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How can I boost the power of my junk box H-bridges?

Discussion in 'Electronic Basics' started by Owen Lawrence, Sep 4, 2003.

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  1. Hi. I want to drive a bipolar stepper motor marked 1.4V 3.8A. I
    have built a PIC16F628 based circuit to drive two H-bridges made up
    of 2N3904/06 transistors. Clearly they will not supply enough
    power. The PCB is already made, so I won't be changing the
    transistors on it.

    What are my options for boosting the 5V low current output of the
    H-bridges? (There are 1K resistors to the bases of all the
    transistors. If I lower them, how low can I go before I start to
    cause problems with the PIC?) I'm thinking of rummaging around for
    four relays, and using one diode per coil to choose which coil gets
    energized, based on the current direction. I'm worried about speed
    (1.8 degrees per step!), and burning out the relay contacts, but I'd
    rather get results at all.

    Would I be able to use the same diode trick with power
    transistors? If so, which ones would you recommend? Note: So far I
    spent only $1, which was for the super glossy paper I used for the
    toner transfer for etching the PCB. Everything else came from my
    junkbox, so I'm loathe to spend any (significant) coin. It's a
    matter of pride, you understand. :)

    Assuming I zipped the right files, you can download the Eagle
    project from

    This circuit can drive small toy motors, but not this big stepper
    motor (it just "clicks" but doesn't move).

    Thanks for any advice.

    - Owen -
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