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Driving LED with NPN

Discussion in 'LEDs and Optoelectronics' started by Gr0zn1, Feb 21, 2014.

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  1. Gr0zn1


    Jan 17, 2014

    I have a question about driving LEDs using NPN transistor.

    I've made square signal generator that works at 10Hz and 25:75 duty-cycle and on 5V. With that output I would like to drive the base of NPN 2N2222. Let's say I would like to drive 6 LEDs with typical Vf=2V at 20mA. I would like to know the right values for base and LED resistors, if I use a secondary supply for LEDs with 30V.

    Thanks for your help!

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    One place to start would be here:

    Use a series resistor as if they are being powered from the 30V supply. Have the transistor's emitter grounded and use a base resistor which is about:

    0.5 * (Vin - Vbe) * hfe / Iled

    For hfe use the lowest value specified in the datasheet for an Ic near 20mA.

    If it's 100, the resistor would be about 0.5 * (5 - 0.7) * 100 / 0.02 = 10,000, so 10k would be fine.
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