Fig.1: the revised USB Power Injector is essentially a switch and a 5V regulator. The Vbus supply from USB socket CON1 turns on transistor Q1 which then turns on power Mosfet Q2. This then feeds a 6V DC regulated supply from an external plugpack to regulator REG1 which in turn supplies 5V to USB socket CON2.
For some time now, the author has used a portable USB hard drive
to back up data at work. As with most such drives, it is powered directly from
the USB port, so it doesn’t require an external plugpack supply.
In fact, the device is powered from two USB ports, since one
port is incapable of supplying sufficient current. That’s done using a special
USB cable that’s supplied with the drive. It has two connectors fitted to one
end, forming what is basically a "Y" configuration (see photo).
One connector is wired for both power and data while the other
connector has just the power supply connections. In use, the two connectors are
plugged into adjacent USB ports, so that power for the drive is simultaneously
sourced from both ports.
An external USB hard drive is usually powered by plugging two
connectors at one end of a special USB cable into adjacent USB ports on the computer. This allows power to be sourced from both ports.
According to the USB specification, USB ports are rated to
supply up to 500mA at 5V DC, so two connected in parallel should be quite
capable of powering a portable USB hard drive – at least in theory.
Unfortunately, in my case, it didn’t quite work out that way.
Although the USB drive worked fine with several work computers, it was a "no-go"
on my home machine. Instead, when it was plugged into the front-panel USB ports,
the drive repeatedly emitted a distinctive chirping sound as it unsuccessfully
tried to spin up. During this process, Windows XP did recognise that a device
had been plugged in but that’s as far as it went – it couldn’t identify the
device and certainly didn’t recognise the drive.
Plugging the drive into the rear-panel ports gave exactly the
same result. The problem wasn’t just confined to this particular drive either. A
newly-acquired Maxtor OneTouch4 Mini drive also failed to power up correctly on
my home computer, despite working perfectly on several work computers.