Plz help me calculate USB power needed

Discussion in 'Mac Accessories' started by n8236, Nov 11, 2008.

  1. n8236 macrumors 65816

    Joined:
    Mar 1, 2006
    #1
    I got this 4 port usb hub, but it's unable to power all my devices connected to it. It currently has a printer, ext 3.5" hd, int. 2.5" hd and a Logitech MX Revolution mouse adapter on it. The only thing that works properly is the 3.5" ext. I can't use the mouse adapter to its full functions, can't print in normal speed or access the 2.5" int hd.

    I know this is due to the lack of available power on the hub. The hub does have a hole for power, but I don't know which power adapter I should be buying for it (in terms of voltage and amperage). What I do know is, a USB port has 500 mA of power on tap while the USB hub itself requires 100 mA leaving 400 mA.

    I just don't know which power adapter to get. :confused:
     
  2. blodwyn macrumors 65816

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    Portland, Oregon
    #2
    You might find it's cheaper and easier to buy another hub which comes with a power supply.

    If you don't want to do this, then a general purpose 500mA PSU with a variety of tips will probably do the trick. The hub should have the polarity and voltage requirements marked on it, and you could take it to the store with you and make sure one of the tips will fit.
     
  3. n8236 thread starter macrumors 65816

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    #3
    With my 2.5" int hd, lets assume it requires the full 500 mA from my laptop's usb port. If I'm getting only a 500mA power adapter for the hub, won't that still fall short of making my printer or mouse adapter work correctly? Will extra amperage hurt the hub?
     
  4. sickmacdoc macrumors 68020

    sickmacdoc

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    #4
    You are correct- the standard USB spec that most commonly available powered hubs use is that the hub be able to deliver 500Ma@5VDC per port. In most cases not every device hooked to the hub will draw that much, but it is usually provided for. Some of the cheaper powered hubs might not provide the full power spec to shave a few cents off the power adapter of course so it worth looking at!

    I just checked out a spare 4 port Belkin hub and confirmed that the powercube for it is rated at 2.1A@5VDC output, so that meets that-- with an extra 100Ma overspec that is probably for the led indicators. Another manufacturer's 7 port I use (can't remember the name) which would require 3.5A to meet the spec actually has a 3.8A powerblock.

    Since the power rating of the power adapter only represent the maximum current that it can supply (think of it as potential current delivery versus actual which will be lower most likely) extra amperage rated supplies will not be a problem.

    That being said with having to match the rating, tip type and tip polarity (is the + side on the outside or inside of the tip? Answer wrong and either the hub will not be powered or it will die a quick and painless death! :D) and I think you will find that the adapter will cost about as much as a replacement hub that comes with a power adapter made for it since you can get them almost everywhere (even in Walmart and I noticed some the other day at our local drug store!). Also most of the commonly available adapters I have seen available separately have only been rated for 500-800Ma in the 5VDC supply, so finding one with a 2A rating could be a little harder.
     
  5. n8236 thread starter macrumors 65816

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    Mar 1, 2006
    #5
    Thanks, that's some good info! I went by my local Radio Shack and those power adapters cost an arm (~30 bux). So I am going to take your advise and just invest in one that already provides an adapter.
     
  6. blodwyn macrumors 65816

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    Portland, Oregon
    #6
    Oops I goofed, I thought it was 500mA for the whole thing - sorry.

    Having said that, I took a look at some powered hubs I've got, and the PSUs that come with them are nowhere near 500mA per port. I have a 4 port Belkin with a 500mA PSU and a 7 port IOGear with a 1A PSU. These are steady state power ratings, so brief peak consumption during disks spinning up (for example) that go above this should still be OK. None of them get above mildly warm in use, even with a 2.5" disk plugged in, so I think in most real world scenarios a 1A PSU should be more than adequate.
     
  7. sickmacdoc macrumors 68020

    sickmacdoc

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    New Hampshire
    #7
    That is really bizarre! :confused: My 4 port Belkin is as I mentioned is 2.1A@5VDC (just looked again when I read your post as I thought I must be blind!) and in addition to the no-name 7 port I have with a 3.8A one, I just pulled out a recent 7 port stackable Belkin one I have in reserve and popped open the package and-- 3.8A@5VDC!!!!!!!!! That one even has that ps spec on the outside of the package I found out after I ripped it open to check! :eek:

    Can't quite figure that discrepancy out!
     
  8. blodwyn macrumors 65816

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    #8
    Huh! Now you've got me thinking - I'll double check again when I get home just to make sure I got the 5V DC rating and not the 110V AC rating.
     
  9. sickmacdoc macrumors 68020

    sickmacdoc

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    #9
    You may be onto something! I just checked the Belkin 4 port one last time and it lists the INPUT as 1.0A@110-120VAC. I had not thought of that! ;)
     
  10. blodwyn macrumors 65816

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    #10
    That must be it - thanks. I think I'll chalk this up to a bad day and start afresh tomorrow :eek:
     
  11. miiles macrumors regular

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  12. sickmacdoc macrumors 68020

    sickmacdoc

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    #12
    Yes, that was what was earlier suggested and in post 5 the OP said that was what he was going to do. The rest was just discussion/clarification of the issues! ;)
     

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