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 Post subject: USB Power
PostPosted: Mon Oct 09, 2017 5:42 pm 
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Joined: Sat Nov 08, 2003 1:01 am
Posts: 1589
Location: Phoenixville, PA USA
I have a project, possibly projects, that will run on USB power, but am having a rough time finding the "standard" current rating on USB 2.0 and USB 3.0 computer ports. I know the original (well, finalized 1.1 specs) max current was 500 mA, but subsequent specs have eluded my normally adroit search abilities.

Anyone have concrete numbers or an actual link that shows those specs?

Thanks,
CeaSaR

**Data lines will NOT be used. This is strictly for +5V power.



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 Post subject: Re: USB Power
PostPosted: Mon Oct 09, 2017 7:07 pm 
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Joined: Sat Aug 12, 2006 4:17 pm
Posts: 1274
Have you checked in wikipedia?
Here's the Wikipedia page on USB: https://en.wikipedia.org/wiki/USB
The right side column shows the spec's for the different versions.

Signed: Janitor Tzap


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 Post subject: Re: USB Power
PostPosted: Tue Oct 10, 2017 2:21 am 
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Joined: Sat Nov 08, 2003 1:01 am
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Location: Phoenixville, PA USA
I was on several Wikipedia pages, and the Max Current spec wasn't listed. Bizarre.

Thanks!



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 Post subject: Re: USB Power
PostPosted: Tue Oct 10, 2017 6:06 am 
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Joined: Mon Feb 09, 2004 1:01 am
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Location: NY
From the wiki page that Janitor Tzap referenced:

Electrical
Signal 5 V DC
Max. voltage
5.00+0.25
−0.60 V[a]
5.00+0.25
−0.55 V (USB 3.0)
20.00 V (PD)
Max. current
0.5 A (USB 2.0)
0.9 A (USB 3.0)
5 A (BC 1.2)
3 A (type-C)
Up to 5 A (PD)


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 Post subject: Re: USB Power
PostPosted: Tue Oct 10, 2017 12:41 pm 
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Joined: Thu Aug 01, 2002 12:01 am
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Location: San Jose CA
need to check your motherboard manual for that. You can measure directly using a USB power dongle and a resistor load

eBay has them from $2 to $10 each This one has some nice features
http://www.ebay.com/itm/LCD-USB-Detecto ... SwKzpZiBf1

ebay also has "USB Resistor Loads" for $1 to $5 various ranges and power ratings, with and without fans and switches to change values


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 Post subject: Re: USB Power
PostPosted: Tue Oct 10, 2017 2:39 pm 

Joined: Fri Aug 22, 2003 12:01 am
Posts: 1612
Location: (somewhere), Afghanistan; from Rochester, NY
Quote:
You can measure directly using a USB power dongle and a resistor load

You can????????
I thought a power control IC in the USB device negotiated with power IC in USB port for current range when cord is plugged in. If device tries to draw more than negotiated range, then port shuts down to save the PC from damage.

Or, are you saying you have to add an external resistor to the gizzmos on link?

Cheers,

( https://en.wikipedia.org/wiki/USB#Power )



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 Post subject: Re: USB Power
PostPosted: Tue Oct 10, 2017 3:34 pm 
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Joined: Thu Aug 01, 2002 12:01 am
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Location: San Jose CA
I don't know about the shutdown but with the dongle you can monitor voltage and thru current. By attaching a variable load to the output you can turn up the current until either the voltage drops off or the port shuts down. I am not aware of any digital configuration of the port for supply current. If I really needed high current, I would attach a hub with external power supply. One of my keyboards has 2 USB plugs presumably to get higher current for high speed gaming IO (which I don't really need) its a pain that it takes up a port, I stopped using it

For Example 2 to 37 ohms (approx.) 125mA to 2A with 5V supply
http://www.ebay.com/itm/USB-Electronic- ... Sw42dZH6m0

Given the gender of the USB connector on this, I assume it is for loading the host but is also useful for determining the discharge rate or capacity on a battery pack. I suppose that's where the higher current would be more useful. I found the meter dongle useful when charging small batteries with no indicator, you can see the current drop to zero when full.


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 Post subject: Re: USB Power
PostPosted: Tue Oct 10, 2017 3:41 pm 
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Joined: Thu Aug 01, 2002 12:01 am
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Location: San Jose CA
probably a more useful answer. I looked in the Device Manager on my PC and in the tree under USB devices, looking at the properties of the Generic USB Hubs and USB Root Hub There is a power tab and it shows the current rating. It tells you how many ports, current per port and for connected devices, it shows power required. Presumably if I overtaxed the port, it would tell me though I have never experienced that.

I suppose if you were working with a USB dev kit or a uController with a USB port you could configure the required current but I don't know what port or what steps it takes. For devices that cannot tell the system how much current it wants, I'm not sure.


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