USB help!!!

Discussion in 'General Electronics Chat' started by angeline_happyoo, Sep 22, 2007.

  1. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
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    May i know what is the USB input current and voltage?
    How about its output current and voltage?
    :confused::confused::confused:
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Google is a really useful tool for getting information. There are 9,670,000 hits on "usb specifications". http://en.wikipedia.org/wiki/USB should get you started.
     
  3. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
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    USB 2.0 has:
    Output voltage: 4.75-5.25V (5V±5%)
    Output current: 100-500mA

    is the input voltage is also 5V and output voltage is also 100-500mA????
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
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    That is quite it, but remember that in portable computer the limit is just 100mA, while in desktop computer it will reach 500mA (saw some specifications mentioning 250mA, if I recall).
     
  5. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
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    Thanks you...

    That means the USB is only used to draw out the power from computer but not act as an amplifier.
    USB does not, for example, input voltage is 2V, and then it amplify the input voltage to 4V output (example)?

    Am i right?
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
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    The USB protocol doesn't work that way. The supply voltage is fed through the two external pins of the USB plug. The two pins in the middle are for data transmission (refered as D+ and D-).

    Also, there are cases in which no voltage is supplied through the USB connection. This is the case of self powered perifericals or perifericals that are powered externally. I can give two examples: a USB pen would need the USB power to be fed, since it has no internal power source, but a printer is fed by other means, so it doesn't take power via USB. In that case, only three pins are used (the power ground included, but just as a voltage reference, not being biased by a significant current). Vcc is left floating.
     
  7. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
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    Well, in this case, what i concern is the supplying power though USB to the device...
    So i'm not interest on trasfering data through USB.

    How many input current and voltage should i provide to have an output power is 5V and 500mA?
     
  8. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
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  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Your last question is hard to interpret. If you are asking something like "how many devices can I power with the USB as a source?", then you will need to determine the current draw of each individual device/circuit and insure that total does not exceed the ability of the USB plug.
     
  10. cumesoftware

    Senior Member

    Apr 27, 2007
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    It is a strange question, but I will do my best.

    Your figure suggests a simple USB connector. Vcc is 5V and the current doesn't change either. Since USB connectors are simply conductors, you should provide the same voltage and current.
     
  11. GS3

    Senior Member

    Sep 21, 2007
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    using USB for power is generally not a good idea as you may damabe the motherboard. Furthermore, it is not like there is an unconditional power supply there. The power supply is software controlled and to get full power the device must solicit it and the controller must grant it.
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
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    It depends on the device. USB was specially made to provide power to portable perifericals that won't have another power supply, external or internal. An USB pen is an example of such device. If you keep the current demanded lower than 100mA you should have no problem.

    By the way, those are good guidelines.
     
  13. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
    28
    0
    Thanks for the reply... :)

    Actually, in this case, USB will only act as an adapter.
    Means that, one end of USB is connected to power supply, the other end of USB is connected to device. This application can be used to charging handphone..

    So i need to provide 5V and 500mA to get an output of 5V and 500mA.
    Am i right?
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
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    Yes, being that the case. So are you buinding a USB power supply?
     
  15. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
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    In this case, i want to replace the wall 3-pin socket to USB, since USB is getting common nowadays...

    So in laboratory, i need an electrical input to flow into USB to get 5V, 500mA.
    I have to decide what is the input current and voltage to supply that USB's output.
     
  16. tinoest

    New Member

    Aug 17, 2007
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    You don't need to put 5V into the USB port. That is what it will output, or am I misunderstanding?
     
  17. angeline_happyoo

    Thread Starter Member

    Sep 2, 2007
    28
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    No. I means have to put 5V to USB input, so will obtain 5V at USB output port...
     
  18. GS3

    Senior Member

    Sep 21, 2007
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    If you are referring to the text I quoted, those are not "guidelines" but USB specs which explain how USB works.

    As you can see there a device cannot just draw 500 mA but has to request it via software and the controller has to grant the request.

    It is obvious that many USB devices draw their power from the USB bus. That's what the quote text explains. What I meant is that it is not a good idea to use a USB port to power one's own experimental circuits unless one knows what he is doing very well. Fumbling around without knowing exactly what you are doing is risking a damaged or dead motherboard as a result of a short, overload or other accident. (That risk exists even if you DO know what you are doing as mistakes happen but the risk is much lower.) Yes, you can get 5V @ 100 mA from a USB port but I see no reason to run such a risk when you can get more power independently and in a much safer manner.

    I have no idea what the original question is here. The question is confusing and needs to be clarified before any meaningful advice can be given. I do not understand what the OP is trying to do and I have a feeling he may be quite lost in which case the advice to not mess with the USB ports in the computer is the best advice.
     
  19. beenthere

    Retired Moderator

    Apr 20, 2004
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    Are we suffering under a wrong notion here? Are you going to supply power to a device by means of a USB cable? In this case, the USB cable will simply be used as a power cable into the device, and have no connection to a computer?
     
  20. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    If the intention here is to, separately from a computer, power a mobile phone or other similar device which can be powered from a computer USB port then the specifications for this power supply or charger are very simple: 5V, 100 mA. But I cannot see any practical reason to do this, other than learning, because these can be bought for close to nothing and are switching, light and tiny. The only case where it would seem justified would be if we need to get the power from some unsuual power source (say from 48 Vdc). The question needs to be clarified before we can go any further.
     
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