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  1. #1
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    Short circuit on this board somewhere- I am at a complete loss!

    This is a simple H bridge implemented with TIP 122/127.

    The base resistors (560R) terminate in a screw terminal block and the conjoined emitters also both terminate in a screw terminal block through the 4.7R resistor.

    With only the power connector (on the right side) connected to 12V, but nothing else connected, it shorts my power supply.

    1) I have checked for bad transistors and they are all good.
    2) I have checked for solder dags.
    3) I have sanded the heat sink around the holes to make sure there are no Al dags connecting to the back of the transistors through the holes.
    3a) I have used those thermally conductive heat pad things behind the transistors along with the plastic eyelet things for the the bolts.
    4) I have used my continuity checkers to look for connections between the back of the transistors and the heat sink - none.
    5) I have checked for a bad cap, but it is good.
    6) Can't find any soldering mistakes.

    If I put my continuity across the power connection header then I get a brief beep but then it stops and I hear nothing else.

    What am I missing here? What have I not considered?

    Click image for larger version. 

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    Click image for larger version. 

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    Last edited by boylesg; 21st April 2017 at 15:26.

    •   Alt21st April 2017, 15:16

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  2. #2
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    Re: Short circuit on this board somewhere- I am at a complete loss!

    If I put my continuity across the power connection header then I get a brief beep but then it stops and I hear nothing else.
    Errrmmmm..... the capacitor charging up perhaps!

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



    •   Alt21st April 2017, 15:59

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  3. #3
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    Re: Short circuit on this board somewhere- I am at a complete loss!

    Hi,

    your schematic shows TIP128... it should be TIP122...

    Can we see a detailed picture of the backside. and a more detailed for the comonent side?

    ***
    Test:
    When your power supply is current limited to 100mA.. what voltage do you see? - measured across the supply rails

    Klaus



    •   Alt21st April 2017, 16:16

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    Re: Short circuit on this board somewhere- I am at a complete loss!

    But I actually removed that cap from the circuit board and re-tested it but it still shorted the power supply. Any other ideas?

    I see I should be doing my h bridge something like this anyway:


    If I wanted to power my current design from 20V then the load would never see more than the 12V input to the bases anyway......emitter follower.......

    - - - Updated - - -

    Sorry Tip122 yes. 12V DC plug pack thing - it just shuts down due to a safety measure being triggered and it stays that way until I disconnect my board.

    20V laptop plug pack - the voltage seems to oscillate around 200mV to 2V or something like that also due to a shorter lived safety measure being triggered I presume.

    - - - Updated - - -

    How's this?

    If you roll the top edge of the board down and towards you in this photo then the front of the TIPs will be facing you and I have the TIPs in this order: TIP122...TIP127...TIP122...TIP127.
    Click image for larger version. 

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    Last edited by boylesg; 21st April 2017 at 17:02.



  5. #5
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    Re: Short circuit on this board somewhere- I am at a complete loss!

    Hi,

    really hard to verify.
    Please check for an accidental short at the power connector.:
    Click image for larger version. 

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    but yes, there are complete H bridges in one IC. This is easy to use and there is less voltage drop and less heating.
    You could do with a much smaller heatsink.

    Klaus



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