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is there a difference between an optocoupler and an optoisolator?

Discussion in 'Electronic Design' started by Michael, Dec 31, 2003.

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  1. Michael

    Michael Guest

    Hi - is there any difference between an optocoupler and an optoisolator?
    Looking at datasheets for them they seem to be the same thing. Also when
    would you use an optocoupler/optoisolator and when would you use a solid
    state relay? Again all three seem to serve about the same purpose. I'm
    guessing that solid state relays are normally intended for higher power
    applications - but that's just a guess.

    Also is there a classic optoisolator or optocoupler? The one I have seen
    used the most is the 4N27 - but my experience is very limited.


  2. A solid state relay is optimized for on off operation with larger
    voltages and currents that couplers carry, including ones that handle
    line voltage and 10s of amperes. Opto couplers are intended to carry
    information in the form of either a digital or analog signals.
    Digital is most common, including ones that have gate outputs that
    look just like any other logic. But most have photo transistor or
    darlington outputs, but some have photo diode, mosfet, triac, photo
    resistor or photo voltaic (solar cell) outputs. The gate and triac
    outputs are strictly off on operation, but most of the others and the
    photo resistor specifically can be used for analog signals to some

    Inputs are generally either a single infrared led or a pair in inverse
    parallel (to respond to current in either direction) but some of the
    photo resistor types still have light bulbs as input. There are
    single, dual and quad channel devices. There are also matched two
    channel devices so you can isolate an analog signal with one and use
    the other for feedback to linearize the output.

    With all this variety, there is no point listing a part number or two
    and expecting that to be representative. Ask Digikey for a catalog
    and browse through pages of choices arranged by category. Then
    download some data sheets from their web site and study the specs. It
    is a pretty dense subject.
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