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Optocoupler Teaser

Discussion in 'Electronic Basics' started by Bart, Mar 2, 2006.

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  1. Bart

    Bart Guest

    Hi All,
    I've been in contact with a guy who has a CNC machine who's controller uses
    optocouplers. Here;s a picture:
    http://home.fuse.net/consultec/misc/images/optocouplers_.JPG

    The one on the right works and there WAS an identical one on the left that
    failed and the manufacturer sent a new one that looks different and doesn't
    work. You can see the new one on the left does have a different design. Upon
    finding the new one doesn't work, the manufacturer sent another, identical
    to the first one they sent and it still doesn't work. The guy is convinced
    they're sending the wrong style because he's swapped optocouplers and
    wherever he puts the original (the one on the right) it works.
    Is there something obvious about what is going on? Is one for RS232 and the
    other for TTL or 0-20milliamp? Is the output side of the chip the problem?
    Is there enough info here to even make a determination?
    This is beyond me, hoping someone recognizes the two different circuits and
    can give some input.
    Thanks in advance,
    Bart
     
  2. John Fields

    John Fields Guest

     
  3. Deefoo

    Deefoo Guest

    Well, it would help if you could supply the part numbers of the two opto
    couplers. Then it would help also if you could tell us what they get at
    their inputs. I see a 1k resistor at the left. If we assume about 10mA
    minimum for the opto coupler LED for it to work then you would need at least
    about 11.5V at the input (10mA*1k + Vled). But there is another LED, looks
    like it is in series with the opto coupler LED, so that would add another
    1.5V, say. Do you have that much (13V) at the input? If not then you should
    probably replace the 1k series resistor by a lower value one.

    --DF
     
  4. Bart

    Bart Guest

    Here is my simple take on it so far,
    POINT: The working one (on the right) uses a voltage divider on the input
    side so maybe some characteristic of the input side it was designed for
    makes
    the new/replacement one (on the left) fail immediately up energizing.

    COUNTERPOINT: They both use what looks like 1/4 watt resistors and
    TO-92 devices which probably means we're just dealing with smaller simple
    signals like data transfer. Even the optocouplers I looked up on DigiKey
    were rated in the milliamps so I don't think the new/replacement ones are
    failing, they're just designed for a different application. Physically seen
    differences are apparent on both the input AND output side.

    MURPHY'S-POINT: The boards have just one simple task, when one
    side is energized (yes, low power) then the other side is allowed to have
    IT'S
    energy flow (also low power). One board works and the other one doesn't.
    Non-working board has transistor mounted backwards from working board.
    Non-working board has transistor mounted backward from silkscreen image
    on board. Non-working board has never worked, will never work, and
    slipped through Brazil's (yes, that's where its made) stringent quality
    checks.
    My guy has received total of three of these replacement boards, all
    identical,
    none work.

    Did they run out of NPN transistors and thought they'd use PNP and just
    mount
    it backwards?

    Glad I'M not paying for the replacement boards,
    Bart
     
  5. Jamie

    Jamie Guest

    hmm interesting devices? i noticed A1 A2 ? these for brush motors
    by any chance? :)
    in anycase, i did notice the transistor in the left unit looks
    like its not in there correctly if you were to follow the board lay out?
     
  6. That doesn't work. You have to pull them inside out to reverse them
     
  7. Deefoo

    Deefoo Guest


    Oh, I see, you call the whole board an opto coupler, I thought you meant the
    6-legged components on the boards. So he swapped boards, not just
    components. Ah, language...

    --DF
     
  8. Bart

    Bart Guest

    My bad...you're right, I should have called it an optocoupler interface
    board
    throughout the post. I'm still working on the exactness of vocabulary this
    discipline demands. Funny thing, when I WAS married my wife would
    blunder with words, confusing me to where I'd ask a simple question
    for clarification....that's when she'd exclaim with head cocked and hands
    on hips "You KNOW what I meant!"
    I never want to be like that,
    Bart
     
  9. John Fields

    John Fields Guest

     
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