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Monday, September 1, 2008

Optocouplers in detail

What is present inside an optocoupler?
Optocoupler typically come in a small 6-pin or 8-pin IC package, but are essentially a combination of two distinct devices: an optical transmitter, typically a gallium arsenide LED (light-emitting diode) and an optical receiver such as a phototransistor or light-triggered diac. The two are separated by a transparent barrier which blocks any electrical current flow between the two, but does allow the passage of light. The basic idea is shown in the figure below, along with the usual circuit symbol for an opto coupler.
Physical seperation
Usually the electrical connections to the LED section are brought out to the pins on one side of the package and those for the phototransistor or diac to the other side , to physically separate them as much as possible. This usually allows optocoupler to withstand voltages of anywhere between 500V and 7500V between input and output.

Signals that are given as input
Optocoupler are essentially digital or switching devices, so they’re best for transferring either on-off control signals or digital data. Analog signals can be transferred by means of frequency or pulse-width modulation.

How it works?
A common implementation involves a LED and a phototransistor, separated so that light may travel across a barrier but electrical current may not. When an electrical signal is applied to the input of the opto-isolator, its LED lights, its light sensor then activates, and a corresponding electrical signal is generated at the output. Unlike a transformer, the opto-isolator allows for DC coupling and generally provides significant protection from serious over voltage conditions in one circuit affecting the other.

With a photodiode as the detector, the output current is proportional to the amount of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode.

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