Max current from USB port?

  • Thread starter Thread starter Piotr Makley
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Piotr Makley

What is the maximum current or power which the USB specs say can be
drawn from a USB port?
 
| What is the maximum current or power which the USB specs say can be
| drawn from a USB port?

500mA I believe
 
DCA said:
| What is the maximum current or power which the USB specs say
| can be drawn from a USB port?

500mA I believe
Click to expand...

ISTR that USB is at 5v so that is about 2.5 Watts.

Do you know what the figure is for Firewire?
 
DCA said:
| What is the maximum current or power which the USB specs say can
| be drawn from a USB port?

500mA I believe
Click to expand...

Or 100mA for:

- a downstream port on a bus-powered hub,
- a port on a root port hub on a battery-powered system (possibly), or
- a downstream port on a battery-powered self-powered hub (possibly).

And, as noted by another poster, this is at 5V, so the maximum power is 2.5W
(500mA) or 500mW (100mA).

Alex
 
Alex said:
Or 100mA for:

- a downstream port on a bus-powered hub,
- a port on a root port hub on a battery-powered system (possibly), or
- a downstream port on a battery-powered self-powered hub (possibly).
Click to expand...

- a port which doesn't think it's talking to anything (according to
the spec anyway, in reality you can often get 500mA or more just by
hooking up to the power lines).


Tim
 
Tim Auton said:
- a port which doesn't think it's talking to anything (according to
the spec anyway, in reality you can often get 500mA or more just by
hooking up to the power lines).
Click to expand...

True, but in normal operation this is a temporary state of relevance only to
developers of USB devices. And maybe hubs... are they supposed to limit the
current?

Alex
 
So you could power devices needing far more power on Firewire than
on USB?
Click to expand...

Not necessarily. Sometimes it supplies NO power whatsoever, like on a
laptop with only 4 pin, not 6 pins. Other times it may supply less than
the 1.25A, perhaps only 400mA... don't remember if there's a lower limit
to how much it "must" supply if it supplies *any* power. It may be best
to always use an external power source whenever possible.
 
Alex said:
Tim Auton said:
Alex said:
news:tlNec.16$KC6.12@newsfe1-win... [USB max current]
500mA I believe

Or 100mA for:
Click to expand...
[...]
- a port which doesn't think it's talking to anything (according to
the spec anyway, in reality you can often get 500mA or more just by
hooking up to the power lines).
Click to expand...

True, but in normal operation this is a temporary state of relevance only to
developers of USB devices. And maybe hubs... are they supposed to limit the
current?
Click to expand...

It's not totally irrelevant to us consumers though, as there are an
increasing number of USB-powered phone chargers and the like. AFAIK,
most don't bother talking to the USB host, they just suck power.

No idea about hubs.


Tim
 
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