digital circuits stages. Additionally, Optocoupler frequency response prove to be incomparable in analog circuits. This IR LED is optically coupled to an adjacent silicon photo-detector device which is generally a photo-transistor, a photodiode or any similar photosensitive element.
These two complementary devices are hermetically embedded in an opaque light proof package. The above figure shows a dissected view of a typical six pin dual-in-line DIP optocoupler chip. When the terminals connected with the IR LED is supplied with an appropriate forward biased voltage it internally emits an infrared radiation in the wavelength of to nanometer range. As can be seen in the image the IR LED and the phototransistor are mounted on adjacent arms of a lead-frame. The lead-frame is in the form of stamping carved out from fine conductive sheet metal having several branch like finishing.
The isolated substrates which is included to reinforce the device are created with the aid of the inner branches. The respective pinout of the DIP are correspondingly developed from the outer branches. Once the conductive connections are established between the die case and the appropriate lead-frame pins, the space surrounding the IR LED and the phototransistor is sealed within an transparent IR supported resin which behaves like a "light pipe" or optical wave-guide between the two IR devices.
The complete assembly is finally molded in a light proof epoxy resin forming the DIP package. At the finish, the lead-frame pin terminals are neatly bent downward. Optocoupler Pinout The diagram above shows the pinout diagram of the typical optocoupler in DIP package.
The other popular names associated with this device are photocoupler or photoncoupled isolators. We can see that the base of the internal IR transistor is terminated at pin 6 of the IC. This base is normally left unconnected since the main purpose of the devices is to couple the two circuits through an isolated internal IR light signal. Likewise the pin 3 is an open or an unconnected pinout and is not relevant.
It is possible to transform the internal IR phototransistor into a photodiode simply by shorting and connecting the base pin 6 with the emitter pin 4. However, the above feature may not be accessible in a 4-pin optocoupler or multi channel optocouplers. Optocoupler Characteristics Optocoupler exhibit one very useful characteristic and that is its light coupling efficiency termed as current transfer ratio, or the CTR.
This ratio is enhanced with an ideally matching IR LED signal spectrum with its adjacent phototransistor detection spectrum. CTR is thus defined as the ratio of output current to input current, at a rated bias level of a specific optocoupler device.
The factors that may vary the CTR depends on the instantaneous specifications of input and output supply voltage and current to the device. Important OptoCoupler Specifications A few of the essential optocoupler specification parameters can be studied from the below given data: Isolation voltage Viso : It is defined as the absolute maximum AC voltage that can exist across the input and output circuit stages of the optocoupler, without causing any harm to the device. Typically this may range between 30 to 70 volts.
It is the standard values of current handling capacity specified to a phototransistor output of the optocoupler, which may range between 40 to mA. This may be typically from 2 to 5 microseconds for both rise and fall. This also tells us about the bandwidth of the optocoupler device. Optocoupler Basic Configuration The figure above shows a basic optocoupler circuit. The amount of current that may pass through the phototransistor is determined by the applied forward bias current of the IR LED or the IRED, despite being entirely separated.
While the switch S1 is held open, current flow through the IRED is inhibited, which means no IR energy is available to the phototransistor. This renders the device completely inactive causing zero voltage to develop across output resistor R2. However, if required the circuit can be modified to work with analog input signals and generate corresponding analog output signals.
Types of Optocouplers The phototransistor of any optocoupler may come with many different output output gain and working specifications. The schematic explained below depicts six other forms of optocouplers variants which have their own specific combinations of IRED and output photodetector.
The next type above illustrates an opto-coupler whose output is enhanced with a silicon based photo-darlington amplifier. This allows it to produce higher output current compared to the other normal opto-coupler.
This magnitude appears to be about ten times higher than a normal optocoupler. However, these may not be as fast as the other normal devices and this may be a significant tradeoff while working with a photodarlington coupler. Also, it may have a decreased amount of the effective bandwith by about a factor of ten.
You can also get them as Dual and quad channel photodarlington couplers. The isolation voltage range of this variant can be as high as volts RMS. The breakdown voltage range can be within 15 to 30 volts, while the rise and fall times is around 15 microseconds each. The next variant above demonstrates a basic SCR or thyristor based opto photosensor. Here the output is controlled through an SCR.
It features a minimum blocking voltages of to V. The highest turn ON currents Ivr can be around 10 mA. The image above displays an optocoupler having a phototriac-output. Optocouplers featuring Schmitt trigger property are also available. This type of optocoupler is displayed above that includes a IC based optosensor having a Schmitt trigger IC that will convert a sine wave or any form of pulsed input signal into rectangular output voltage.
