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In Rush Current

Discussion in 'Electronic Basics' started by Richard, Apr 20, 2005.

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  1. Richard

    Richard Guest

    I posted about a SBC P4 that randomly failed to boot (Power up) and want
    some ideas how to get accurate current measurements. You kind folks helped
    out by explaining the Shunt deal.

    Since then I have taken numerous measurements and I want to share my setup
    in case someone has so ideas.

    First, the spec of the SBC is Max. 4.5 amps on Start up, Normal 3.2 amps.
    I am using a 5 vdc DC COnverter located 14" away using 18 ga Instrument wire
    14" long to carry the power. I "thought" is was large enough, but now I
    have second thoughts.

    When I first power up the DC Converter, I see a small spike which is right
    at 10 amps, and it only last for a few ms. Then the power drops to about
    1.5 amps. When the computer beeps, I jump up to 3.5 amps then to 7.5 amps,
    then immediately back to 3.5, then it slowly settles down to around 2 amps.
    Now I only have 10% Processor Utilization at any given time.

    I take it, the 10 amp jump is the initial in rush current? then the 7.5
    amps is a very breif spike from the SBC kinking off?

    Does this mean my 18 ga is border line and I would really need 16 ga for the
    power distribution wires? I did note that with the 18 ga wire there is 90
    millivolt drop from the DC Converter terminals to the SBC Power Terminals so
    I think that is a good indication I need to step up a size.

    Any thoughts,
  2. I suspect your current spike corresponds to some
    extra activity. Is the internal clock speed varying,
    as uP's intended for mobile use often do?
    Yes. You are charging the bulk bypass caps with it.
    Hard to say (even if I wished to guess what "kinking off"
    is, but I don't want to know ;-).
    That's a little much for a 14 inch wire run. At those
    currents, heating is not really an issue, but drop may
    be depending on your supply tolerance and how the
    SBC is specified. 16 guage should be plenty big.
    Is there a reason to go for the smaller wire?
  3. Richard

    Richard Guest

    Sorry, typo, I meant that was the CPU kicking off or Starting up. The SBC
    has it's on DC Converter inside so it could be it starting up.
    No, we just had a 100 premade harness kits made because I looked at the
    original motherboard specs and then looked up the 18 ga. and it seemed to be
    sized ok, and our other harnesses were 18 ga.

    I made up a 16 ga harness to see if it helps. Besides the fact that the
    voltage drop will be much less, will I see differences with the inrush
    currents as well? I think the wire being too small and the high inrush is
    what keeps the computer from booting sometimes.

  4. If the spec on your supply output voltage is enough tighter
    than the spec on your SBC supply requirement so that
    the 90 mV drop is not a problem, then I would use those
    100 harnesses. And have more made up with bigger wire.
    The inrush current is most likely limited by the supply
    rather than the wire. I would expect no real change.
    That seems unlikely to me. Your SBC probably has
    a spec on how fast the supply should come up. Be
    sure you meet it. Also, be sure there is no overshoot
    just after that initial current spike driving the supply
    voltage above the SBC voltage spec.

    You might want to look carefully at the supply voltage
    waveform as it comes up. Maybe the DC-DC converter
    is initially putting the supply into current limit and causing
    a wobbly turn-on waveform rather than what the SBC
    designers assumed (and should have specified). You
    might try a higher current limit supply, experimentally,
    to see if that reduces the misbooting incidence.
  5. Lord Garth

    Lord Garth Guest

    It occurs to me that the modern ATX power supply has extra power supply
    lines. These
    are intended as a parallel path to supply the CPU with the extra power it
    needs. You
    may have noted the (usually) two odd power connectors coming from the power

    Are you using these?
  6. Richard

    Richard Guest

    No Sir,

    This is an Industrial, Embedded Motherboard running a Via 667mhz on a single
    5 vdc supply. We are using a Datel 18-36vdc input to 5 vdc output rated at
    5.0 amps. It's 14" away from the SBC.

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