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  • Power question

    I have a self-powered usb hub that operates at 5 volts, obviously I can't wire this up directly to my car's battery. I was wondering what I would have to do to drop the input power from 12volts to 5 so it is usable by my hub. Thanks for the insight

  • #2
    Run 5v from a molex connector?

    Ratshack cigar lighter adapter set to 4.5V might work.
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    • #3
      a voltage regulator. 7805 is the chip number. Has 3 pins, input, output, and ground. You can run these in parallel for higher amperage, and may require a heatsink.
      2007 Honda Fit Sport 1.5L SOHC-VTEC

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      • #4
        the 7805 yields 5 volts at 1amp so if I needed 5volts at 2amps I would have to wire two 7805s in parellel?

        Code:
        (assuming the pins are in, ground, out from left to right 
        & that the middle pins are grounded)
          ___      ___
         |   |    |   |
         |_ _|    |_ _|
         | | |    | | |
         |   `----)---`------(+5v to usb hub)
        _|________|
        ^
        |___(+12v from battery)

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        • #5
          You'd also want to use some caps to even out the power. Don't forget an inline fuse.

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          • #6
            what size capacitor(s) would I be needing then? Also would a 5amp fuse suffice or wouldn't that be effective since I'll only be drawing 1 or 2amps (I don't remember the number of amps my hub requires...I'll need to check on that when I get home)

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            • #7
              for 2 amps i would run 4 chips maybe 6. They are fairly cheap and small so you can get away with it. Yes, your drawing looks good. As far as the caps go, put one large cap and one smaller cap across the input source and the output source. A 5A fuse should work, just make sure your wire is rated for more than 5 amps.
              2007 Honda Fit Sport 1.5L SOHC-VTEC

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              • #8
                how large and how small should those caps be? my ac adapter for my hub states the following:
                input: 100-240V~0.5A 47-63Hz
                output: 4.0~5.5V = 2.4A max

                then further down it lists:

                input: AC 100-240V 32-43VA,50-60Hz
                output: DC 5.0V,2.1A

                so I'm assuming I need to present 5.0volts at 2.1amps for my hub to operate properly

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                • #9
                  I don't know if I would do that with the 7805. I have seen instances where they do not share current that well, and one hogs more than the others. It is better to build a switching regulator or use this mondo regulator
                  http://www.national.com/pf/LM/LM323.html
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                  • #10
                    so if I went with this regulator I'd still need the capacitors on the input and output?

                    No external components are required for operation of the LM123. If the device is more than 4 inches from the filter capacitor, however, a 1 F solid tantalum capacitor should be used on the input. A 0.1 F or larger capacitor may be used on the output to reduce load transient spikes created by fast switching digital logic, or to swamp out stray load capacitance.
                    sorry for all the questions but my field of knowledge dealing with circuits is very basic

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                    • #11
                      They make 5V 5A regulators (I think Linear might make one).

                      As mentioned don't try to parallel them.

                      Also note, if your hub draws 2A you will be dropping ~9V across your regulator generating 18W of heat. You will definitely need a heat sink for that.

                      Without knowing the USB spec off the top of my head, and without knowing how many ports you have, I am going to guess USB provides for 500ma per port (and a guess is you have 4 ports) for 2A, and then 100ma to run the logic in the hub.

                      It is entirely possible you will need a very miniscule heat sink for this, just be aware of it.

                      For the capacitors put a .01uF and a 10uF on both input and output (maybe a bit bigger one on the output) and you should be fine.

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                      • #12
                        The caps I would got with a .01uF and a 1000uF rated AT LEAST 24V


                        you could rip apart the wall adapter and see what kind of goodies it has.
                        2007 Honda Fit Sport 1.5L SOHC-VTEC

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                        • #13
                          Ok, I went by rat shack and picked up a .01uF and 1000uF capacitors as well as a small square of bread board, small project box, and heat sink and thermal paste. I looked at voltage regulators and all they had were 5v at 1amp so I found a 5v 3amp on the internet and ordered it. One more question, which capacitor goes where? 1000uF on the input side or the output?

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                          • #14
                            Hey Cyber let me know how this works out. I have the same issue.
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