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

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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
    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.
    2000 Subaru OBS

    Dell P3 @ 900 Mhz
    7" Lilliput TS w/DigitalWW in-dash mount
    80GB External HD

    I am Zero Bitrate....

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    • #3
      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?
      2004 Matrix XR A7N8X-VM/400 AMD XP-M 2500+, DS-ATX
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      • #4
        I think the diode is the cheaper and easier way...
        Fiat Punto 176 Cabrio CarPC Projekt
        Status: finished (you never really finish... lots of new ideas)

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        • #5
          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.

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

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            • #7
              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.

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              • #8
                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?

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                • #9
                  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

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                  • #10
                    A zener diode only allows current to flow if the voltage is higher than the listed value.
                    2000 Subaru OBS

                    Dell P3 @ 900 Mhz
                    7" Lilliput TS w/DigitalWW in-dash mount
                    80GB External HD

                    I am Zero Bitrate....

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                    • #11
                      Thanx guys. Always wondered but never really bothered to find out - my dad would be ashamed of some of my electronics knowledge!

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                      • #12
                        If you are powering the USB hub straight from a 5V power supply lead, you can disconnect the +5V lead running into the motherboard altogether. At least, you could run a diode to prevent the backflow into the motherboard, but if your devices are being powered by the power supply, there is no sense in having a current draw from the motherboard anyways. I would run the signal wires and the ground wire from the USB socket/hub back into the motherboard, then tie the ground from your power supply into the USB ground wire, and connect the +5V pin of the USB directly to your 5V power supply.

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                        • #13
                          This sounds like it might work, but looking at my hub that I'm working on currently, there are some circuit connections to the 5v line coming from the computer. The hub may use these for knowing if the computer is on or not. I really couldn't tell you though.
                          2000 Subaru OBS

                          Dell P3 @ 900 Mhz
                          7" Lilliput TS w/DigitalWW in-dash mount
                          80GB External HD

                          I am Zero Bitrate....

                          Comment


                          • #14
                            Add a relay between your hub and its 5v source and control it with the +12v from a mobo fan header.

                            Mobo shuts off, fans off=>Relay off=>hub off.
                            Now Galileo is real. Muhahahahaha :p

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                            • #15
                              Not to argue, but I really think you shouldn't use a relay. Especially something connected directly to the mobo. Reason being is that the relays have a tendency to throw immense amounts of interference back into the trigger line. This is why you see alot of schematics with diodes placed across the ground to trigger.

                              If you do choose to go with a relay, don't connect it to the mobo. Connect it to something else that comes on with the computer. Maybe your inverter, or the supply line from an opus.

                              Just use the diode idea (my favored method) or the no-connection-to-the-5v-line-on-the-computer method. Both of those sound pretty safe, and if you do either of those, you will only be out a cable or two until you get it right.
                              2000 Subaru OBS

                              Dell P3 @ 900 Mhz
                              7" Lilliput TS w/DigitalWW in-dash mount
                              80GB External HD

                              I am Zero Bitrate....

                              Comment

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