"Michael A. Terrell" wrote in message
m...

Arfa Daily wrote:

Anyone know if any USB pins other than 1 and 4, are used as part of the
USB
charger protocol ? I needed a small 5 volt PSU for a little project, so I
bought one of the latest generation 'Apple' chargers from eBay, where
it's
all built into a standard sized UK 13A mains wall plug. It came with a
USB
to Apple lead, so I cut the Apple connector off, fully expecting there to
be
only two wires - Gnd and +5v - in the cable, but there was actually 4
thin
wires. So I then assumed that they had maybe just doubled up on the
wires,
as this thing is rated at 1 amp output. But again, no. All four wires
were
separate, so I checked what was coming out of it. Just over 5v between
black
and red, so that's ok, but measuring between black and green, and black
and
white, there is also voltage. Two point something on one pair, and three
point something on the other, I don't recall the exact figures. Could
they
be just 'pullup' values to keep the "data +" and "data -" lines quiet
when
it has a genuine USB device connected to it ?

I'm not particularly concerned at this, because I am only interested in
the
5 volts, so cut the other two wires off. There is one thing that slightly
concerns me, though. A couple of months back, I had a similar need, and
purchased one of the slightly larger ones that you see on eBay - like a
small wall-wart. It went into service powering an ethernet wireless
bridge
dongle to allow my sat box to connect to the network. It's not a feature
that I use often, so I hadn't noticed that it was no longer working, and
you
can't easily see the supply, as it is plugged into an extender block
fixed
to the back of the TV. When I came to plug in my new supply to the same
extension block, I noticed that the other one looked 'skewed' in the
socket,
and the LED wasn't on. When I pulled it out, it was obvious that it had
'exploded' internally, blowing the lid half off. When I took it apart, it
had indeed had a major failure, with a big silvery blast mark where the
line
power came in via a small resistor.

When I looked at the USB connector on the bottom of this one, pins 1 and
4
were the 5v output and gnd, but pins 2 and 3 were joined together, and
had a
couple of resistors hooked to them, and disappearing into the circuitry.
So
why is this ? Do they perhaps make 'alternate' use of these pins for
flagging when charging is complete or something ? Even if they did, I
can't
see that it would be an issue to have the supply running continuously. I
have a similar one for my iPod that I leave plugged in all the time, and
90%
of the time, it's just a dangling USB lead so no load at all.

Was I just unlucky that a crappily built Chinese switcher failed on me in
just a couple of months, or did it get upset because it was not being
used
for its intended purpose, leaving pins 2 and 3 always open ? I just don't
want the same thing happening again to my new one ...


Check out this page for the resistors that are needed to turn on most
USB sources.

http://www.beyondlogic.org/usbnutshell/usb2.shtml

Cheers

Arfa