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Transient Noise causing MAX7221 7-segment Display to Blank Out

Discussion in 'General Electronics Discussion' started by SignalFlow, May 29, 2015.

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  1. SignalFlow


    May 29, 2015
    I designed an industrial control board with a PIC24F microcontroller. It communicates with various chips via SPI. It has some digital inputs that are optoisolated now. These inputs are subject to noise generated from contactor coils turning ON/OFF (that's why I optoisolated them).

    My board has a 7-segment display that is driven by a MAX7221 IC (via SPI). Before I optoisolated in the digital inputs (that are just reading switches), the 7-segment display was blanking out on me (going into shutdown mode). After I optoisolated I don't have this problem anymore...until I connect the board standoffs to chassis ground (earth ground). If I connect the board standoffs to chassis ground (earth) and turn a contactor on/off, then the display blanks out.

    It is strange because the board standoffs are not touching any trace or board ground on the PCB. The board ground polygon pour has cutouts in it for the standoffs so the standoffs aren't electrically connected to the board ground.

    The only thing I can think of is that the transient noise is capacitively coupling from the earth ground to the board ground due to the proximity of the standoffs to the board ground. This could be causing the ground to bounce and therefore losing power to the MAX7221 momentarily which would put it into shutdown mode.

    Any ideas out there on what could be going on? I'm thinking of trying an AS1116 7-segment driver instead of the MAX7221 since I have read a lot of other people having similar problems (EMI related issues) with this chip.

    I know if the chip loses power, then it goes into shutdown mode and has to be re-initialized. So I'm not sure if it is losing power. I have the recommended caps nearby (even a 10uF cap that I thought would act as a tank cap in case of power blips) but the caps aren't working.
  2. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    You must always supply a circuit diagram and PCB layout if you stand any chance of us answering your question.
  3. Tha fios agaibh

    Tha fios agaibh

    Aug 11, 2014
    Couple of things I'd try/check;
    Try plastic standoffs instead of steel.
    Try shielding the bottom of pcb with mylar, pvc or similar insulating layer.
    Check that wiring to your board is properly shielded with drain wire grounded at source end only.
    If any signal wiring is running near ac line voltage wiring, make sure it is not running parallel to them. Ideally, isolated or crossing at 90degrees. Use twisted pairs for dc wiring.
    Suppression is added to inductive loads (contactors, relays) such as diodes or movs.
    Arouse1973 likes this.
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