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I use a non-self-powered USB modem device for a solutions I provide to multiple customers, and very often I encounter a problem in which the devices stops working due to many reasons, one of which I suspect is insufficient USB power, but can't confirm. I could purchase a powered USB hub which I do sometimes, but the problem bugs me because I can't confirm that power is the problem and got to make trips to the customer's office at my own expense.

I've read the question How do you check how much power a USB port can deliver?, and followed the instructions in the answers.

The device I'm using is Sierra Wireless GL6110 USB modem. The vendor specification (see spec link below) says that the modem can draw up to 400mA max. Under Device Manager -> Generic USB Hub -> Power Tab, it says that the modem is drawing 20mA, which I believe is lower than the actual maximum. It also says that each port can draw up to 500mA.

https://www.sierrawireless.com/productsandservices/AirLink/Programmable_Modems/~/media/Data%20Sheet/Sierra_Wireless_AirLink_FXT_Series_Programmable_Gateways_GL_Series_Modems.ashx

I believe that the information provided under Power tab is not sufficient for me to gauge whether the system can provide enough power for my modem device because:

Firstly, even though the hub will provide up to 500mA per port, this is shared across multiple ports and there may not be enough power to provide 500mA for every single port.

Secondly, the power required by a device as indicated in Under Device Manager -> Generic USB Hub -> Power Tab, may not be accurate in the first place, and it is only a power required figure and not the actual power drawn.

So to solve my problem, I'm trying to find out if there a way for me to detect ACTUAL maximum power that can be supplied to a USB hub? What I intend to do is to activate all the USB devices connected to the system, e.g. if it is a USB speakers, I would turn on the speaks at full volume, and then measure the maximum power a port can deliver using some instrument or software.

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There is an easier solution then what you want to do. Purchase a USB Hub that has an external power source. This way you are guarantee that the USB Hub will be able to supply 500mA which I believe is the maxium amount a single port can provide. –  Ramhound Jul 11 '12 at 12:06
    
That is what I usually do but sometimes this doesn't solve the problem as the modem might have hung due to other reasons. I want to be able to rule out USB power problem from the start. –  Joshua Lim Jul 11 '12 at 12:42
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1 Answer 1

The only way to measure this exactly is using purely hardware, eg measuring the current using a multimeter. The basic concept would be to apply a resistive load to the Vcc and GND usb power pins and measure the current. Increase the load until the underlying usb power circuitry shuts down because you're drawing too much current. The value on the multimeter at that point would be the max power you can effectively get. If the port can supply 500mA, this would mean the lowest resistor value is about 10ohms due too Ohm's law. So a variable resistor ranging from 100 to 0 ohm is usable.

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Thank you, is there a device that I can buy? ;) –  Joshua Lim Jul 11 '12 at 10:35
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Note that the "shut down" of the USB port may be a permanent because a melting fuse or something else that breaks. –  Robert Jul 11 '12 at 11:53
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@Robert afaik by the time this would happen (if the hardware itself already hasn't a safty built-in that ties vcc to gnd or so) the os would already have disabled the port –  stijn Jul 11 '12 at 13:08
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@stijn But then your answer will not work because AFAIR an USB 1/2 port does only provide 100mA by default. If a device needs more it has to request it and is only allowed to consume more if the USB system has granted it. Therefore the shut down should occur much earlier if the port is really disabled by software. –  Robert Jul 11 '12 at 14:17
    
@Robert the numbers in my answer are merely an example. Want 100mA? then using ohm's law a load of 50ohm will draw max power allowed. –  stijn Jul 11 '12 at 14:54
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