amperage double take

[sr1200]

Ive had this tiny little USB hub sitting behind my desk for about a year and a half now.  I recently replaced it with a new hub since i had reason to suspect the old one was beginning to fail.  When i disconnected it, i looked at the power supply (a small lightweight wall wart) and saw the rating on it... 5v 3.8A.  Could that be right?  3.8Amps.. my television doesn't even pull that, why would this little USB hub need that much room?

 

[zebra50]

20 Watt is not a huge power supply - possibly a standard unit.

USB is supposed to draw 500mA to 900 mA if we believe wikipedia...

http://en.wikipedia.org/wiki/Universal_Serial_Bus

So it its a hub it may need to send that out to four USB sockets.

 

[Speedskater]

Remember that power is Volts (times) Amps = Watts.

So:
USB is 5V (times) 3.8A = 19W

TV is 120V (times) 3.5A = 420W

 

[sr1200]

good point... just kinda took me back a bit since im used to seeing everything in mA

 

[JohnRoberts]

The external power supply has a maximum output rating. Conservative design uses a larger-stronger supply than needed. Any standardization within a company will get more volume from putting oversized supplies on products that don't really need them.

I used a 1A 16VAC wall wart on lots of (tens of ) different SKUs, most of which didn't need anywhere near the full amp. Using different smaller supplies might save pennies but create a major PIA for users and the company keeping them all straight.

JR

 

[Andy Peters]

Ive had this tiny little USB hub sitting behind my desk for about a year and a half now.  I recently replaced it with a new hub since i had reason to suspect the old one was beginning to fail.  When i disconnected it, i looked at the power supply (a small lightweight wall wart) and saw the rating on it... 5v 3.8A.  Could that be right?  3.8Amps.. my television doesn't even pull that, why would this little USB hub need that much room?

For a hub to be truly useful, it needs to be able to supply enough current for downstream self-powered devices to work. Each port, then, has to be able to source 500 mA for a high-power device (500 mA after enumeration). Recent revisions to the USB spec include a "battery charger" mode, in which a non-enumerated device identifying as a battery charger can draw up to 1.8 A.

Finally, if the hub supports USB 3.0 (they're blue), then the maximum current each port should be able to provide is 900 mA, so multiply that by the number of ports and add 100 mA for the hub's own use.

-a

 

[PRR]

7 ports at the old 500mA/each spec is 3.5 Amps; 3.8 covers the hub's own needs too.

> my television doesn't even pull that

No, but it sucks 120V, so each Amp counts 24 times more power.

> took me back a bit since im used to seeing everything in mA

I'm doing calculations on a 830 Amp (and 240V) system.

840 Amps is the Bolted-Short current, the Major Screw-Up condition. I need to know that the 100A breaker will blow fast-enough to protect $4,000 of wire to the street.

If I lived 10X closer to the street it would be 8,000A fault current. (Then the breaker *would* blow fast.)

If I lived 20X closer the fault current might be over 10,000 Amps, and then I would have to go to a different class (22KA) of service equipment. At these currents it is not just the heat: electromagnetic force rips the bus bars out of their mounts, then the breaker trying to stop the flood busts a gut (possibly never opening).