USB Power supply

Discussion in 'The Projects Forum' started by RodneyB, Jan 24, 2013.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I Want to extend my usb wireless LAN by the maximum of 5 meters. I want to supply the modem with a seperate 5 Volt power supply. And only have the D+ and D- Going back to the USB socket on the computer. The External power supply is to ensure there is no Voltage drop and the wireless Lan gets the full5 Volts required. I have severalHubs and USB appliances already on my computer and often have to un plug devices so others work. I suspected a voltage drop thus the reason for a seperate power supply for the device.

    Many thanks

    Rodney
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Is there a question in there?
     
  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    ooops sorry the question is "Is it possible to have a seperate 5 volt supply to the wlan and still be able to get the data connected at the pc socket, would I have to use the power supply fromthe PC as well.

    Thank you
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Ah, got it. There are powered hubs, so it must be possible to do what you want. But I'm afraid I don't know how this is done.

    In general it can be a problem to connect two power supplies - in this case the PC USB power and the powered hub's power supply. Current can flow between them - even over a single connection - if care is not taken, if they are not isolated from each other.

    I was going to say that the hub can be powered completely separately, but I think the data lines must somehow be referenced to "ground", and I don't see how that works if ground is not common. But once you connect grounds, you risk the problem of current passing from one supply to the other.

    We need somebody that knows something about this. :(
     
  5. wayneh

    Expert

    Sep 9, 2010
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    I guess this would be the time to ask, why not just buy a self-powered hub? They're pretty cheap and I believe would solve your problem. Just watch out for the maximum total current. Many hubs can supply 500mA on a port or two but not on all of the ports at the same time.
     
  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I live in Zimbabwe, we donthave the luxuary of just walking into a shopand buying no ebay or on line shopping we make a plan or go without, as far as stripping old PCB's for components. A PIC 16F88 cost me US$12.00 fromour localelectronics shop.

    So The need to make came about because I coudnt buy one
     
  7. wayneh

    Expert

    Sep 9, 2010
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    OK, well the thing to investigate then is IF your separate power supply can safely share a ground with your PC, without significant current flowing. I'm sure someone else may have a more elegant idea, or can tell us if it is even necessary to connect the grounds at all, but here is how I would do it:

    Identify a possible supply for your self-powered hub. It should be regulated 5V and, in my opinion, it should be rated to at least 1A.

    Connect a voltmeter across a 200Ω (or 150-1000Ω) resistor, and use that resistor to briefly connect the test power supply's ground to the ground of your PC's USB port. Watch for a voltage across the resistor, hopefully well under 1V, and no heating of the resistor. Be prepared to quickly break the connection if either of these is not true.

    If you get past that test, there is hope. Now try placing a load on the test power supply, perhaps a 12V light bulb (it will be dim at 5V). Check again for current flow between the two power supplies. If there is little voltage across the resistor and it stays cool, I think you're good to go.

    When you connect your self-powered hub to your PC, make sure that the positive conductors don't touch, only the grounds and the data lines.

    If this testing is uncomfortable for you, do a bit of research to determine if connecting the data lines alone - with no common connection of the power lines - will allow USB to function reliably. It might.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    You may want to read this for more background. Or this.
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I think I follow you. For the most part yes you can do this. I am assuming the power you use at the wlan is an isolated supply such as a "wall wart" or other transformer coupled supply.

    You can just open the power line from the UBS connector, and then splice in your replacement power.

    The ground or return line should stay intact, the +D and -D depend on that.

    Before you modify anything you can check this by connecting the minus out of the new power to the PC's case, which is typically the same ground as on the USB. If nothing sparks you are good to go.
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    That's a grey area. I've seen a few computers with dead USB ports from using cheap powered USB hubs. When the computer is turned off, the port tries to power the motherboard for some reason. A combination of bad motherboard/USB port design and bad powered hub design.
     
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