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BJT Transistor w/ motor

  1. Oct 30, 2011 #1
    1. The problem statement, all variables and given/known data

    You want to use a PIC microcontroller to drive a motor in a clockwise direction at either 0% speed (when the PIC output is 0V) or full speed (when the PIC output is +5V). This motor draws 500mA at +5V while the output of the used PIC is limited to +5V and 20mA. To solve this problem you decided to use a BJT transistor that withstands 500mA as an interface between the PIC and the motor as shown in Fig 2. This transistor will play the role of a switch that turns on and off the motor while it is also able to handle the required current by the motor.

    Notice that there is no need to add a limiting resistor RC in series with Q1 collector because the motor internal resistance will limit the collector current IC to 500mA at VCC = +5 V and VCE(Sat) = 0.21V.

    The selected transistor has the following specifications extracted from its datasheet:
    - VBE = 0.7V when the EBJ is forward biased
    - 300 < hFE = Beta < 400, means that you can obtain a current gain of at least 300 (for Beta minimal) at the edge of saturation (EOS).
    - max IC (Sat) = 4000mA, meaning that this transistor can withstand max 4000mA.
    - VCE(Sat) = 0.21V an estimate of VCE at saturation
    - VCEO = 40V, means that this transistor can handle VCE voltages up to 40V when it is
    turned off
    (a) Select a standard value RB that guarantees the operation of your transistor in saturation.

    2. Relevant equations


    3. The attempt at a solution

    I am not 100% sure, because there are a lot of numbers being thrown at me, of what to do.

    How I tried it was: When PIC = +5V, the current can not be larger than 20mA.

    5 - VBE / R = Ib = 4.3/R < 20mA

    Imotor = 500mA = Ic
    Ic = (Beta)Ib

    I plugged in random Resistor values to obtained an Ib less than 20mA and Beta to be between 300 and 400. I got 3.3kOhm (there are a whole range of them.

    What I need to know now (if that was correct) how to prove everything else to ensure 3.3kOhm was correct.
  2. jcsd
  3. Oct 31, 2011 #2


    User Avatar

    Staff: Mentor

    For that 3.3k, determine base current. With that base current, determine minimum collector current and show transistor is saturated. Hence, determine collector current.
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