MCT2E connections help

Discussion in 'General Electronics Chat' started by onlyvinod56, Jan 9, 2013.

  1. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008

    see the attachment please..

    Why is the R3 value selected as 560.
    The supply voltage for MCT2E is taken as 12v.
    can anybody explain this using the MCT2E datasheet?

    If the mosfet is changed to IRFz44, do i need to change the 560 ohm?
  2. bertus


    Apr 5, 2008

    Why do you use such a low value for the current limiting resistor (R2) for the led?


    With that resistor the led current will be almost 80 mA, as the absolute max current in the data sheets states 60 mA.
    Also this current will probably be to muh for the MCU.
    A resistor of about 150 Ohms would be more suitable.

    Last edited: Jan 9, 2013
  3. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008
    That schematic is not mine. And iam not building that. Iwant the information about the transistor details of the optocoupler. But anyway MR.Bertus, you raised a similar point.
    I have an explanation. If i want to design that circuit, here goes my calculations.....

    Vcc of MC = 5V
    from MCT2E datasheet, Vf = 1.5 (max) and If = 20mA;

    Therefore, R = (Vcc - Vf)/ If = (5 - 1.5)/ 20mA = 166 ohm.
    I think i am correct.:D

    I know how to work with the led.
    My question is , how to calculate the R3 value for the transistor part?