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[HELP] Optoisolators in series

Discussion in 'General Electronics Discussion' started by eight08, Nov 25, 2012.

  1. eight08

    eight08

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    Apr 16, 2010
    i'm currently designing a PCB, and due to design considerations, i am faced with a problem.

    I used to wire an optoisolator traditionally by arranging them in parallel (see pic "old"), now, i want to wire them in series. Is this possible? (see pic "new")
     

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  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi there eight08

    both circuits, parallel or series, are pointless as neither of them actually do anything

    have a look in a datasheet for any opto-isolator and see examples of how they are commonly used
    Any questions dont hesitate to ask :)

    cheers
    Dave
     
  3. eight08

    eight08

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    Apr 16, 2010
    Thanks for the reply dave, but i forgot to put the output pin on the schematics, but, the outputs would be taken from the collector of the optoisolators. uploaded new pic, marked in red.

    Thanks!
     

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  4. davenn

    davenn Moderator

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    Sep 5, 2009
    it mite be an idea to tell us what you are trying to achieve.

    the output as you have marked will be a voltage that is present when the opto is turned off, The current available will be determined by the resistor value above that tap point.

    as you have wired it BOTH optos will be permanently turned on and you will never see an output voltage at that point

    Dave
     
  5. CocaCola

    CocaCola

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    Apr 7, 2012
    What is the point of the circuit? There is no isolation, so it's really useless...

    As Dave said please review the datasheet on application...
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    to expand on CC's first comment.... "There is no isolation, so it's really useless..."

    this was going to be my next comment. you have no isolation as both parts of the opto are powered by the same voltage source.
    In pratice, you will find that the source power supply that switches the opto LED on is a separate power supply to what the load is being supplied with

    Dave
     
  7. eight08

    eight08

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    Apr 16, 2010
    sorry guys, wrong definition of the part, it's a "reflective optical sensor".
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    have a look at this circuit of mine and note how there is no connection between the positive or AC rails of the input and output sections of the 3 opto's

    in fact the 2 upper opto's are using low voltage DC to switch 240V AC on the output

    and THAT is one of the primary purposes of opto's, to isolate 2 sections of a circuit that may have VERY big differences in voltage

    [​IMG]

    Dave
     

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  9. davenn

    davenn Moderator

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    Sep 5, 2009
    OK.....
    but my previous comment still stands..... you have no isolation between the input and output stages. and the way it is wired the output will be permanently turned on and no voltage will be predent at the point you have indicated


    Dave
     
  10. eight08

    eight08

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    Apr 16, 2010
    It's just for a low voltage microcontroller application, in which, after the optical sensor is triggered, the input voltage would be sent to the uC...

    I've been thinking about it, i guess the series thing wouldn't really work, but i might probably place the emitter terminal straight to the main ground, rather than the series rail.

    Thanks for the reply!
     
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    OK....

    using an opto on the input of a micro isnt a bad thing, specially if you want to protect the micro from stray and possible excessive voltages :) thats cool

    you just need to separate the input and output supply rails. So that the input to the LED of the opto is being supplied from your signal source ... whatever that is and the photo transistor in the opto is being supplied from the same source as the micro

    like this....

    [​IMG]

    now in this case the pin of the micro is held high till the opto is turned on
    you may want it the other way in which case move the collector resistor and tap point to the emitter leg and the tap point between the emitter and the resistor



    Dave
     

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  12. davenn

    davenn Moderator

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    Sep 5, 2009
    Ohhh ....

    if this is a system like where you have a opto chopper wheel, as in an old style mouse thats cool.
    The LED of the opto will have a separate battery supply or a isolated supply from the micro supply... use a voltage regulator to do that isolation.

    edit .... after thought ..... in this situation, it may work with out isolating the LED supply :)

    the output of the opto is still the same arrangement

    Dave
     
    Last edited: Nov 25, 2012
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