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Thread: Can someone tell me what Kind of Circuit I need to build for this to work

  1. #1
    Variable Bitrate
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    Can someone tell me what Kind of Circuit I need to build for this to work

    Can someone tell me what I need to make something keep voltage from going 2 ways in a wire.

    Here is the setup of the problem.

    I have a USB 2.0 Hub wired directly to the 5v line on my computer power supply, my problem is that when the computer is turned off, the lights on the CD-Rom, HDD LED and Power Led would all light up, as they were being powered from this 5v line back through the USB cable into the case.

    Does anyone know some type of Diode, Resistor or some other form of electrical part that I could purchase and Place on the wire, so that the power will not flow back through the cable into the 5v line. I am pretty sure that the Power is flowing from the USB Ports, through this Hub and then back into the 5v line of the Power Supply, which is then supplying power to the leds mentioned above.

    If I remove the Power Plug from the USB hub, then the Leds mentioned above no longer light up, so I am trying to stop the back flow of power into the Power Supply.

    Thanks for any help,
    Bryan
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  2. #2
    Maximum Bitrate kiltjim's Avatar
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    You need a simple diode "pointing" to the hub from the computer on the host(?) cable. That should solve your problem.

    However, with all diodes, they have voltage drop across the connection. I don't know if that is going to be a problem. I don't think it should, since the hub is going to be recieving data over the data lines, not the 5 volt line.
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  3. #3
    MySQL Error MatrixPC's Avatar
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    How about a relay?
    Use the PC power (5V or 12V from the drive connector) to power the relay. The relay then connect or disconnect the "power" to the USB hub base on whether the PC is on or off.
    Make sense?
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  4. #4
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    I think the diode is the cheaper and easier way...

  5. #5
    Maximum Bitrate starfox's Avatar
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    Yeah just buy a schottky diode (A power diode might drop the voltage a little too much). You'll need to take apart the USB cable running from the motherboard to the USB hub and put the diode in series with the V+ line. Make sure the diode's facing the right direction.

  6. #6
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    anyone run into problems with the 0.7 Volt drop across a diode?

  7. #7
    Maximum Bitrate starfox's Avatar
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    Schottkys have a lower voltage drop (0.2V to 0.3V), and should give you less problems in this situation where 0.7V is quite a bit.

  8. #8
    MySQL Error Scouse Monkey's Avatar
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    What else is the difference between a normal and shottky diode? i used to use them on the motors of my RC cars. Also what is the difference with a zener?

  9. #9
    Maximum Bitrate starfox's Avatar
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    Schottkys have a lower dropout voltage, but may have a higher reverse bias current leak.

    Have a look at this page, i think it summarises things well:
    http://www.play-hookey.com/semicondu...ed_diodes.html

  10. #10
    Maximum Bitrate kiltjim's Avatar
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    A zener diode only allows current to flow if the voltage is higher than the listed value.
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