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 Post subject: Custom laptop stand USB power question...
PostPosted: Wed Jun 02, 2010 5:53 am 
8086
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The Dell XPS m1530 has always been notorious for running hot. I'd like to build a custom laptop stand that incorporates a fan to help keep things cool.

But I want to do something more...

I want to know if I can power a fan from two usb ports. I would like to be able to have a fast/slow toggle and IIRC a usb port only outputs at 5v. So I would like to be able to have a 5v/10v switch...you know, for when I need a lot of air flow in a hurry...

I know they have such cables for hard drives, but would it work (or be advisable) for a toggled fan?

Thanks for any help! Have a great day!


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PostPosted: Thu Jun 03, 2010 4:28 pm 
Bitchin' Fast 3D Z8000
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What I think you are trying to build has already been built.

The Thermaltake A1888 80mm Mobile Fan II External USB Cooling Fan uses only 1 USB port, has a variable speed knob, and only costs around $15.

http://www.thermaltakeusa.com/Product.aspx?C=1165&ID=1801

I've seen 120mm fans that are USB powered and use only 1 USB port. Just splice in a single RheoBus fan controller which can be found for less than a $1 on the net.

I would forget the toggle switch idea. Why just have two speeds?

I hope I understood your post correctly and this helps.

-ddb


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PostPosted: Fri Jun 04, 2010 1:56 am 
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couple issues with roll your own USB projects.. one is (according to USB standards) that its only 100 ma per port unless the device requests full power (500 ma). some ports are dumb and will give 500 ma at any time though, depends on the manufacturer. some notebook USB ports wont even give the whole 500 ma.. again, depends on the manufacturer/model.

most stock computer fans need 7v to start up. check the fan specs, most will list start up volts. once running they can run on less but 7v start up is very common.

never tried daisy chaining (series connection) USB ports for 10v but it (in theory) should work. but I would try it out on a cheap USB port add in card 1st. just in case the magic smoke escapes.


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PostPosted: Fri Jun 04, 2010 6:39 am 
8086
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ddbEntertainment:

Yes, you helped quite a bit! I just have several old case fans laying around and wanted to use them. The rheostat idea is fantastic...don't know why I didn't think of it earlier.

I *was* going to build my own laptop stand...but that didn't happen...WalMart had a sale on some nice mesh ones and so I picked it up. Still going to mod it for better airflow.

CharBroiled:

I have a modded usb powered fan cooling the computer now. When I get the gumption I'm going to take the laptop to work and have one of our maint guys take a voltmeter to a dual usb power cable I have on hand. From what I've read it should work...but that is why I'm going to test it with a voltmeter first...

Thank you both for your help! I'll keep y'all posted.


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 Post subject: Re: Custom laptop stand USB power question...
PostPosted: Wed Jun 09, 2010 2:30 pm 
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hbar98 wrote:
The Dell XPS m1530 has always been notorious for running hot. I'd like to build a custom laptop stand that incorporates a fan to help keep things cool.

But I want to do something more...

I want to know if I can power a fan from two usb ports. I would like to be able to have a fast/slow toggle and IIRC a usb port only outputs at 5v. So I would like to be able to have a 5v/10v switch...you know, for when I need a lot of air flow in a hurry...

I know they have such cables for hard drives, but would it work (or be advisable) for a toggled fan?

Thanks for any help! Have a great day!


OK, few problems here with the concept. Yes, the USB ports offer some 5 volt power, with very limited current. However, it is all the same 5 volt power meaning you can't in any way, shape, form or manner do a 5 + 5 = 10 Volts trick. Since the source is the same you just can't do it.

Next problem is yes, they do make 5 volt fans. However, these fans are generally very small and produce at best a breeze.

Now, as was mentioned, the USB ports have limited current. For certain devices that demand it, on some motherboards, they can deliver up to 500 mA (0.5 Amp). This is only available on boards supporting it and the device must request it.

A simple, accidental short of the 5 volts on a USB port can kill the port, as in really bad things can happen. That is the sole main reason I never advocate people plugging in home brew devices to a USB port.

Just My Take
Ron


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