H-bridge R values

Discussion in 'The Projects Forum' started by dqadri, May 2, 2011.

  1. dqadri

    Thread Starter New Member

    Apr 30, 2011

    Im making a H-bridge for my school projekt. It will power a 12V 3A motor.

    Q1 and Q4 are Tip127 5A PNP transistors Hfe = 1000
    Q2 and Q3 are Tip112 5A NPN transistors Hfe = 1000
    (Multisim dident have them)

    First problem:
    Will this H-bridge work? The inputs are a comparetor with Isource max = 60mA and 5V.

    Second problem:

    I found R3 and R4 this way:

    Ib= Ic/Hfe = 3A/1000 = 30mA
    Rb= (Vin-VbeQ2-VbeQ5)/Ib = (5-0,7-0,7)/30mA = 1,2KOhm (dont mind the wrong value in the pic.)

    Is this right??

    Third Problem:

    R2 and R3 are parallel so same voltage but i dont know how Big I is over R2 and how to calculate R6. My teacher said something about a voltage drop over R6 so Q4 would turn on!

    Thx for you time, Dennis!

    !!! EDIT !!! Gain for BC547A is 100!
    Last edited: May 2, 2011