usb device failure / related to AC adapter?

Glasers

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Apr 14, 2014
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I have a Lenovo Twist computer/laptop running windows 8

I had a crazy USB issue. I had 4 USB devices plugged in. Hub, attached to printer keyboard and printer and a camera in the other USB port.

Always works fine in this config and suddenly I set it up and the hub connected components won't work and Windows says the USB device malfunctioned.

I tried a different hub and same thing, restarted everything....

Then tried unplugging laptop and running on battery power and everything went back to normal. But plugged into the wall the messages come back..... I happened to have another ac adpter and tried that and no plroblems....

My question is how is the AC adapter causing problems? Any ideas?
 
Hi,

Can you remember at School were you connected a light bulb to a circuit.
With a battery as the source.

It was nice and bright, then you added another light bulb and with two each was half as bright.

That is because they were both drawing power from the same circuit.
So 12v example would be halved to 6v.
Bulb 1 6v bulb 2 6v. add another and divide again.


So if you have a four port hub that is connected to a 5v single usb port that provides 5v. your deviding it four times when all devices are powered by the single usb port of 5v.

That is 1.25v per device, and way bellow the voltage required to run the devices.
Electrical law.

Each device in single use fine.
Four running at the same time, the board or single usb cannot provide more than 5v.


Four devices that each require 5v at the same time. 5v times 4 = 20v.

What I am saying is a usb port is fixed to provide a maximum of 5v.
Running a hub is drawing two much power.
And why the usb fails. over current.




 
Am on my phone so am not writing it all out but the above is slightly right but mostly wrong. There is a limit in what can be plugged into USB but it does NOT reduce the volts, it increases the Amps drawn. Also the bulb analogy describes a series circuit where USB is parallel.
 
Hi

I think Shaun is confused

USB requires 5.0 volts +_ 0.25 v
If you have 1 or 4 USB devices voltage is still 5.0 volts

However there is a USB standard on 0.5 amp per USB port
Some laptops can not provide 0.5 A per port

Some scanners and 2.5 " hard disks ect need close to 0.5 amps
So external hub may need a 5V psu

Regards
Mike Barnes