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Power additiona usb ports through motherboard fan header

Last response: in Systems
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November 5, 2009 5:20:34 AM

posted this in computer periperals, other users suggested i post it here. replies are below so far....

ok I have made a great media centre pc and have housed it in a great looking box with plenty of usb ports integrated neatly. The problem i have is the board i use for the media centre pc doesnt have enough usb headers on it to cater for the feature of the box. I need an additional header, thus allowing for the 2 spare usb ports currently on the box. I know that usb hubs are available and require an external power source in most situations so with that in mind, i thought that i could wire up the additional ports to an internal heade but instead of including the 5v power and ground, bypass that to a fan header on the motherboard thus catering for the what would be an external power supply for an external usb hub. Anyone know if this is possible? I present a diagram but it is basically a general idea, i still am unsure if it will work.
Have a look below, again i am unsure if i this will work or if i need to include some other components or anything. Any help would be great!!!



reply1

l_r_c_t 10-28-2009 at 05:09:50 PM | BBCode | Report


First of all the question is more suitable for the "Homebuilt Systems" section in the forum; http://www.tomshardware.com/forum/forum-31.html.

If I got you right, and you want to connect the USB hub to the Fan power supply, "on the paper" it should work, but the problem will occur when the fan's RPM will be high (no matter why). In this situation both of the devices (USB and fan) will have to share that same power supply which is not meant for it.

This is my idea, if someone can give a more knowledgeable response, or idea, is welcome to post it.

my reply

there wont be any fan connected to the board, i intend to use that fan header soley for the usb. Also i am wanting to connect the usb ports that come with a motherboard (usually just plug straight into the board) and not an actual hub. The main queries are :
is 12v power too much for the usb power (usually 5v)?
wil it be ok to run 2 usb ports that usually require 2 5v power sources off the 1 12v power source?
Do i need to add in any resistance between the fan header and the usb power connection?
will the data connections be ok to add another ?

It is unlikely at any time that all usb ports will be running, but i need to have that option available being a media center pc with lots of multiplayer games.

reply 2

Look, I don't think that it should be very complicated, if it is as easy as a simple circuit you just have to put 2 resistors (one for each port) that will take part of the voltage supplied.

Since the connection is serial I think you will need a resistor that will be able to "take" 2 volts, which are those left from the 12 (12-5-5=2).

Again, I don't think it should be very complicated, but I still recommend you to post it, including my replies, in the "Homebuilt Systems" section.
a c 122 B Homebuilt system
a c 156 V Motherboard
November 5, 2009 5:53:44 AM

From the above post (too hard to figure who said what):

" i intend to use that fan header soley for the usb. Also i am wanting to connect the usb ports that come with a motherboard (usually just plug straight into the board) and not an actual hub. The main queries are :
is 12v power too much for the usb power (usually 5v)?"

Yes. 12 volts will fry whatever is plugged into a USB port.

"wil it be ok to run 2 usb ports that usually require 2 5v power sources off the 1 12v power source?"
No.

"Do i need to add in any resistance between the fan header and the usb power connection?"
Won't work. You cannot depend on a resistor to drop a constant amount of voltage when you have a variable load.

"reply 2

Look, I don't think that it should be very complicated, if it is as easy as a simple circuit you just have to put 2 resistors (one for each port) that will take part of the voltage supplied."

Same answer as above: You cannot depend on a resistor to drop a constant amount of voltage when you have a variable load.

"Since the connection is serial I think you will need a resistor that will be able to "take" 2 volts, which are those left from the 12 (12-5-5=2)."
Same answer as above: You cannot depend on a resistor to drop a constant amount of voltage when you have a variable load.

Sigh. Why not just take the 5 volt power that you need from the 5 volt line in one of the drive molex connectors?
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November 5, 2009 6:52:07 AM

ok so it should be ok to acquire power for the two additional usb ports via the power supply 5v line? Also will the data aspect of my design be ok? i assume when using a third party usb hub it acts in the same way, or does it?
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a c 122 B Homebuilt system
a c 156 V Motherboard
November 6, 2009 2:27:57 AM

What data aspect? You have two separate USB connectors. Unless there's more going on here than I see, it should be no different than the other USB connectors.
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