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  • need help with Resistors.

    alright, i dont know if this is the right place to put this...but its dealing with power....so here goes.

    i have an external usb hub that i 'hacked' into the front of my emachines case.
    right now i have it powered by 'self' instead of 'bus', however, i only have 5v tapped from PS going to the dc input on the hub...its rated for 7.5v.

    would anyone be so kind to explain what resistor(s) i need to take 7.5v from the 12v coming from the PS?

    thanks.
    -Keaton.
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  • #2
    If you want a simple resistor-divider circuit, you need more info... how much current does the hub draw? Is it constant current or does it vary with load? How much does it vary? How large of a input voltage window can the hub take?

    Since this probably isn't a constant current draw, you might want to look into voltage regulation instead of resistor-divider.
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    • #3
      okay, then whats the easiest way to get 7.5v dc @2100ma to the hub?(from the interanl PS?)

      im not picky...i just need it to be able to power/run my ipod/high power usb devices...

      hers a link to the hubs descripition
      Code:
      http://www.techexcess.net/belkin-4-port-usb-hub-f5u021.aspx
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      • #4
        Originally posted by keatonreckard View Post
        okay, then whats the easiest way to get 7.5v dc @2100ma to the hub?(from the interanl PS?)

        im not picky...i just need it to be able to power/run my ipod/high power usb devices...

        hers a link to the hubs descripition
        Code:
        http://www.techexcess.net/belkin-4-port-usb-hub-f5u021.aspx
        v=ir
        r=v/i
        You need to drop 4.5v at 2.1A, so R=4.5/2.1 = 2.14ohms.
        However, if the hub draws less current (very likely), the voltage drop will be less and you'll be feeding more than 7.5v into the hub and the magic smoke might escape.

        Safest way is a 7.5v regulator, a resistor is not the right solution here.
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        • #5
          well, im 16 and have no job... so im not gonna go buy anything huge.....i have a ton of old resistors in the barn...so...i just need it to work...not be perfect...so would it be better if i took it down to like 6.5v? so then it would blow up if i unplugged my ipod?
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          • #6
            do it right, do it once.
            do it wrong, buy a new hub.

            my advice, don't bodge it, do it right.
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            • #7
              Just buy a new hub that takes 5v. They are what, $10, $15 new? Maybe less used somewhere. That's 2 hours work at minimum wage. Im sure you could mow 1 lawn and get yourself a new hub. Or before you start hacking things up, sell yours on ebay, and buy a different one that takes 5v.
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              • #8
                well...this hub is already crammed into the front area of my case...(like the plastic part where the status leds are and such...seperate from the inside of the case..) and im trying to spend as little money as possible....ive gotten everything in this compy for free....except a few leds...

                the hub is constantly connected to
                (1) a usb to ide adapter for my dvd burner...
                (2) a usb bluetooth dongle...
                (3) and ipod usb cable that comes out to the front of the case..
                (4) a hacked usb extension cable to keep the use of the front mounted usb port...

                the 5v i have connected powers everything fine except high power devices...
                such as usb to sd/mmc/MS/xd adapter, ipod, keyboard...etc...

                so would the voltage really differ that much with the ipod(connected or unconnected) being the only changing variable?
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                • #9
                  Originally posted by keatonreckard View Post
                  the 5v i have connected powers everything fine except high power devices...
                  such as usb to sd/mmc/MS/xd adapter, ipod, keyboard...etc...

                  so would the voltage really differ that much with the ipod(connected or unconnected) being the only changing variable?
                  You already defined the ipod as a high power device. Therefore it will have a significant effect on the current, and therefore the voltage drop.

                  So the answer to your question is "yes".
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                  • #10
                    Yes, the power will fluctuate too much. Actually, just had a thought. Take the 12v and 5v from your PSU and you'll have your 7v. I don't recommend it, but it's been done. Don't blame me if you screw up your PSU, though!
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                    • #11
                      Originally posted by psyrex View Post
                      Yes, the power will fluctuate too much. Actually, just had a thought. Take the 12v and 5v from your PSU and you'll have your 7v. I don't recommend it, but it's been done. Don't blame me if you screw up your PSU, though!
                      DO NOT DO THIS...!

                      If your hub ground is actually 5v, then you just shorted out your power supply's 5v line to 0v through the usb cable between the hub and the pc.
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                      • #12
                        im pretty sure my computers grounds are all the same....(connected)...so that wouldnt work.
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                        • #13
                          Originally posted by Rob Withey View Post
                          DO NOT DO THIS...!

                          If your hub ground is actually 5v, then you just shorted out your power supply's 5v line to 0v through the usb cable between the hub and the pc.
                          Well a simple chopping of the hub's usb cable to take out the ground and +5v, and leave data +/- only fixes that.

                          But yes, dont do this unless you are extremely lazy.
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                          • #14
                            Like the other guys said resistors are the wrong way to do this. A resistor is a current limiting device not a voltage limiting device. It only drops voltage once you have reached the circuits current limit. You cant guarantee the same load all the time so you wont get the same voltage.
                            Options are buy a proper regulator or buy a new hub.
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                            • #15
                              It's possible that the first component off the 7.5v is a 5v regulator. If you could identify it, you could rip it out and tap in there. But that may not be the case.

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