# Using 5V supply from USB header



## EnglishLion (Oct 23, 2007)

I've just bought a red lcd temp display to replace the blue one in my case.  They are identical except for colour and run from a molex connector (5V).  I want to run the new one from my motherboard instead as cables up to my psu make it a bit more messy.

I have a spare USB header and I understand that they supply 5V, can anyone see a problem with supplying a small lcd display, designed to run off a molex, with power from a USB header?

http://www.watercoolingshop.com/catalog/product_info.php?cPath=62&products_id=294&osCsid=e939fc1f598041fb2e02bdfc31d03eb6  This is a link to the lcd display.


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## Kreij (Oct 23, 2007)

You should be able to hook it up to a USB port as long as you do not exceed a total current draw of 500ma (which is the specified current limit per port). 

I think you can actually exceed that, but I am not sure by how much.

The link did not show specs on the lcd displays voltage and current requirements (unless I am just blind), so I would make sure that when you try it you are using a port with no other USB devices on it to add to the load.


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## Deleted member 24505 (Oct 23, 2007)

I'd say its fine.The current draw from that will be negligable.


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## ktr (Oct 23, 2007)

I see no problem. I made a small adapter for my cell phone so i can charge it from a usb port.


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## ktr (Oct 24, 2007)

Its really simple. 

1) Get a usb cable, cut it. There will be 4 wires, a red, green, white, and black. Obviously, red would be positive, and black is negative. 

2) Then get a power jack that fits in your phone. Cut its wire. 

4) Finally, attach the red to red, and black to black. I would recommend soldering it. And for professional look, you can use heat shrink wrap and make it look like one piece.


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## Random Murderer (Oct 24, 2007)

Kreij said:


> You should be able to hook it up to a USB port as long as you do not exceed a total current draw of 500ma (which is the specified current limit per port).
> 
> I think you can actually exceed that, but I am not sure by how much.
> 
> The link did not show specs on the lcd displays voltage and current requirements (unless I am just blind), so I would make sure that when you try it you are using a port with no other USB devices on it to add to the load.



if the device exceeds 550mA, the computer will automatically cut power to that port and the port will require a reset. If you're running xp or vista, you'll get a popup saying "power surge on USB port" and will allow you to reset it there.


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## Necrofire (Nov 2, 2007)

Actually, the limit is 100mA per port, up to 500mA if the device negotiates for 500mA. I don't think negotiating is an easy task though.

Random Murderer is sorta right, some setups will power cycle the port, but in my case, the motherboard shuts down instantly.

You 'should' be safe running that from usb.


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## Random Murderer (Nov 2, 2007)

Necrofire said:


> Actually, the limit is 100mA per port, up to 500mA if the device negotiates for 500mA. I don't think negotiating is an easy task though.



wrong.
the limit is 500mA per port, that is exactly what is supplied to each usb port, and the device only draws what it needs.


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## Necrofire (Nov 7, 2007)

Random Murderer said:


> wrong.
> the limit is 500mA per port, that is exactly what is supplied to each usb port, and the device only draws what it needs.



So what does this excerpt from Wikipedia mean.


> Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 100 mA up to a maximum of 500 mA.



I'm confused...


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## Random Murderer (Nov 7, 2007)

this is why i hate wikipedia :shadedshu
That's actually bass ackwards, and only works that way if the computer's usb hubs are set to low power mode, which they are not by default.
usb is preferred by modders because of this constant supply of 500mA, if you don't believe me, take out your multimeter and measure the damn things yourself.


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## Necrofire (Nov 7, 2007)

My amp meter only does 200mA, and there's a setting for 10A, but the fuse blew for it, and I stuck a monster one in there from a dead power supply. Can I still measure it even if the fuse is dangerously too big, since it's "only" 500mA?


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## Random Murderer (Nov 7, 2007)

Necrofire said:


> My amp meter only does 200mA, and there's a setting for 10A, but the fuse blew for it, and I stuck a monster one in there from a dead power supply. Can I still measure it even if the fuse is dangerously too big, since it's "only" 500mA?



i wouldn't recommend it. electricity is an unforgiving bitch.


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## Steevo (Nov 7, 2007)

Random Murderer said:


> i wouldn't recommend it. electricity is an unforgiving bitch.


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## Necrofire (Nov 7, 2007)

Ok, that's more than enough to go for it. I'll post again with results.


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## Necrofire (Nov 7, 2007)

Very interesting.....
My front ports only put out 350mA, that strikes me as odd.

EDIT: also, what a VERY random time for me to look at Random Murderer's number of posts...has it always been EXACTLY 1600?

EDIT 2: I just found out, the negotiation for the 500mA is on the software side... The system really has no control if the device decides it wants 500mA without "negotiating"
So Wikipedia is half-right. If you go above 550mA, a resettable fuse is triggered, though.


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## EnglishLion (Apr 9, 2008)

Just thought I'd update this thread with the results of my mod.  I went ahead and connected my LCD temperature display to one of my internal USB connectors.  







What I noticed was that the 5V in the USB header is supplied on two pins seperated by two other pins.  ie (5v X X Grd) which is the same configuration as a standard motherboard speaker connector.  So I just cut off a speaker cable and soldered it on to the LCD display power connector.


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