24-01-2013, 02:15 PM
OPTOCOUPLERS
OPTOCOUPLERS.ppt (Size: 2.3 MB / Downloads: 81)
INTRODUCTION
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.
Often this is because the source and destination are (or may be at times) at very different voltage levels, like a microprocessor which is operating from 5V DC but being used to control a triac which is switching 240V AC.
In such situations the link between the two must be an isolated one, to protect the microprocessor from overvoltage damage.
Relays can of course provide this kind of isolation, but even small relays tend to be fairly bulky compared with ICs and many of today's other miniature circuit components.
Because they’re electro-mechanical, relays are also not as reliable , and only capable of relatively low speed operation.
Where small size, higher speed and greater reliability are important, a much better alternative is to use an optocoupler.
These use a beam of light to transmit the signals or data across an electrical barrier, and achieve excellent isolation.
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.
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),
How They're Used
Basically the simplest way to visualise an optocoupler is in terms of its two main components: the input LED and the output transistor or diac.
As the two are electrically isolated, this gives a fair amount of flexibility when it comes to connecting them into circuit.
All we really have to do is work out a convenient way of turning the input LED on and off, and using the resulting switching of the phototransistor/ diac to generate an output waveform or logic signal that is compatible with our output circuitry.