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Theory of the Optocoupler

There are many situations where signals and data need to be transferred from one subsystem to another within a piece of electronics equipment, or from one piece of equipment to another, without making a direct ohmic electrical connection. Optocouplers 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 Fig.1, along with the usual circuit symbol for an optocoupler. 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 optocouplers to withstand voltages of anywhere between 500V and 7500V between input and output. Optocouplers 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.

 Key Parameters:
The most important parameter for most optocouplers is their transfer efficiency, usually measured in terms of their current transfer ratio or CTR. This is simply the ratio between a current change in the output transistor and the current change in the input LED which produced it. Typical values for CTR range from 10% to 50% for devices with an output phototransistor and up to 2000% or so for those with a Darlington transistor pair in the output. Note, however that in most devices CTR tends to vary with absolute current level. Typically it peaks at a LED current level of about 10mA, and falls away at both higher and lower current levels. Other optocoupler parameters include the output transistor.s maximum collector-emitter voltage rating VCE(max), which limits the supply voltage in the output circuit; the input LED.s maximum current rating IF(max), which is used to calculate the minimum value for its series resistor; and the optocoupler.s bandwidth, which determines the highest signal frequency that can be transferred through it . determined mainly by internal device construction and the performance of the output phototransistor. Typical opto-couplers with a single output phototransistor may have a bandwidth of 200 - 300kHz, while those with a Darlington pair are usually about 10 times lower, at around 20 - 30kHz.