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CMOS 555 driving BC337 - base resistor

Discussion in 'General Electronics Discussion' started by Cirkit, Aug 1, 2016.

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  1. Cirkit

    Cirkit

    132
    10
    Oct 28, 2015
    Hello,

    I am trying to determine the correct base resistor value to drive a BC337 transistor into saturation. The transistor is connected to the output of a CMOS 555 timer via the resistor. The transistor is driving a 3V solenoid which consumes around 200mA when active.

    The entire circuit is powered from 3V.

    With VBE = 1.2V and hFE = 60, I have chosen a 500 Ohm resistor. Is this correct?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,316
    2,593
    Nov 17, 2011
    It depends on the operating voltage (or the high output voltage, respectively) of the 555. hFE = 60 means Ib ~ 3.5mA. The required value for Rb is determined by:
    Rb = (Vout_high - Vbe)/Ib
     
  3. Cirkit

    Cirkit

    132
    10
    Oct 28, 2015
    The 555 is operating in monostable mode and the high output voltage is around 3V.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,316
    2,593
    Nov 17, 2011
    Then 500 Ω is a good choice.
     
    Cirkit likes this.
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