Optocoupler help

Discussion in 'The Projects Forum' started by pa99, Apr 21, 2013.

  1. pa99

    Thread Starter New Member

    Mar 23, 2009
    4
    0
    Hi hopefully someone can help out.

    I've built an ECG project based on AD6424AD. It's running on 9volt battery supply. Output to DAC and signal processing done in C.. I'm considering adding the optocoupler PS2506. Having difficulty understanding the resistance values required in this circuit. The high CTR of the device means only low current is needed at the LED. Any help would be much appreciated, I don't get to these resistor values with their datasheet.

    Thanks again..

    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    9V - 1.4V for the LED = 7.6V
    7.6V/.01A = 760 ohms

    That's what I can do with no schematic and no idea how much current you want the output to pass.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Your image stored at photobucket does not show in Firefox and shows an error in Konquerer.
    Please store the schematic over here:
    Attachments and Images

    Bertus
     
  4. pa99

    Thread Starter New Member

    Mar 23, 2009
    4
    0
    Thanks for the replies..

    Sorry about the schematic , should be there now. Output is to a USB DAC so I don't think current out is a major worry. The +9 ,-9 throws me in calculating the resistor values..

    Thanks again..
     
  5. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    18Vmax-1.4 = 16.6V
    16.6V/.01A = 1660 ohms minimum

    Apparently the ADC can output anything from +9 to -9V
    1660 ohms will limit the current in the driver side to 10ma.
     
  6. pa99

    Thread Starter New Member

    Mar 23, 2009
    4
    0
    OK thanks , so the increased resistance on the input of the optocoupler will limit current further. Is this due to the high CTR for this particular optocoupler. 2000% ?

    How did you come up with those ADC values ? As it turns out , the ADC I'm using has a -10v to 10v range..
     
  7. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Increased resistance? You already have 3.5k in that circuit. 1660 ohms is less than 3500 ohms. It's due to the datasheet telling me to use 10 milliamps for the emitter side of the optoisolator.

    I came up with the voltages by looking at the schematic where they are clearly labeled.
     
  8. pa99

    Thread Starter New Member

    Mar 23, 2009
    4
    0
    :)

    OK thanks, I meant the increased resistance , from the minimum value you suggested to the actual value used on the input side. 3.5K..

    Thanks , sorry if I'm making a meal of it ..
     
  9. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Well, now you know the math and the limitations of the opto. You can adjust if necessary.
     
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