Help with H-bridge Design

Discussion in 'The Projects Forum' started by MEnotsogoodwithEE, Apr 1, 2009.

  1. MEnotsogoodwithEE

    Thread Starter New Member

    Apr 1, 2009
    2
    0
    I am currently an ME student working on a project and I need some EE advice for my electronics.

    I will be using two motors at 12VDC that draw 20A max (at stall) and would like for both of them to be bi-directional. I would like to use a programmable (USB interface) motor controller that I have, but it is only rated for 1.2A max current. It's user manual can be found at http://www.pc-control.co.uk/images/Motor-Bee%20Manual.pdf and it's IC's spec sheet at http://www.pc-control.co.uk/images/L293D%20motor%20driver.pdf.

    I know that I will have to use the motor controller to control a transistor H-bridge circuit that will handle the higher current. The controller I am using outputs a PWM (pulse width modulated) signal and I do not know the frequency of the signal.

    I am trying to figure out what transistors to use in the H-bridge. Using BJTs would probably be simplest, but I've heard there are advantages to using MOSFETs instead. However, I've also heard that MOSFETs have trouble handling PWM signals at some frequencies (and since I don't know the frequency of the signal from the controller, this may be a bad idea).

    Can anyone give me some advice about any of this?

    Thanks in advance for any replies, I really appreciate it.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Since the LD293 won't switch faster than 5kHz, MOSFETs will work fine. High power BJTs will also work fine.
     
  3. MEnotsogoodwithEE

    Thread Starter New Member

    Apr 1, 2009
    2
    0
    I am going to go with some BJT's for simplicity.

    Here are my schematics...let me know how they look:
    http://img5.imageshack.us/img5/8713/schematic2i.png
    http://img19.imageshack.us/img19/9620/schematic3.png


    I have a bunch of questions:

    1. The OP1,2,3, and 4 stand for the outputs of the motor controller. What voltage should I run for those outputs?

    2. The transistors I plan to use are:
    http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMuDHCWHhuFg%2bdgDlTRSjcHDH5Ubcn35Cdw=
    and
    http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMuDHCWHhuFg%2bdgDlTRSjcHDOZe2zzJ4e4U=
    Will these work for my application?

    3. I am wondering how to determine what the values of the protection diodes, the capacitor, and the base resistors should be?


    Thank you for your help in advance, and I look forward to your replies!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your PNP transistors are not connected correctly; the emitters need to be connected to the more positive side, collectors towards the middle of the bridge.

    1) You'll need to get within 0.5v of the PNP emitter's voltage to turn the PNP's off, and within 0.5v of the NPN emitter's voltage to turn them off.

    2) If you can supply around 3A to 4A to the base, they might. However, your driver isn't capable of that. You will need big heat sinks, and you will lose several volts when the transistors are saturated.

    3) If you were using MOSFETs, you wouldn't have to worry about the protection diodes, because those are built-in to the MOSFETs themselves. But anyway, use protection diodes that are rated for 25A or more. The cap can be pretty small.
     
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