"William Sommerwerck" wrote in message
...
Perhaps I should have read a bit further. This is probably what you're
looking for...

Charging ports and accessory charging adapters
The USB Battery Charging Specification of 2007 defines new types of USB
ports, e.g., charging ports.[44] As compared to standard downstream ports,
where a portable device can only draw more than 100 mA current after
digital
negotiation with the host or hub, charging ports can supply currents above
0.5 A without digital negotiation. A charging port supplies up to 500 mA
at
5 V, up to the rated current at 3.6 V or more, and drop its output voltage
if the portable device attempts to draw more than the rated current. The
charger port may shut down if the load is too high.

Charging ports exist in two flavors: charging downstream ports (CDP),
supporting data transfers as well, and dedicated charging ports (DCP),
without data support. A portable device can recognize the type of USB port
from the way the D+ and D- pins are connected. For example, on a dedicated
charging port, the D+ and D- pins are shorted. With charging downstream
ports, current passing through the thin ground wire may interfere with
high-speed data signals. Therefore, current draw may not exceed 900 mA
during high-speed data transfer. A dedicated charge port may have a rated
current between 0.5 and 1.5 A. There is no upper limit for the rated
current
of a charging downstream port, as long as the connector can handle the
current (standard USB 2.0 A-connectors are rated at 1.5 A).

Before the battery charging specification was defined, there was no
standardized way for the portable device to inquire how much current was
available. For example, Apple's iPod and iPhone chargers indicate the
available current by voltages on the D- and D+ lines. When D+ = D- = 2V,
the
device may pull up to 500 mA. When D+ = 2.0 V and D- = 2.8 V, the device
may
pull up to 1000 mA of current.[45]

Dedicated charging ports can be found on USB power adapters that convert
utility power or another power source - e.g., a car's electrical system -
to
run attached devices and battery packs. On a host (such as a laptop
computer) with both standard and charging USB ports, the charging ports
should be labeled as such.[44]

To support simultaneous charge and sync, even if the communication port
doesn't support charging a demanding device, so called accessory charging
adapters are introduced, where a charging port and a communication port
can
be combined into a single port.

The Battery Charging Specification 1.2 of 2010 [12] makes clear, that
there
are safety limits to the rated current at 5 A coming from USB 2.0. On the
other hand several changes are made and limits are increasing including
allowing 1.5 A on charging ports for unconfigured devices, allowing high
speed communication while having a current up to 1.5 A and allowing a
maximum current of 5 A.




Thanks for that William

On the face of it, then, it would seem that a voltage on the data pins is
used to inform the USB device how much current it can take, which in the
case of the 'port' being on a 1 amp charger, would be the full spec whack of
1000mA. Since posting this, I had a closer look at the blown PSU, and the
two resistors form a potential divider across the 5v rail, so that would tie
in nicely. At least I know now that the extra wires are for the benefit of
the device hanging on the charger, and not to inform the charger about
anything, so I guess that confirms that I was just unlucky with the supply
that blew.

Arfa