AC Optocoupler

AC optocoupler for switching DC circuit
In the upper circuit a Photo-Transistor based optocoupler circuit is used. It will act like a typical Transistor switch. In the schematic a low cost photo-transistor based opto-coupler PC817 is used. The infra-red led will be controlled by the S1 switch. When the switch will be on, the 9V battery source will provide current to the LED via the current limiting resistor 10k.The intensity is controlled by the R1 resistor. If we change the value and make the resistance lower, the intensity of the led will be high making the transistor gain high.
On the other side the transistor is a photo-transistor controlled by the internal infra-red led, when the led emit infra-red light the photo transistor will contact and the VOUT will be 0 turning off the load connected across it. It is needed to remember that as per the datasheet the collector current of the transistor is 50mA. The R2 provide the VOUT 5v. The R2 is a pull-up resistor.

Optocoupler for detecting AC voltage
Here another circuit is shown to detect the AC voltage. The infra-red led is controlled using two 100k resistor. The two 100k resistor used instead of one 200k resistor is for extra safety for short-circuit related condition. The LED is connected across wall outlet Line (L) & Neutral line (N). When the S1 is pressed the led start to emit infra-red light. The photo transistor makes a response and converts the VOUT from 5V to 0V.
In this configuration the opto-coupler can be connected across low voltage circuit such as microcontroller unit where the AC voltage detection is required. The output will produce square High to Low pulse.
As of now the first circuit is used to control or switching the DC circuit and second is to detect the AC circuit and control or switch DC circuit. Next we will see controlling AC circuit using DC circuit.

Optocoupler for controlling AC Circuit using DC voltage
In the upper circuit The LED is again controlled by 9V battery through 10k resistor and the state of the switch. On the other side a photo-TRIAC based opto-coupler is used, which control the AC optocoupler from the 220V AC outlet. The 68R resistor is used to Control the BT136 TRIAC which is controlled by the photo-TRIAC inside the opto-coupler unit.
This type of configuration is used to control electrical appliances using low voltage circuitry. The IL420 is used in the upper schematic which is a photo-TRIAC based Opto-coupler.
Other than this type of circuitry an opto-coupler can be used in SMPS to sending secondary side short-circuit or over current condition information to the primary side.

Photo-transistor optocoupler
The Transistor type can be anything whether PNP or NPN.Photo-Transistor can be further of two types depending on the output pin availability.This pin 6 is used to control the sensitivity of the photo-transistor. Often the pin is used to connect with ground or negative using a high value resistor. In this configuration, false triggering due to noise or electrical transients can be controlled effectively.
Also, before using Photo-transistor based AC optocoupler, the user must know the maximum rating of the transistor. PC816, PC817, LTV817, K847PH are few widely used photo-transistor based optocoupler. Photo – Transistor based opto-coupler is used in DC circuit related isolation.

Photo-darlington transistor optocoupler
Darlington Transistor is two transistor pair, where one transistor controls other transistor base. In this configuration the Darlington Transistor provide high gain ability. As usual the LED emits infrared led and controls the base of the pair transistor.
This type of opto-coupler also used in DC circuit related area for the isolation. The 6th pin which is internally connected to the base of the transistor, used to control the sensitivity of the transistor as discussed previously in photo-transistor description. 4N32, 4N33, H21B1, H21B2, H21B3 are few photo-Darlington based opto-coupler example.

AC optocoupler
AC optocoupler is mainly used where AC based control or switching is needed. The led can be controlled using DC, and the AC optocoupler used to control AC. Opto-coupler provide excellent isolation in this case too. Here is one Triac Application. The AC optocoupler examples areIL420, 4N35 etc are example of TRIAC based opto-coupler.

Photo-SCR based optocoupler​
SCR stand for Silicon controlled rectifier, SCR also referred as Thyristor. Same as like other opto-coupler the LED emit Infrared. The SCR is controlled by the intensity of the LED. Photo-SCR based Opto-coupler used in AC related circuitry. Learn more about Thyristor here.
Few Examples of photo-SCR based opto-couplers are:- MOC3071, IL400, MOC3072 etc.

How they operate
Both AC optocouplers and optoisolators allow the transfer of signals and data from one system to another within a piece of electronic equipment without a direct electrical connection. This is done optically by using a beam of light to an optical receiver in a single package with a light-conducting medium between the emitter and detector. This allows the total electrical isolation of electronic circuits while transmitting information from one voltage potential to another. In all optocouplers and optoisolators, input signals are converted to a pulse of light from an LED. This pulse of light is transmitted to a silicon photosensor.

Design considerations
The photosensor can be either analog or digital depending on the type of input signal to be transferred across the device. When an application requires an analog signal, such as for 4 to 20 mA, the photosensor can be either a photodiode or phototransistor. Both of these devices provide an analog output signal that can be used for a variety of analog applications.
An analog response is required for those applications where the amount of signal is critical to the operation of the system. The amount of current on the output of the device referenced to the amount of light into the LED is called the current transfer ratio (CTR), the output current divided by the input current. CTR values may vary from 10% to over 5,000%, depending on the gain of the system. Typically the lower the CTR, the faster the rise and fall times.
Analog devices normally require additional circuitry to be utilized by other components of the system. Digital output devices are best suited when the application requires a pulse, or bit output.
These devices are most often easier to use while providing a reliable signal that can interface directly with other system components. As long as sufficient light is shining on the photosensor, the output signal will be either high or low depending on the logic configuration.
A consideration facing designers is the speed of transmitting the digital data while minimizing the propagation delay. Most optoisolators/optocouplers are used for data rates below 10 Mbits/s, and the propagation delay may be in the 2 to 20-µs range.

The optocoupler
Optocouplers are specifically designed as small package devices similar to a Dual In-Line Package (DIP) or a surface-mount device (SMD), so that they take up the least amount of space while using light to transmit the data. The designer should pay attention to the definition of the isolation voltage potential because it may be identified as an ac, rms, or dc value.

The optoisolator
Optoisolators are available in a multitude of package styles including rectangles, cylinders, and specialty configurations. These package types are designed to provide higher isolation voltages than what can be achieved with DIP and SMD packages.
While using the optoisolator, the designer may need to take into consideration the environmental conditions. For example, percentage of humidity in the environment would be a key consideration if the system requires 50,000 V of dc isolation.
If the humidity in the air is too high, an arc may occur around the optoisolator or along the surface of the PCB, resulting in a conductive path and shorting around the device. Distance, temperature, barometric pressure, type, and amount of contaminate in the air, as well as humidity, define the breakdown potential exterior to the optoisolator.