current limitation in optocoupler

Discussion in 'Electronics Resources' started by hse, Jul 28, 2010.

  1. hse

    Thread Starter New Member

    Jul 28, 2010
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    Hi
    I'm protecting some in/output pins by use of optocouplers. The optocouplers to be used are 74ol6000. Both in- and output side is connected to 5V. These are isolated from each other. I'm not sure how to calculate The current limiting resistor on The input side.....

    Could anyone please provide some hints to get The calculation right?

    The microcontroller is able to sink/source 3mA.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    There is no need for the current limiting resistors with these optocouplers.
    The input and output are TTL compatible.

    [​IMG]

    See also the complete datasheet attached.

    Bertus.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    The 74ol6000 contains an LED driver circuit that will handle the current limiting.

    As for standard current limiting calculations, you will use Ohms Law.

    Have a read through here:
    Bills Index...Chapter 2: Current Limiting
     
  4. hse

    Thread Starter New Member

    Jul 28, 2010
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    0
    I'm not familar with TTL logic....so what you are saying is that the TTL compatibility means that the it internally handles the limitations.

    What about my microcontroller input....should I just connect it to the opto output directly? They are Both supplied from The same source.... or should the current here be limited by a resistor? If Yes, i guess this will affect the rise/fall times....or?
     
  5. hse

    Thread Starter New Member

    Jul 28, 2010
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    0
    Actually it seem s like the 74OL6000 is obsolete (or at least they are hard to find), so I think need another that could replace ir but having the same characteristic (high speed). Going to Fairchilds homepage I found the FOD0710 (12,5Mbit/s) or maybe the FOD0720 (25Mbit/s).

    Still I don't know if this one need a current limiting resistor......
    Both on in-/output side???? or none? It states an average output current at 10mA. Is that too much for an µC input?

    Could you help me there?
     
  6. hse

    Thread Starter New Member

    Jul 28, 2010
    8
    0
    I think I might did not make my point all clear so lets sum up:

    My µC has 2 inputs, and 2 outputs. All 4 must be connected through optocouplers.

    1.The µC is capable of sink/source 3mA. The output is connected to an optocoupler (probably FOD0710 from fairchild), but if the output is low then it needs to sink some current. The spec. for the opto states +/- 10mA for the input current, but i guess that would be the maximum tolerances, is there not a minimum amout of current to be draw for the optocoupler to work properly???

    In case that the input current is -10mA, I guess my output that is able to sink 3mA will be damaged or what?

    2. The opto average output current is 10 mA. I have no clue if that is too high for an input (since it is not stated in the datasheet).
    What I thought was, that an input is at very high impedance (to not load) and thus only a very small amount of current (a couple of µA maybe) was able to flow into it (just as for op amps). How does that sound in context to the average output at 10mA??

    3. Does I need any current limiting resistors on either side ot he optocouplers?
    I know you said not for the 74OL6000, but what about the FOD0710???

    Best Regards
    Hse
     
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