H-Bridge - removing resistor to base of MOSFET decreases power to motor

Discussion in 'The Projects Forum' started by Don23, Feb 14, 2012.

  1. Don23

    Thread Starter New Member

    Feb 22, 2011
    I've built an H-bridge circuit on a breadboard that powers a Traxxas EZstart motor. The motor runs optimally at 7.2 volts. I'm running it at 6v. It's based on the attached schematic. I'm using TIP122, TIP127 mosfets and 2N3904 transistors instead of what the schematic shows, but it all works fine.

    I'm puzzled by one thing. I noticed that if I replace the resistors leading to the base of the PNP mosfets, I actually get less power out of the motor. See the attached schematic. I don't get the logic of why increasing the current to the base of the PNP Mosfets would have this effect. Can anyone explain?
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Did you check if you still have 6V power supply voltage?
  3. mcasale

    Senior Member

    Jul 18, 2011
    So, you are running this at DC -- not PWM, correct?

    Can you post your "new" schematic? I'm guessing there are some unseen voltage drops that are preventing the MOSFETs from turning completely on.

    Also, what voltages are you driving the inputs with?
  4. russpatterson

    Senior Member

    Feb 1, 2010
    Just for the sake of accuracy, your schematic doesn't show any MOSFET's, just BJT's. Also MOSFET's don't have a base, they have a gate. The TIP122, TIP127 look like darlington pair BJT's. MOSFET's outperform BJT's for high power stuff. Here's a good article on MOSFET's http://reibot.org/2011/09/06/a-beginners-guide-to-the-mosfet/
    Last edited: Feb 14, 2012
  5. crutschow


    Mar 14, 2008
    If you changed from BJTs to MOSFETS int that circuit, then you need to add a resistor from each gate to source to insure proper turn-off of the MOSFET. The resistor value depends upon whether the bridge is just on/off or PWM.
  6. Don23

    Thread Starter New Member

    Feb 22, 2011
    You guys are all over this! Thanks for all the intel. I have to confess, I retried this test and the motor works fine with straight wires or resistors up to about 3K ohms. I'm assuming loose connections in the breadboard were the cause of my original problem.

    This is straight DC power supply. No PWM. My most recent test was with 5.7 volts from 4 D cell batteries, before being connected to the circuit. At this voltage level, I had to twist the gear of the motor to get it run, but once it ran, it ran well.

    FYI, see the attached image of my breadboard. I'm connecting a wire between the left and middle arrow, or the right and middle arrow to complete the circuit.