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Thread: Teensy 4.1 - Optoisolator 4N25

  1. #1

    Teensy 4.1 - Optoisolator 4N25

    Hi all,

    Today I using 10 Arduino Mega outputs (5V) to drive 10 4N25 with a 390 series resistor to drive the IR and works perfectly.
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    I intend to migrate to Teensy 4.1 with the same 10 outputs and same circuit, but I know the output voltage from Teensy is 3.3V, and in-circuit I will change the series resistor to 200 ohms, which maintains the 1.3V and 10mA in IR.

    Click image for larger version. 

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    My question is: can I drive direct from Teensy to my 10 outputs or I will need another circuit because of Teensy current limit per pin?



  2. #2
    Senior Member
    Join Date
    Mar 2016
    From the 4N25 data sheet, the LED has a Vf of around 1.1V at an If of 10mA - range looks to be 1 to 1.25V. So with a 200 ohm resistor and 2.2V (3.3V - 1.1V) you get 11 mA. A bit high for a Teensy. I would probably use a 270 ohm resistor for an If of 8.15 mA. 300 ohm gets you to 7.3 mA.

    Note that there are much better optos out there (4N25 is pretty old). I'd look at the EL817/LTV817, LTV-356/7. Better CTR and thus lower If needed to get a good transfer. Cheaper. I use a 357 a lot and it works quite well plus it is really small (I can pack a lot in a small space and the 1 channel version allows me to chase very favorable volume discounts, probably not your concern).

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