how much current do I need


18 posts
by scales11 » Sat Nov 03, 2012 3:55 pm
I am trying to figure out how many amps I will need to power my small setup, can anyone offer some suggestions? Here is what I am attempting:
1x Raspberry Pi
1x USB wireless network card
1x USB hub (I suppose I need this)
1x USB to 2.5 sata drive & adapter
(like this http://www.newegg.com/Product/Product.a ... 6812226048)

I think everything runs on 5V so I think I just need enough current....Thoughts?
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by joan » Sat Nov 03, 2012 4:11 pm
Guesstimates.

700mA - Raspberry Pi
50mA - USB wireless network card
10mA - USB hub (I suppose I need this)
500mA - USB to 2.5 sata drive & adapter
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by Onira » Sat Nov 03, 2012 4:21 pm
my card shows 500mA
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by scales11 » Sat Nov 03, 2012 4:43 pm
how are you measuring the current draw?
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by Onira » Sat Nov 03, 2012 6:45 pm
Code: Select all
$ lsusb -vv
and it just shows me
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by scales11 » Sat Nov 03, 2012 7:57 pm
my card shows 500mA

which card are you referring to?
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by Onira » Sat Nov 03, 2012 8:17 pm
Edimax EW-7811un or D-Link DWA-131
both show 500mA
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by Wendo » Sat Nov 03, 2012 8:33 pm
Thats max power, not what it will actually use. I'm happily powering my Edimax EW-7811un from the pi itself on a rev 1 board, so that's absolute proof it doesn't need 500ma, the polyfuses only allow 100ma per port
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by Onira » Sat Nov 03, 2012 9:20 pm
I thought that they allow bit more
but I know that my RPi was freezing sometimes and disconnecting maybe cos of power...
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by pluggy » Sun Nov 04, 2012 2:23 pm
The current wifi adapters use depends on what kind of link it needs to maintain. Close to the access point / router with a 'clean' RF environment, it will use very little, at the limits of range and / or a noisy RF environment, the thing will automatically turn up the power to try and maintain a connection and will use a lot more, probably up to the stated 500 mA.

I run a couple of them headless with wifi all the time and I don't own a USB powered hub....

One is on a Rev one with 'butchered' polyfuses and the other is a Rev2. Neither is particularly stressed, distance or workload wise.

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by Cloudcentric » Sun Nov 04, 2012 3:17 pm
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by scales11 » Mon Nov 05, 2012 10:47 pm
OK, so I figure I could power the PI and those accessories with maybe 2A, maybe 3 to be safe?

Now for the rest of my story...
I also would like to power a pair of bookshelf speakers (and whatever amplifier they use). My project is a rev2 of my wifi radio that I built using the MightyOhm guide. I am not really happy with the performance of my rev1, I used the following:
-asus 520GU
-lepai 2020 mini amplifier (I did take the amp out of its aluminum case in an attempt to make the overall housing compact, not sure what the effects of this are)
-cheap ac/dc molex adapter (came with an external ide/sata kit), outputs 5v and 12v

After the PI came out, I found squeezebox and before I knew it, I craved a rev2 of my wifi Radio. I am not against building my own speakers, but I was eyeing the creative t12 or t10 speakers since they would probably be a quick solution. Unfortunately they require 12v and in an effort to keep this project compact, I am left wondering what kind of power supply to use.

I am getting a bit off topic, but figure that in order to get a better answer, I should share my full intentions.

So, any suggestions on how I should power all of this?
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by Alfadaz » Tue Nov 06, 2012 5:33 am
I have been looking at power solutions as i may also have some 12v stuff i want to power.

My latest thought was to use a laptop or LCD power supply as you can get a 12v 3-4A one for very little money, then butcher an in-car Aux fitting to get the 5v for the Pi.

I have one like this in my office not being used, it was off an old LCD and the bonus is that it screws together rather than plastic sealed, so will be easy to take apart.
http://compare.ebay.co.uk/like/27088177 ... sbar&cbt=y

I use this in my car for charging my iphone and my tablet.
http://www.amazon.co.uk/TRIXES-Charger- ... 94&sr=1-19

Daz
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by joan » Tue Nov 06, 2012 8:36 am
I use a 12V 3A laptop supply. It directly supplys a mini screen (car rear view mirror type). It also feeds a 5V UBEC to supply the Pi and other 5V gadgets.

UBEC http://www.ebay.co.uk/itm/2-6S-Lipo-Battery-Input-4A-UBEC-5V-4A-5-5V-26V-rc-heli-/270740352010
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by Alfadaz » Tue Nov 06, 2012 9:54 am
Why i didn't think of using a UBEC i will never know... :oops:

Daz
(ex RC heli/plane pilot)
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by scales11 » Tue Nov 06, 2012 1:37 pm
Wow that UBEC looks like an excellent solution, Thanks! I am still trying to read up on how it works, but it looks perfect and at such a good price. I definitely have some of those 12v adapters laying around so I think I may go that route.

Any drawbacks to this method? Any idea how I can determine the amount of power that the UBEC will need to get 5v@3A?
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by Alfadaz » Wed Nov 07, 2012 5:39 am
A UBEC takes any voltage you put in which is between 6v and 25v and gives you a steady 5v (or 6v) output.

For RC use, if you had an electric model, it meant you didnt have to have 2 battery packs, 1 set for the motor and one set for servo's etc. You just had you main battery pack and a UBEC to split off your 5v for them.

Daz
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by garycocs » Mon Nov 26, 2012 2:14 pm
Hi Guys,

Great thread going here, a lot of stuff I need.

One question I have is, what if I wanted to run two of these off one PSU? Or three? (Thinking about a multi zone setup)

Would multiple molex from a computer PSU work or would you be better off going for the 12v plus UBEC?

Thanks for any advice you can give, I should know a lot more about the power draw of these things but it's been years!!
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