optocoupler (MCT2E) resistor?

Discussion in 'The Projects Forum' started by onlyvinod56, Apr 23, 2014.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Hello.

    I need to connect MCT2E optocoupler to F28335 DSP board whose logic high voltage is 3.3V.

    What value of resistor should be used on emitter side?
    Thankyou.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    The MCT emitter has a forward drop of 1.25V, so I would design for a current of 10mA.

    Thats 3.3 -1.25 = 2.05/.01 =~200R, use a 180R or 220R.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,556
    2,375
    ~ 330Ω.
    Max.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The "3.3v logic high" sourcing 10mA will be more like 2.8 or 2.9v DC.

    2.8v - 1.25 LED Vf = 1.55v
    R = E/I = 1.55/10 = 155 ohm.

    I would probably go with 180 ohm like Ericgibbs suggested and see how it works.

    The moment you connect it to the logic high pin, you can measure the volts across the resistor because the LED is on and the resistor volts can tell you the LED current; I = E/R = ?/180 ohm
     
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