h_bridge (slow motor)

Discussion in 'General Electronics Chat' started by z3ngew, Apr 21, 2013.

  1. z3ngew

    Thread Starter New Member

    Apr 18, 2013
    Hello everyone,
    I need help with h bridge circuit using power transistor,
    i am using 2 pnp(TIP127) and 2 npn transistors(TIP122), the problem is that the motor is driven but with small speed, the load voltage(motor voltage) is less than the voltage that it should be.

    the circuit can be found in the following file
    View attachment circuit.zip

    Thanks in advance,
  2. panic mode

    Senior Member

    Oct 10, 2011
    those transistors are darlingtons and have huge Vce when on.
    for example if Ic=3A, Ib must be 12mA and Vce=2V
    if Ic=5A, Ib must be 20mA and Vce=4V.

    Voltage you see on the motor is Vbatt-2*Vce. If you use say 13.8V supply and motor that draws 5A, you will be loosing 8V on those transistors, only 5.8V will be across the motor. another thing is dissipated power. at those conditions motor will get 5A*5.8V=28W but transistors will need to get rid of 40W of heat. that is terrible efficiency.

    If the supply voltage is even lower such as 9-12V, you get even worse results.

    now to get 20mA through 1k to a Darlington, you need signal level of control voltage to be
    Vcontrol = Vbe + 1k*20mA = 1.4V + 20V = 21.4V.
    I am assuming that your MCU output is maybe 5V at best (if not 3.3V). you see where this is going?

    Also, for H-bridge to work properly you have to have either:
    1. all transistors off (no current through motor)
    2. Q1 and Q3 on (motor runs forward)
    3. Q2 and Q4 on (motor runs reverse)
    4. Q3 and Q4 on (breaking)

    only transistors that are mentioned to be "on" should be really on AND, they have to be in saturation (work like a switch).

    I see no control signals but given arrangement, the only sensible inputs would be two wires on the very left. Anything else would short or overload driving output (such as MCU). however there is a problem. suppose wire at left top node (connects to R1 and R4) is signal "A" and wire at the bottom is "B" (connects to R2 and R3).

    then we can create a state table:
    A=0, B=0, this turns on Q1 and Q2 so this is "breaking" (but motor leads are positive)

    A=1, B=0, Q4 and Q3 are on, you get short circuit through two transistors, current does not go through motor.

    A=0, B=1, Q2 and Q3 are on so you get short circuit without current ever going through motor.

    A=1, B=1, Q3 nd Q4 are on so you get "breaking"

    the only reason you get ANY movement out the motor is because the transistors are not saturating, they are all turning on but operating in linear region. They must be getting crazy hot and your battery or power supply are getting killed for nothing or in case of power supply, current limiting must be kicking in.

    who ever designed this piece of ....... art, had no idea about circuit design.

    my condolences...