Inexplicable fault. Any tips?


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  1. #1
    Fanatical Contributor Tim's Avatar
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    Default Inexplicable fault. Any tips?

    Hi

    I have a board that has a DRV8871 (h bridge chip) on board. I have used them before. I have them on a board from a previous batch (same pcb just soldered at another time)
    I have use them on a previous board

    They are dead simple
    Power + Ground
    Signal on each input to decide direction
    Output changes depending on input

    Today only one of the outputs worked, 3 new boards and all the same. Fitted a new chip, still the same fault. Buzzed it out all ok on connections. Wrote really simple code to just drive one line high, then the other keeping the opposite line low when the other high. Still the same

    I dug out a previous board that works, swapped the chip for the one pulled of the non working and it works. Put the chip from the previous board on a new one. Does not work.
    Lifted the output legs so cannot be pulled low by board (there was no continuity before) still the same.

    I have wasted 6+ hours on this and no futher.

    Any ideas what to do next?

    Thanks

    Tim
    Tim

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  3. #2
    Prolific Poster charliecoultas's Avatar
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    Default Re: Inexplicable fault. Any tips?

    A piece of insulated cable that has a break in it?

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  5. #3
    Fanatical Contributor Tim's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Thanks

    No cables used and I'm testing with a scope directly on the board.
    Tim

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    Prolific Poster normnet's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Perhaps this has been tried but desolder the H bridge chip and check continuity on all relevant traces.
    Test continuity with a light bulb so as to include a load for amps?
    Add leads to the chip from known good sources and move each to the board one at a time.

    Norm

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    Senior Member GAMBOA's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Tim,
    I see the following possible failures:
    1.- Welding failure. Check that all pins are well welded.
    2.- Microcontroller failure. Check signals IN1 and IN2. Test these signals directly wired without the microcontroller.
    3.- Check Rilimit. Check the resistance value and the welding of your pins. Try with another value.
    4.- Check the suitability of the VM voltage. Check the capacitors or even remove them to make a test.
    5.- Check the motor if it is ok, and even put a resistive load with an LED to see the behavior.
    6.- Check that the integrated circuit has been purchased in an authorized channel and is not a counterfeit piece

    Regards,

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    Fanatical Contributor Tim's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Hi Norm and Gamboa

    I have pulled the chip and checked the traces all are good.
    As I said I swapped the chips between good and bad boards, Chip works on the good not the bad
    3 new boards worked before
    Chips are from digikey, the same batch worked before not now
    Rlimit is the same value as good board, odd thing is one channel works other does not

    A comment
    On the scope you can see Out2 working fine 12v is there, Out1 does not. But on the scope you see a tiny spike as it tries to turn on. This made me think it was a grounding issue on track so lifted leg on chip to free air. Still the same

    I'm going to try next Norms last suggestion of attaching wires and see what happens as I attach each to board.

    Thanks
    Tim

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    Prolific Poster See_Mos's Avatar
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    Default Re: Inexplicable fault. Any tips?

    My first guess would be ground bounce. try beefing up the ground track with some fine wire. Could also be a layout problem, the PIC ground should go direct to the decoupling 'lytic and not via the H-bridge common. The ground track to the current sense resistor need to be thick enough not to add to the current sense voltage.
    Last edited by See_Mos; 4th September 2018 at 11:25.
    My RAM is failing

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    Fanatical Contributor Tim's Avatar
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    Default Re: Inexplicable fault. Any tips?

    I think now it's a faulty batch. I took a proto board with pads to suit the chip. Wired up the device and tested it using the 12v 0v and 5v signal lines from my normal board and the result was the same. One line worked the other did not.

    My problem is nobody carries it in the UK I had to go to Digikey, Mouser has them as well.

    Fing pain as I wanted to ship in couple of days
    Tim

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    Henk57
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    Default Re: Inexplicable fault. Any tips?

    "An assortment of protection features prevent the device from being damaged if a system fault occurs."
    Important line on page 7.

    Most Driver's must have a load (eq. Stereo set's without loudspeakers connected!).

    So try connect a "load" to the output, a 12V bulb or a (toy)motor, only then you can say it is defect, or not.

    Henk

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    Prolific Poster charliecoultas's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Some of today's music sounds better "without loudspeakers connected!".

    Charlie

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    Fanatical Contributor Tim's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Henk

    There was load to start with and I have tried many times with load without load.

    I purchased 20 direct from TI then I found Farnell sell an equivalent. DRV8870DDAR I cannot actually see any difference! So I will have 50+ when I only wanted 15
    Tim

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    Junior Member Thoranaga's Avatar
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    Default Re: Inexplicable fault. Any tips?

    I would check the board for metal debris , solder bridges , flux etc. and clean it thorowly with isopropyl alcohol .

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    Fanatical Contributor Tim's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Hello Thoranaga

    I have 4 boards all the same.

    New parts are arriving today so will see what happens.
    Tim

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    Fanatical Contributor Tim's Avatar
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    Default Re: Inexplicable fault. Any tips?

    Well it looks like I'm doing something wrong. I swapped the part for a DRV8870DDAR. I modified the Irlim etc to suit and exactly the same. Only Out2 is being driven.

    If I had any hair left I would be pulling it all out

    I have a dev board coming and will try that on its own to see what happens then slowly integrate it in until it does the same.
    Tim

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  29. #15
    Prolific Poster See_Mos's Avatar
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    Default Re: Inexplicable fault. Any tips?

    I just read the data sheet. The typical low logic level has to be less than 0.5 volts so that might be worth checking.

    Probably a silly question, did you 'scope the input pins to be sure the logic is correct

    What voltage do you have on the V.Ref pin and what is your current sense value?

    I assume you are using PWM to control the speed so it might be worth trying hard driving to see if that gives any clues.
    Last edited by See_Mos; 5th September 2018 at 15:20.
    My RAM is failing

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