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USB current??? 2

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Andrea1984

Andrea1984

Electrical
Jan 20, 2010
6
IT
Hello to everybody,
it's the first thread i open here, and i hope you'll be able to help me...

I always knew that USB standard allows a current to be sinked of 500mA...great surprise today when i realized that connecting my HTC Tatoo to the Notebook USB port it sinked 780 mA...
Am i crazy? Is this normal? Shouldn't the USB port be limited to 500mA? Otherwise...what am i missing?

Thanks in advance for your helpfull answers..

Best regards,

Andrea

 
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If it's not communicating at the time, you're allowed to supply as much current as the connector can handle (assuming it's a host or a hub downstream charging port). For USB 2.0, the standard socket is designed for 1.5A.

Dan - Owner
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Now, that's a piece of information that I also could use!
Thanks a lot.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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If I remember correctly the device has to ask for the power.
It asks for it in units of 100ma in 2.0 and 150ma in 3.0(usb on the go)(Up to 5 units per device). I also think the max power can change depending on the communication speed.
 
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I have kept power below 200 mA in all applications and ramped up to a maximum of 500 mA when device found by the PC. Is that old, outdated, no-good-anymore information?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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As stated above, there are restrictions as to how much current inrush current is allowed. Similarly, there are requirements on the host side that govern the transient response (sag) of the +5V line. The specs even indicate the load characteristics that it must hold up to. From what I recall from when I last did a USB project, this is a common source of trouble when attempting to certify a USB complaint device or controller.

Under normal operating mode, the device must request how much current it wants to draw up to 500mA. Since the value used to communicate this is byte wide, each unit of specified current represents 2mA.

 
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I've got a wifi router that permits higher than normal current on the USB port because the designers know that 3G Internet sticks need more than 500mA.

And many such sticks come with a 2-plug cable to draw power from 2 sockets if connected directly to older laptops that enforce the 500 mA limit.

Another older stick I used to have contained a battery to solve the peak power issue.

 
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Thanks a lot for your very fast replies...but i explain you better why i'm worried about this argument.

I'd like to recharge the battery of my HTC using the AUX input (actually a USB Socket)of my Ford Fiesta car , but i'm worried that HTC asking for more current than the standard one of USB may results in damaging the AUX input in my car...

Do you think the current would simple be limited by the USB (AUX) controller or that some damage may occur?

Thanks a lot again,

Andrea
 
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I'd be a lot more worried about a laptop than a Ford in terms of the amount of current that can be provided. Automakers usually try to make systems fairly robust with a healthy safety factor due to liability and the "wide range" of people who buy cars.

I'm guessing the Fiesta's USB port can take care of itself.

There is an Automotive forum on this site you could try.

David Castor
 
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Some replies above describe hand shaking, but don't clearly mention that it's optional.

USB ports will provide power to even the simplest device. A simple, completely mindless, USB-powered device can draw power from any common (USB 2.0) socket. Hand shaking is obviously optional.

In fact, some USB-powered toys have two wire (power only) cords, no data wires. There's even a USB-powered coffee cup warmer, nothing more than a 10 ohm, 2.5 watt resistor.




 
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Hi VE1BL,

uyou percetly took the point...I don't mind about the communication while charging the device...i just wanted to know what is the current limit of a USB host...
So do you all agree that it should be 1,5A when there is no communication? May you tell me if there is some official document on the internet saying this?

Thanks a lot to everybody for helping

Bye bye

Andrea
 
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"So do you all agree that it should be 1,5A..."

Not me. I've seen too many instances where the limit (for USB 2.0) was described as being "500 mA".

And I've seen several explicit exceptions where the product info states that the ports provide more than (again) "500 mA" - typically to support 3G Internet sticks.

But USB power ports, such as the little white power cube provided with iPhones, provides 1A.

It's a classic case of YMMV.

Have you checked the car's manual?

 
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There are rules to the power capabilities I listed above, but they're in the spec, so read it.

Noway, 3.0 ups the specs a bit, but only a few compliant devices have hit the shelves yet... I think only a single motherboard has been offered, and I've seen a handful of devices like hard drive docks, etc.



BTW, I just noticed this is not an engineering design, sounds like a home project... this thread may disappear soon.

Dan - Owner
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I see no reason to RF this thread. Good information that seems to be new to a bunch of people - including me.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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I see no reason to RF this thread


Sorry but...what does RF stands for in this sentence? :)
 
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Gunnar,

I didn't for the reason you mentioned, but I wanted to warn in case someone did (as well as remind Andrea that this site is for practicing engineers and hobby projects as a first post can get you booted).

Dan - Owner
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Gunnar,

I didn't for the reason you mentioned, but I wanted to warn in case someone did (as well as remind Andrea that this site is for practicing engineers and hobby projects as a first post can get you booted).

MacGivers...
Sorry but i don't understand what you mean...warning about what:)?
 
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