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H-Bridge troubles (driving a stepper motor)

  1. Feb 22, 2012 #1
    I'm trying to drive a stepper motor with an h-bridge. The motor has four input leads, so uses two h-bridges. I need to be able to reverse the current through the coils (thats why I'm using the h-bridges), and control the mosfets on either side of the h-bridge with one 5V output pulse from an Arduino.

    My first attempt was to put P-Channel mosfets on the top half, N-Channel mosfets on the bottom half, and have one signal feed the gate of the correct N-Channel and go through an inverter and feed the corresponding P-Channel. That didn't work, because (for some reason unknown to me) the Hex Inverter IC's LOW was about 1V, and didnt put the P-Channel into conduction.

    Then I tried the circuit that I have attached, to actively drive the P-Channel gate low. The output (either side of the coil) looks fine from an oscilloscope when the motor isn't connected, but when it IS, the voltage at these points dies out.

    I've seen a half dozen H-bridge circuits, and tried most of them (the simpler ones), but none of them have worked. I think that the problem may be that the P-Channel drain voltages are about equal to the gate voltage...but the gate threshold voltage is 2V, so it doesn't turn on.

    I've been working on this thing non-stop for weeks, asked a handful of EE students and teachers, and no one knows....any help or suggestions would make you a celebrity around here.

    (P.S. The "resistance" of the coil is 12 ohms.)


    Attached Files:

  2. jcsd
  3. Feb 23, 2012 #2
    A few observations:

    1. The high side P-channel fets in your schematic are upside down. The source pins should be connected to VCC (12V) and drain is connected to the motor.

    2. Your schematic depicts N-channel fets on the high side. But since you indicated you’re using P-channel on the high side, be sure your schematic reflects this. Using high side N-channel fets is a common approach, particularly in higher power motor controls, but this would require different high side gate drive circuitry.

    3. The Hex inverter would not have worked as the high side fet driver. When the hex inverter is logic low (0V), then Vgs = -12V and the high side fet is on. When the hex inverter is logic high (5V), then Vgs = -7V and the high side fet is still on.

    Once you turn those P-channel high side fets around, the circuit as you’ve drawn it should work fine.
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