Optoisolator (4N33) Question

Discussion in 'General Electronics Chat' started by bob332, Feb 23, 2011.

  1. bob332

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    Feb 14, 2011
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    finally got caught up w/ some other stuff and had a question in regards to this:
    [​IMG]

    the datasheet - http://www.fairchildsemi.com/ds/4N/4N33M.pdf

    claims forward current up to 80ma and forward voltage of 1.2-1.5V. i am considering that my main input will be 13V max, so the R2 resistor for 1.3V/5ma to the incoming side will be of 2.7KΩ, why does the schematic say 10KΩ is necessary? is my math incorrect or am i missing something? are the leds in these to be used like a normal led? even if 15V was coming in, i am still well under anything near max if i am reading the datasheet correctly :confused:

    edit:
    also, i don't have to have a common ground between the sides according to the datasheet....again if i am reading it correctly...just wondering why that schematic was created that way? what should i expect for a V drop across the diode? 1V? if i am using only +DC, is there any possibly way for it to go under 0V since the original power is coming from a wallwart, but i just didn't think they could go less than 0V.

    thanks,
    bob
     
  2. blueroomelectronics

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    Where's that schematic from?
     
  3. bob332

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    Feb 14, 2011
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    it was on an arduino page
     
  4. blueroomelectronics

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    Which Arduino page? Post a link.
     
  5. bob332

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    Feb 14, 2011
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    let me go through my history and find it...just out of curiosity, why do you need the link? i am not making up where i found it at...the original had 2 schematics on it, i took off the other circuit..
     
  6. bob332

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    Feb 14, 2011
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  7. blueroomelectronics

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    I'm curious to why it was designed that way, the tied ground defeats using an optocoupler and the 10K is very high. What voltage range do you want for the input?

    Edit, thanks for the link.

    13V is this for an automotive application? Some designs would use a pair of transistors as a constant current source for the LED.
     
  8. bob332

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    Feb 14, 2011
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    i kind of thought the same thing after i read the data sheet, and i just want 12-14Vmax, that is why i am a bit confused. the arduino can handle 5Vinput on its side so i was too wondering why such a high resistance on that side too.
     
  9. blueroomelectronics

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  10. bob332

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    Feb 14, 2011
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    will take a look, thanks :)
     
  11. blueroomelectronics

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    Don't connect the grounds, and for 14V a 1K resistor should be ample.
     
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  12. bob332

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    Feb 14, 2011
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    is there an issue w/ the grounds connected as i did follow that arduino schematic at the moment...i just thought i read the data sheet wrong :(. i can change it on the next board i make and will. thanks for clearing it up.

    now is there any way for a complete +dc power to get -? is the diode necessary? i know they are cheap and not a problem, but for education is the diode necessary?
     
  13. bob332

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    Feb 14, 2011
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    not an automotive application, just happens that the V is 12V.
     
  14. blueroomelectronics

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    The diode is only needed if you have an AC input or reverse the +/- inputs.
     
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  15. bob332

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    Feb 14, 2011
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    thanks, may as well leave the diode in case +/- gets reversed :) appreciate the info
     
  16. bob332

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    Feb 14, 2011
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    this schematic would be more accurate:

    [​IMG]
     
  17. blueroomelectronics

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    Again get rid of the connection between 2 & 4 of the 4N33
     
  18. bertus

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    Apr 5, 2008
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    Hello,

    Here is the corrected circuit:

    [​IMG]

    I also corrected the current for the input led.
    In the datasheet is given that it should be between 10 and 50 mA.
    Now it will be about 8 mA at 5 Volts and 23 mA at 12 Volts.

    Bertus
     
  19. bob332

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    Feb 14, 2011
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    excellent. just to verify, and i couldn't find it on the datasheet (maybe i did see it but didn't know??), but how long can the led stay on? are they designed to be on for say days at a time like a regular led? of course, staying under the 50ma as suggested and now put into place :)
     
  20. hgmjr

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    Jan 28, 2005
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    Bertus' design should be more than capable of withstanding 12V continuously on its input for many years without failure of the OPTO.

    hgmjr
     
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