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Thread: need help with Resistors.

  1. #1
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    Unhappy 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.

  2. #2
    Maximum Bitrate psyrex's Avatar
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    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.

  3. #3
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    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

  4. #4
    Raw Wave Rob Withey's Avatar
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    Quote 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.
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  5. #5
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    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?

  6. #6
    Raw Wave Rob Withey's Avatar
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    do it right, do it once.
    do it wrong, buy a new hub.

    my advice, don't bodge it, do it right.
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  7. #7
    Fusion Brain Creator 2k1Toaster's Avatar
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    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.
    Fusion Brain Version 6 Released!
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  8. #8
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    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?

  9. #9
    Raw Wave Rob Withey's Avatar
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    Quote 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".
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  10. #10
    Maximum Bitrate psyrex's Avatar
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    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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