use of photocoupler in this H-bridge design....

Discussion in 'The Projects Forum' started by jaydag77, Jun 3, 2007.

  1. jaydag77

    Thread Starter Member

    May 28, 2007
    Hi again folks, been busy farting away on this project for last day or so- managed to get uC mainboard done yesterday and this morning am looking to put the main h-bridge together. If anyone here has seen my other two posts, the first has details regarding the design, but I'll quickly go back over them here:

    The main (rear motor) h-bridge design is typical for the most part (please check out attach and focus on left side H only). The H bridge itself was basically copied from the radioshack PCB which occupied the platform I am targeting. I have seen varieties of this design all over the internet in the past,, the part which is starting to bother me right now is the small transistor that switches a pair of larger opposing transistors. The original uses SS9014, mine the 2N3904, both of fairly similar values to switch the large pairs- in my particular implementation I would like to isolate the uC from the H(s) so want to use some photocouplers I have salvaged from old modems (PC817). I've never tried using them before,.have checked out the sheet and some other projects which used them but in my case still am unsure about implementation due to the small transistors in place already- in this case would I be right to cut out the small signal transistor(s) and replace with photocoupler transistor(s) respectively?
    How about connections driving from uC? low or high side, I thought low? Would be easier to layout also based on my existing design/layout scribbles.

    Any tips with this would be great, have other portions to work on as well but this is what I wanted to build today, thanks in advance folks! jd
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005

    When printing that large of a schematic, it might be wise to use the landscape. Using landscape will allow you to open up the diagram without it being so bunched up.

    Also, on all the TO_UC ... it would be better to label them as as FM_UC [and pin number] for signals originating at the UC and TO_UC [and pin number] for signals going to the UC.
  3. jaydag77

    Thread Starter Member

    May 28, 2007
    apologies for the crappy schematic, it was drawn up quickly and with little regard for anyone else's viewing but my own. Usually I would take the time to label everything properly etc but I think at that time I was unsure about which IC to go with, I ended up picking the pic16f886.

    I have actually been putting better schematics together this afternoon of the overall system in blocks, everything nice and spaced out, most things labeled, etc. I will post when finished.

    Any tips regarding the photocoupler connections? regards, jd
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    Why is isolation required?:confused:
  5. jaydag77

    Thread Starter Member

    May 28, 2007
    That is a good question! As I stated earlier, I have never used them before,,with that being said I decided to try them because it sounds like a good preventative measure to help protect the microcontroller from spikes that may be caused from the H's switching, also help prevent noise on those signal lines? I also thought they might make a better replacement than the driving small signal transistors that were in their place? I have just finished breadboarding the thing up and am about to test the theory out in a few minutes.>> I have been thinking bit more about that question and went back to find other designs which use similar approach, I managed to find one very close to this design I'm trying at

    the attached H is also very similar but for the poles and parts.

    If there is zero need for my isolating the signal lines then please lemme know! One other thing that I am concerned with is causing forbidden switch combos!! Apparently this H has only 1 combo which can cause smoke (11) Any tips for making a more robust design would be greatly welcomed at this point! regards, jd..,.