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Thread: Standby power pull

  1. #1
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    Standby power pull

    How much power does a computer running only an MSI MS-6368 on windows 98 take when it is on standby. I was thinking about only turning the computer to standby when I go out to eat or somewhere where I won't be gone. I'd still turn it all the way off at night and when I'm not going to drive the car for a couple hours.

    ~Lucius

  2. #2
    Constant Bitrate
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    The amount of power used in standby is mostly dependent on the power requirements of your RAM.

    The best way to determine current draw is put a meter in-line and measure it with the computer in standby.

    I have 512MB of DDR RAM and it uses around 125mA at 12v.

    Watch out for USB devices using power while in standby.

  3. #3
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    The 5VSB I believe is rated only to 500mA, however the problem is that most power supplies (yes, even commercial ones I'm afraid) use a 12V step-down linear regulator, so basically it uses the same current at 12V as it does at 5V. So if you see your 5VSB using 125mA, it's really drawing 1.5W of power. Not terribly much, but that will drain your battery after a while. Not to mention that trickle draining your battery like that will shorten the lifespan.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  4. #4
    Constant Bitrate
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    I measured current usage at 300mA on the 5v rail and then estimated the usage at the 12v input. Basically ignoring the ineffeciency of the stepdown. So in reality usage will be higher.

    The newer ATX specs boost the 5v stby spec to > 1A I believe. But it all depends on your power supply specs.

    My Opus DC-DC powersupply is 1A (1.2A peak) for instance.

    On my old Sproggy, I used a 1A linear regulator for the 5v stby.

  5. #5
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    So at 300mA 5V, you're using 300mA at 12V which is 3.6W, quite high actually... But if you got a 5W solar battery charger and put it in your rear it'd probably allow you to run it indefinitely like that. Additionally if you made yourself a switched 5VSB instead of linear, you can bring it down to 1.5W which can easily be covered by a small solar charger.

    The opus is most likely a linear regulator for the 5VSB as well.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  6. #6
    Constant Bitrate
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    No, In either case, it's still 1.5W used, so
    300mA @ 5v = 1.5W is the same as 125mA @ 12v = 1.5W

    Again, this ignores the linear conversion loss. so the 12v measurement might actually be 40% higher with a linear.

    In fact, I think I did actually measure my Opus, and it was drawing something like 150-165mA in standby. So the Opus must have an approx 30% conversion loss.

  7. #7
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    My calculation is correct for most PSUs that use a linear regulator. I haven't actually seen any PSUs at all that use a switching regulator for the +5VSB, and if they do then it'll rate a lot more than 1A.

    So just to clarify, you measured the +5VSB output on your Opus and got 300mA, and on the exact same power supply in the exact same scenerio you measured at the +12V rail and got 150mA?
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  8. #8
    Constant Bitrate
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    Originally posted by Telek
    My calculation is correct for most PSUs that use a linear regulator. I haven't actually seen any PSUs at all that use a switching regulator for the +5VSB, and if they do then it'll rate a lot more than 1A.

    So just to clarify, you measured the +5VSB output on your Opus and got 300mA, and on the exact same power supply in the exact same scenerio you measured at the +12V rail and got 150mA?
    Yes, measured at both places using same config. It sounds like you are applying the 40% extra power requirements in your calculation to the 5v side, rather than the 12v side.

  9. #9
    Constant Bitrate
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    or actually maybe you are multiplying an extra factor of 2.4?
    2.4x125=300

    Here is how I calculated:

    P=I*E
    .300*5=1.5W

    to find I at 12v:
    1.5=I*12 or I=1.5/12 or I=.125 or I=125mA

    Now figure in an extra 40%
    125mA*1.4=175mA

  10. #10
    Constant Bitrate
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    You can also use the conversion factor 2.4

    12/5=2.4

    then convert between the two easily

    From 5v to 12v:
    300mA / 2.4 = 125mA

    or from 12v to 5v:
    125mA * 2.4 = 300mA

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