optocoupler resistor

Discussion in 'General Electronics Chat' started by jrekha, Apr 29, 2010.

  1. jrekha

    Thread Starter New Member

    Mar 31, 2010
    11
    0
    i am using hcpl2630 opto. my input is 3.3V and i need to put a resistor at cathode to gnd, to limit the current.how do i fix its value.how do i get this opto's CTR value?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A datasheet is here: http://www.fairchildsemi.com/ds/6N/6N137.pdf

    5mA input current results in up to 13mA output current.

    Typ Vf @ 10mA = 1.4v.
    Rlimit = (3.3v-1.4v)/5mA
    Rlimit = 1.9v/5mA
    Rlimit = 380 Ohms.

    If possible, I suggest using the resistor on the anode side.
     
  3. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    The datasheet for this device has all the info you need to figure things out....

    hcpl2630.jpg

    B. Morse
     
  4. kkazem

    Active Member

    Jul 23, 2009
    160
    26
    Hi,
    I disagree with the sergeant. If you look carefully on the first page of the datasheet, they are recommending that you use a minimum of 6.3 mA, so that over time as the LED output diminishes by up to 20%, you'll still have the same output reception. Notice that 6.3mA * (1-0.2) = 5mA. The max input voltage of the device is 1.4 V forward on the LED, so R= (3.3V-1.4V)/6.3mA = 301 ohms, which is a standard 1% value.

    You can probably get away with using 380 ohms, but an old engineer's rule of thumb is always design for the worst case. If you use 300 Ohms, you're covered; if you use 380, you may or may not have problems now or later as the Opto degrades, which they all do over time (mainly the LED section).
    Regards,
    Kamran Kazem
     
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