The main advantage of opto-couplers is their high electrical isolation between the input and output terminals allowing relatively small digital signals to control much large AC voltages, currents and power.Īn optocoupler can be used with both DC and AC signals with optocouplers utilizing a SCR (thyristor) or triac as the photo-detecting device are primarily designed for AC power-control applications. Optocouplers and Opto-isolators are great electronic devices that allow devices such as power transistors and triacs to be controlled from a PC’s output port, digital switch or from a low voltage data signal such as that from a logic gate. Also unlike a thyristor (SCR), a triac is capable of conducting in both halves of the mains AC cycle with zero-crossing detection allowing the load to receive full power without the heavy inrush currents when switching inductive loads. This type of optocoupler configuration forms the basis of a very simple solid state relay application which can be used to control any AC mains powered load such as lamps and motors. Optocouplers and opto-isolators can be used on their own, or to switch a range of other larger electronic devices such as transistors and triacs providing the required electrical isolation between a lower voltage control signal, for example one from an Arduino or micro-controller, and a much higher voltage or mains current output signal. Obviously, tubing with a reflective inner would be more efficient than dark black tubing. The advantage of this home-made optocoupler is that tubing can be cut to any length you want and even bent around corners. An Led and a photo-transistor are inserted into a rigid plastic tube or encased in heat-shrinkable tubing as shown. Simple home made opto-couplers can be constructed by using individual components. There are many other kinds of source-sensor combinations, such as LED-photodiode, LED-LASER, lamp-photoresistor pairs, reflective and slotted optocouplers. The photo-transistor and photo-darlington devices are mainly for use in DC circuits while the photo-SCR and photo-triac allow AC powered circuits to be controlled. The four optocouplers are called the: Photo-transistor, Photo-darlington, Photo-SCR and Photo-triac as shown below. Optocouplers are available in four general types, each one having an infra-red LED source but with different photo-sensitive devices. Since there is no direct electrical connection between the input and output of an optocoupler, electrical isolation up to 10kV is achieved. The spectral response of the LED and the photo-sensitive device are closely matched being separated by a transparent medium such as glass, plastic or air. The photo-transistor can be used to switch current in the output circuit. When the current flowing through the LED is interrupted, the infra-red emitted light is cut-off, causing the photo-transistor to cease conducting. The base connection of the photo-transistor can be left open (unconnected) for maximum sensitivity to the LEDs infra-red light energy or connected to ground via a suitable external high value resistor to control the switching sensitivity making it more stable and resistant to false triggering by external electrical noise or voltage transients.
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