These IC photodetectors based devices are actually designed to work like a multivibrator circuit. Isolation voltages may range between to volts. Turn-on current is usually specified between 1 to 10 mA. The minimum and maximum working supply levels are between 3 to 26 volts, and the maximum speed of data rate NRZ is 1 MHz. Application Circuits The internal functioning of optocouplers is exactly similar to the working of an discretely set up IR transmitter and receiver assembly.
This resistor can be connected in two basic ways with the optocoupler LED, as demonstrated below: The resistor can be added in series either with the anode terminal a or cathode terminal b of the IRED. In our earlier discussions we learned that for AC input, the AC optocouplers is recommended.
However, any standard optocoupler can be also safely configured with an AC input by adding an external diode to IRED input pins as proven in the following diagram. This design also ensures safety for the device against accidental reverse input voltage conditions. This configuration causes a significant increase in the rise time of the input signal, but also results in a drastic reduction in the CTR value down to 0.
Optocoupler Digital Interfacing Optocouplers can be excellent when it comes to digital signal interfacing, operated at various supply levels. Optocouplers are also the favorites when it comes to interfacing personal computers or microcontrollers with other mainframe computers, or loads like motors, relays , solenoid, lamps etc. The below shown diagram illustrates the interfacing diagram of an opto-coupler with TTL circuits.
This is because the TTL gates are rated to produce very low output currents around uA , but are specified to sink current at a fairly high rate 16 mA.
However this also means the output response will be inverted. It must be at least 4. The collector output pin of the optocoupler can be seen is connected between the input and ground of the TTL IC. This is important because a TTL gate input must be adequately grounded at least below 0. It must be noted that the set up shown in the above figure allows a non-inverting response at the output. No matter which configuration is selected at the input side, R2 at the output side must be sufficiently large to enable a full output voltage swing between logic 0 and 1 states at the CMOS gate output.
Interfacing Arduino Microcontroller and BJT with Optocoupler The above figure shows how to interface a microcontroller or Arduino output signal 5 volts, 5 mA with a relatively high current load through an optocoupler and BJT stages. Interfacing Analogue Signals with Optocoupler An optocoupler can be also effectively used for interfacing analog signals across two circuit stages by determining a threshold current through the IRED and subsequently modulating it with the applied analog signal.
The following figure shows how this technique may be applied for coupling an analogue audio signal. The op amp IC2 is configured like a unity gain voltage follower circuit. This pin3 of the is op amp set up at half the supply voltage via R1, R2 potential divider network. This allows the pin3 to be modulated with an AC signals which can be an audio signal and causes the IRED illumination to vary as per this audio or the modulating analogue signal. On the output side of the optocoupler the quiescent current is determined by the phototransistor.
This current develops a voltage across potentiometer R4 whose value needs to be adjusted such that it generates a quiescent output which is also equal to the half the supply voltage. The tracking modulated audio-output signal equivalent is extracted across potentiometer R4, and decoupled through C2 for further processing.
Interfacing Triac with Optocoupler Optocouplers can be ideally used for creating a perfectly isolated coupling across a low DC control circuit and a high AC mains based triac control circuit. It is recommended to keep the ground side of the DC input connected to a proper earthing line. The complete set up can be viewed in the following diagram: The above design can be used for an isolated control of mains AC lamps , heaters, motors and other similar loads.
This circuit is not zero crossing controlled set up, meaning the input trigger will cause the triac to switch at any point of the AC waveform. This voltage is used for triggering the triac through Q1 whenever the input side is switched ON by closing the switch S1.
Meaning as long as S1 is open the optocoupler is off due to a zero base bias for Q1, which keeps the triac switched OFF. Q1 subsequently connects the 10 V DC to the gate of the triac which switches the triac ON, and eventually also switches ON the connected load. This circuits allows the triac to trigger synchronously, that is only during the zero voltage crossing of the AC cycle waveform. When S1 is pressed, the opamp responds to it only if the triac input AC cycle is near a few mV near the zero crossing line.
If the input trigger is made while the AC is not near the zero crossing line, then the op amp waits until the waveform reaches the zero crossing and only then triggers the triac via a positive logic from its pin4. This zero crossing switching feature safeguards the connected from sudden huge current surge and spike, since the turn ON is done at the zero crossing level and not when the AC is at its higher peaks.
This also eliminates unnecessary RF noise and disturbances in the power line. This optocoupler triac based zero crossing switch can be effectively used for making SSR or solid state relays. However, unlike other optocoupler devices, optoTriac or optoSCR feature a rather high surge current handling capacity pulsed which may be much higher than their rated RMS values.
With triac optocouplers, the surge specification may be 1. The following images show a few application circuits using triac optocouplers. In the first diagram, the photoTriac can be seen configured to activate the lamp directly from the AC line.
4N33 Siliconix / Vishay, 4N33 Datasheet
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Optocouplers – Working, Characteristics, Interfacing, Application Circuits
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