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DPRG: Re: HBridge Question

Subject: DPRG: Re: HBridge Question
From: Jim Brown brownjim at airmail.net
Date: Sat Aug 7 17:24:09 CDT 1999

At 01:38 PM 8/6/99 -0700, you wrote:
>I'm working on making a robot at home.  I'm using 12V motors as a drive
>system and I'm using an H-Bridge to give it enough power.  My speed control
>comes through pulsed input into part of the H-Bridge.  I sent you a pic of
>it.  Is there something wrong with the diagram?  Unfortunately it doesn't
>work.  Different transistors in the circuit heat up at different times.  I
>think I'm overloading the first string of transistors (the 2222's and the
>2907's) and thereby maxing out the base current in the second string
>transistors.  Do you have any thoughts or suggestions?
>Thanks
>Colin
>Attachment Converted: "c:\eudora\attach\HBridg.jpg"

On the schematic you sent, it looks to me like what will happen
is:



         +12                                   gnd
          |                                     |
         v                                       ^
A-/\/\--|                                         |--/\/\-C
         \          gnd             +12          /
          |          |               |          |
          |         ^  gnd       +12  v        |
          +-/\/\/--|   |           |   |--/\/\-+
                    \  v           ^  /
                     | |           | |
                     +-+--(motor)--+-+
                     | |           | |           
                    /  v           ^  \
          +-/\/\/--|   |           |   |--/\/\-+
          |         ^  +12       gnd  v        |
         /           |               |          \
C -/\/\-|           gnd             gnd          |-/\/\-A
         v                                      ^
          |                                    |
         gnd                                  gnd





When A is on, the upper left 2907 turns on giving +12 to the
3055 which shuts it off. Also, the lower right 2907 turns on
giving +12 to the 3055 which shuts it off.

When A of off, the upper left 2907 turns off and there's not
a negative polarity voltage so the 3055 is off too.

When C is on, the upper left 2222 turns off, and so is the 2955.

When C is off, the 2222 is on, and sinks to ground, which turns off
the 2955.

   A   C   Direction
   1   0   None.
   0   1   None.


Perhaps from your schematic, both "A" should be on top
and both "C" should be bottom.  Then when "A" was on, one
side goes to ground and the other side goes to 12v.  And
when "C" is on one side goes to ground and the other to 12v.

I'm not sure, but it seems you have the left side inverted 
somehow, sinking where it should be sourcing, but I'm not 
an electronics guru.  Anyway, this is my version of
what I think should work.  

    A    C   Direction
==========================
    1    0   Forward
    0    1   Reverse
    1    1   fuse blow
    0    0   fuse blow



         +12                                   +12
          |                                     |
         v                                       v
A-/\/\--|                                         |--/\/\-C
         \          +12             +12          /
          |          |               |          |
          |         v  +12       +12  v        |
          +-/\/\/--|   |           |   |--/\/\-+
                    \  ^           ^  /
                     | |           | |
                     +-+--(motor)--+-+
                     | |           | |           
                    /  ^           ^  \
          +-/\/\/--|   |           |   |--/\/\-+
          |         v  gnd       gnd  v        |
         /           |               |          \
A -/\/\-|           gnd             gnd          |-/\/\-C
         v                                      v
          |                                    |
         gnd                                  gnd


I don't really like having A and C like above since if you 
get them swapped, then it'll fry because it'll make a 
direct short.  Also, there's no Good way to make it stop.  
It's better to use four I/O lines then you can turn off
the upper lines for stopping or for PWM.


   A   B   C   D   Direction
- -----------------------------
   1   0   1   0   Forward
   0   1   0   1   Reverse
   1   0   1   1   None  (PWM)
   0   1   1   1   None  (PWM)
   1   1   1   1   Break (actually halts the motor)
   

         +12                                   +12
          |                                     |
         v                                       v
A-/\/\--|                                         |--/\/\-B
         \          +12             +12          /
          |          |               |          |
          |         v  +12       +12  v        |
          +-/\/\/--|   |           |   |--/\/\-+
                    \  ^           ^  /
                     | |           | |
                     +-+--(motor)--+-+
                     | |           | |           
                    /  ^           ^  \
          +-/\/\/--|   |           |   |--/\/\-+
          |         v  gnd       gnd  v        |
         /           |               |          \
C -/\/\-|           gnd             gnd          |-/\/\-D
         v                                      v
          |                                    |
         gnd                                  gnd


Anyway, unless you just need that kind of power, you could
do well with a prepackaged H-Bridge chip like the L298
or the LMD18200.  The L298 can handle motors the size
of a coke can or smaller about 1-2 amps and has two h-bridges
built in.  The LMD18200 has only one h-bridge but can
handle 2-3 amps.


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