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    Super Moderator BATcher's Avatar
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    USB 'external' hard drives and DVD writers often do not have a normal power supply, but derive their power from one (and often two) USB ports. However, the amount of current drawn is almost never quoted (possibly because it exceeds the 500mA at 5V per USB socket).

    Does anyone know of a simple way of measuring the total current drawn by these devices? I really don't want to bodge together a USB connector and stick an Ammeter in series...

    Thanks!
    BATcher

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    Quote Originally Posted by BATcher View Post
    USB 'external' hard drives and DVD writers often do not have a normal power supply, but derive their power from one (and often two) USB ports. However, the amount of current drawn is almost never quoted (possibly because it exceeds the 500mA at 5V per USB socket).

    Does anyone know of a simple way of measuring the total current drawn by these devices? I really don't want to bodge together a USB connector and stick an Ammeter in series...
    In Device Manager for each USB root hub look at Properties | Power.

    Joe

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    Super Moderator BATcher's Avatar
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    Quote Originally Posted by JoeP View Post
    In Device Manager for each USB root hub look at Properties | Power.
    Thanks, Joe - but I just don't believe the figures I see. Would a webcam (not switched on!) draw exactly 500 mA, and a variety of USB Flash Drives draw exactly 100 mA, 200 mA and 500 mA? Suspicious...
    BATcher

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    Quote Originally Posted by BATcher View Post
    Would a webcam (not switched on!) draw exactly 500 mA, and a variety of USB Flash Drives draw exactly 100 mA, 200 mA and 500 mA? Suspicious...
    That is suspicious. Looks more like rated power than actual. Sorry about that.

    Joe

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    The simple way is to bodge a USB conn... Oh, you said that!

    cheers, Paul

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    Quote Originally Posted by BATcher View Post
    USB 'external' hard drives and DVD writers often do not have a normal power supply, but derive their power from one (and often two) USB ports. However, the amount of current drawn is almost never quoted (possibly because it exceeds the 500mA at 5V per USB socket).

    Does anyone know of a simple way of measuring the total current drawn by these devices? I really don't want to bodge together a USB connector and stick an Ammeter in series...

    Thanks!
    BATcher,
    Hello, This may not be the best and most accurate way of doing this but...... on my "OS" i have a "APC backup Power Supply" (XS 1300) You can monitor the power usage on the front LED panel. If you have this type of system (every one should) then when you run something or plug in anything you then can observe the power usage or its changes. Then use ohms law P= IE to calculate the current. Regards Fred
    PlainFred

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    Super Moderator BATcher's Avatar
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    Quote Originally Posted by Just Plain Fred View Post
    BATcher,
    Hello, This may not be the best and most accurate way of doing this but...... on my "OS" i have a "APC backup Power Supply" (XS 1300) You can monitor the power usage on the front LED panel. If you have this type of system (every one should) then when you run something or plug in anything you then can observe the power usage or its changes. Then use ohms law P= IE to calculate the current. Regards Fred
    Ah, Fred - what I am trying to measure is not the 230V AC current drawn by a PC or server, but that of a USB device (some hundreds of mA at 5V DC)...
    BATcher

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    Quote Originally Posted by BATcher View Post
    Ah, Fred - what I am trying to measure is not the 230V AC current drawn by a PC or server, but that of a USB device (some hundreds of mA at 5V DC)...
    BATcher,
    Yes, I understand your point . As most PC's use 120V AC ( normal house power) you should be able to see a few Watts change when you plug in a device. It matters not what power supply in you PC the extra current load (mA or A) is coming from. The "Watts" shown on the LED meter will show a increase in overall power usage, then you could approximate the current mA, or a few Amps. EX: If your device was drawing the "500mA" @ 5V, you would see a Approximate 2.5 Watt increase on your power usage meter. Regards Fred
    PlainFred

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    Approximate 2.5 Watt increase on your power usage meter.
    Assuming nothing else stirs when you plug the device in, which is most unlikely. I would expect that sort of fluctuation with the PC idling, and much greater change if you start rattling the hard disk or making the video card work.

    BTW, all the best computers run on 230v. ;-))

    cheers, Paul

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    Quote Originally Posted by P T View Post
    Assuming nothing else stirs when you plug the device in, which is most unlikely. I would expect that sort of fluctuation with the PC idling, and much greater change if you start rattling the hard disk or making the video card work.

    BTW, all the best computers run on 230v. ;-))

    cheers, Paul
    Paul,
    You are correct ,i have tried this on my "PC" with a "Pocket Media Drive " and the background fluctuation obscures the insertion of the drive. Regards Fred
    PlainFred

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    Super Moderator BATcher's Avatar
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    The only device I can find is one from Hong Kong, the USB Port Voltage and Current Tester, but I can't find it being sold anywhere in the UK...
    BATcher

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    BAt-
    You did not get much good info.
    there are several ways you can make a measurement but not very accurate. since you do not want to open any wires for meter insertions, then you need an exteranl dc amp clamp on meter. They make then, just go to your car battery man and watch him stick a clamp on meter to read the dc amps. If you can seperate the wires feeding the device a compass will detect the magnetic field and compare against a know standard. You get the idea I hope.
    By the way, go ask some active radio ham, they should know.

    regards,
    Darrold w6zev

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    IIRC exact power data isn't exposed to the OS, and the USB controller handles over-current shutdowns autonomously. Although you don't want to do it, I think the best way is to get a USB extender cable and a cheap DMM that does DC mA, and make one. $10 tops if you shop well at eBay. Clamp-on meters are great, but they're not cheap.

    I've done the thing with an AC power meter and it works if you're careful, but it's not very accurate. I did it on a test system that wasn't online and didn't get interrupted often.

    -John O

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    Measuring USB device current and voltage

    You can use a multimeter and a hacked up cable to measure actual USB current but its a bit messy.
    I use this meter for a neater solution ...

    http://www.smartronixstore.com/index...&product_ID=53

    I dont have to hack up cables or use long meter leads.

  15. #15
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    First post, link to a page selling stuff. Hmm....
    We don't mind ads but please be honest about it, especially at over $40.

    cheers, Paul

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