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555 RFI problem: reprise

Discussion in 'Electronic Basics' started by Jordan, Jan 31, 2006.

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  1. Jordan

    Jordan Guest

    Some weeks ago I asked for help with a car blinker circuit, that was
    prone to being affected by the (magneto) ignition system.

    I think I've found the cause - the 555 IC itself.

    I've only checked it on a bench so far, using a spark generator to
    simulate the unsuppressed high tension ignition spark.
    After breadboarding the circuit for the umpteenth time, I used a
    different brand of 555 - a National Semiconductor. I'd previously used
    KIA brand, thinking they'd all behave the same. Not the case - the NS
    555 doesn't behave erratically with the spark nearby, whereas the KIA
    555 always does.
    No filtering was used either, just a cap on pin 5.

    Lesson learnt!
    Here's hoping it'll work OK on the vehicle now.

    Many thanks to all who suggested fixes.

  2. Guest

    "Cause" is a little strong here. It's a mix of the brand of 555, the
    circuit you designed, the construction technique, and the environment.
    Some of those are under your control, others aren't!
    Presumably the EMI is being conducted in through either ground, power,
    or load lines.

    Or possibly more than one of the above. (Although common-mode EMI for
    such a simple circuit may not be a big deal.)

    The cap on pin 5 is very important in any but the most gentle
    environments. Bypassing and maybe regulation on the power will help too
    (while not really keeping out any sort of truly harsh RFI).

    Possibly RF chokes or ferrite beads (I'd lean towards the RF chokes at
    first) would help too.

    Certainly not all 555's are equal. Just as not all 7805's etc. In the
    perfect world we'd engineer our designs so that it didn't matter which
    brand the purchasing department bought this week :).

  3. Jamie

    Jamie Guest

    are you sure you wasn't working with a CMOS version?
  4. Jordan

    Jordan Guest

  5. Jordan

    Jordan Guest

    No - it's RFI, or other "wireless" effect. It happens with no wire
    connection, with separate power supply etc.

    I had several suggestions, and tried most of them. Only changing the
    brand of 555 worked.

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