need help in optoisolator!

Discussion in 'General Electronics Chat' started by evander, Mar 31, 2010.

  1. evander

    Thread Starter New Member

    Mar 31, 2010
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    Hi guys, i have a problem with this circuit. I do not know what happened to the opto isolator but when i turn on the switch the "?" will get turned on but when i off the switch the "?" will still be ON. any idea what happened? i suspect is the problem with the 3.9k resistors. Advice pls..thanks!
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
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    You must have a resistor in series with the LED to limit the current to about 5mA. See the data sheet for the opto-isolator to find the optimum current.
     
  3. evander

    Thread Starter New Member

    Mar 31, 2010
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    Sorry for some misunderstanding. Here is a better picture of it. The problem is i do not know what is inside the "?" and what could possibly be the circuit that will cause voltage still floating around that area?
     
  4. evander

    Thread Starter New Member

    Mar 31, 2010
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    anyone can help?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    What is the voltage that you measure?

    If the IR emitter were on, you should measure around 24v at that point.

    If it's off, you should eventually read nearly 0v at that point.

    If the box had a capacitor in it, it may take a while for the charge to drain via the 3.9k resistor.
     
  6. KansaiRobot

    Active Member

    Jan 15, 2010
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  7. Electrobrains

    New Member

    Mar 27, 2010
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    Hm, such black-box questions are very difficult to answer without more information (maybe there is a small green man inside, who has brought a battery with him..)!

    Seriously, the sergeant has the answer, there must be a fat capacitor inside or a power source connected with some latching circuitry.

    @KansaiRobot : no-one here knows if a diode is necessary or not. If there is an inductor built-in, the stored current would (quickly) discharge through the 3.9k resistor.
     
  8. evander

    Thread Starter New Member

    Mar 31, 2010
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    We observe that the circuit was latched on for a long period of time(not aware that it will switch off subsequencely). What other scenario could this happen other than having a big capacitor in the black box.
     
  9. evander

    Thread Starter New Member

    Mar 31, 2010
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    This is a reworked circuit please take a look.
     
  10. Electrobrains

    New Member

    Mar 27, 2010
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    It can be almost anything inside that box!
    For instance, here is an example with a relay.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    What kind of current flow do you get when you manually connect "the box?" to 24v?

    Optoisolators/optocouplers generally have a rather limited current capacity. If subjected to excessive current, they may fail by shorting out, which would cause you to always measure voltage on the output-side emitter.
     
  12. evander

    Thread Starter New Member

    Mar 31, 2010
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    Btw one simple question, how to i halve my resistor value?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    The 3.9k resistor?

    Connect two 3.9k resistors in parallel. Or, you could get roughly the same by using a 2k resistor. You can get quite fiddly with being precise, or you can get the job done.
     
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