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Low Voltage Suppression

Discussion in 'Electronic Design' started by Matt, May 25, 2007.

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  1. Matt

    Matt Guest


    I have a bit of an odd problem with voltage suppression. I am
    trying to get rid of a small (700mVpp on a 12VDC line) spike coming
    from a DC fan that can possibly feed back into sensitive circuits. I
    have designed with circuits that needed protection from kV+ spikes but
    nothing this small. Do they make TVSs with a very tight control
    (+-100mV) around the breakdown voltage or is there another component I
    should be looking at?

  2. Eeyore

    Eeyore Guest

    You need to filter the wire going to the fan.

  3. Matt

    Matt Guest


    Thanks for the reply. Will a simple LC low pass work on such a
    small spike at such a low frequency (~110Hz) without the components
    getting large?

    Thanks again
  4. Eeyore

    Eeyore Guest

    I take it you mean the repetition frequency is 110Hz ?

    The 'frequency' you need to filter is that of the spike. How wide is in in
    microseconds ?

  5. Phil Hobbs

    Phil Hobbs Guest

    Alternatively, run the sensitive stuff off a capacitance multiplier.


    Phil Hobbs
  6. MooseFET

    MooseFET Guest

    Can you afford to lose some of the 12V? A source follower made from a
    power MOSFET can make physically small parts filter quite low
    frequencies. A 10V regulator would also work.

    Also, try putting a capacitor right on or near the fans electrical
    connections. You want to provide a low impedance path for the spikes
    that doesn't go near the small signal paths.
  7. Eeyore

    Eeyore Guest

    Agreed. The 'filter cap' can easily be quite a large value e.g. 100uF/16V
    electrolytic ( high frequency low ESR type).

    If the filter can't be installed right next to the fan itself - do also ensure
    that the fan wires are tightly twisted to reduce radiation of the pulses.

  8. Don't supress the noise, slow it down with enough capacitance and then
    regulate it out by running the sensitive circuitry off of a lower
    voltage (say 8VDC).

    Best regards,
    Spehro Pefhany
  9. D from BC

    D from BC Guest

    IIRC..ittty biity spikes are not controlled by transient suppressors.
    An ideal zener might..but doesn't exist.

    Heres some ideas...
    * series regulator with high PSRR
    * shunt regulator
    * inductor
    * capacitor
    * Replace the fan with heat sinking or make a cooler design.
    * Make the electronics more noise immune <<some poor designs are just
    noise magnets
    * Improve the PCB layout

    D from BC
  10. Rich Grise

    Rich Grise Guest

    I've even seen a .1 or so ceramic actually crimped into the fan lugs with
    its power wires. That, and maybe a couple of ferrite beads, should make an
    observable difference.

  11. Eeyore

    Eeyore Guest

    Ferrite beads !

    Yes. you can slip them over the wires too. Excellent idea.

  12. Jasen

    Jasen Guest

    try this:

    12V ------>|---+------ to fan

  13. Eeyore

    Eeyore Guest

    In addition, place the cap as close as possible to the fan and twist the fan

  14. Fred Bloggs

    Fred Bloggs Guest

    Anyone who would ask a question like that has no business posting to SED
    or even touching anything electronic...
  15. D from BC

    D from BC Guest's a lame Q but how about this for the hot shot designers..

    Can the ideal zener be created?
    And would it be useful?

    I propose this recipe:
    1 super fast op amp #????
    1 super fast transistor

    It's basically just a super fast version of a TL431.

    World's fastest transistor: (old article)

    1Ghz Op Amp (at Av=1)

    If everything is fast could clip slow spikes.
    A programmable TVS circuit!
    D from BC
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