Very OT: USB 2 ports

500mA - about 4W in total power.

P.

I have a PSTwo, and a USB tv in card.

My plan was to run the now 12V console of a USB port.............

Well thats that out of the Q then!!

Cheers,

Mark.

Nope, and you won't get a Jump Lead adapter either!

What ? Surely 500mA is enough to run a knackerd starter motor with a sticky solenoid and turn a V8 over!!

Mark Solesbury wrote:

Funny you should be talking about USB current, because I am up to my eyes in USB documentation designing a new system at work.

AFAIK, although according to the USB spec, USB devices aren't supposed to draw more than 500mA**, and only then if they ask for it nicely, there are few USB ports that actually implement it properly. That said, you may well blow fuses set in the PCB design if you try it.

Steve

** From Maxim Semiconductors app note 3241.

What Your Mom Didn't Tell You About USB With any standard, it's interesting to see how actual practice diverges from the printed spec or how undefined parts of the spec take shape. Though USB is, with little doubt, one of the best thought out, reliable, and useful standards efforts in quite some time, it has not been immune to the impact of the real world. Some observed USB characteristics that may not be obvious, yet can influence power designs, are:

• USB ports do NOT limit current. Though the USB spec provides details about how much current a USB port must supply, there are mile-wide limits on how much it might supply. Though the upper limit specifies that the current never exceed 5A, but a wise designer should not rely on that. In any case, a USB port can never be counted on to limit its output current to 500mA, or any amount near that. In fact, output current from a port often exceeds several Amps since multi-port systems (like PCs) frequently have only one protection device for all ports in the system. The protection device is set above the TOTAL power rating of all the ports. So a four-port system may supply over 2A from one port if the other ports are not loaded. Furthermore, while some PCs use 10-20% accurate IC-based protection, other will use much less accurate poly-fuses (fuses that reset themselves) that will not trip until the load is 100% or more above the rating. * USB Ports rarely (never) turn off power: The USB spec is not specific about this, but it is sometimes believed that USB power may be disconnected as a result of failed enumeration, or other software or firmware problems. In actual practice, no USB host shuts off USB power for anything other that an electrical fault (like a short). There may an exception to this statement, but I have yet to see it. Notebook and motherboard makers are barely willing to pay for fault protection, let alone smart power switching. So no matter what dialog takes place (or does not take place) between a USB peripheral and host, 5V (at either

500mA or 100mA, or even maybe 2A or more) will be available. This is born out by the appearance in the market of USB powered reading lights, coffee mug warmers, and other similar items that have no communication capability. They may not be "compliant," but they do function.

You can get powered USB hub's can't you?. Would it not be possible to pull more current out of one of them? (and also cheaper to replace if you try to pull too much and break stuff!)

No, because the spec says 500mA per device.......

The point of the powered hub is to deliver that 500mA per port without overtaxing the root hub on the machine it's attached to.

Sorry.

P.

Modern USB ports usually have a self resetting "polyswitch" fuse. So nothing happens...when the developers did their homework :-)

regards - Ralph

...if they did their homework, a USB device can only draw 100mA, unless it asks for more and the system can grant it, up to 500mA..

Then again, you can get coffee warmers that plug into USB ports....

Steve

Hold on a minute - You lot gave me a right earful for posting some details of an XJS part I have for sale. Mr Solebury here posts a question that is not even about cars (No offence sir) and not only do you lot not chastise him - You positively help him with good advice.

Starting to think you lot have something against me.

Stu

On or around Mon, 13 Dec 2004 18:29:19 +0000 (UTC), "Stuart Adair" enlightened us thusly:

It is flagged "very OT", which allows anyone who don't want to look to ignore it.

IIRC, yours was neither OT nor FS or any such?

Personally, I look at most things, but some folk don't want to do that.

Hand Bags a 12 paces!!!!!

The one who said "Spam" was hardly supported here though was he ? Cheer up you old bugger.

Steve

The polyswitch is there to prevent the system from damage when more than 500mA are drawn. Of course usually it does not come to this point, because the system recognizes a power surge and shuts off the supply.

regards - Ralph

Why ? Do you want to run a winch off it ?

Peter R.