The term Universal Serial Bus (or USB in short) has become ubiquitous in
the parlance of computer peripherals. We hear about USB Mouse, USB pen
drives and many such stuff. All these devices operate with USB power.
This article is dedicated to a high-level overview of USB power and the
underlying technical concepts.
At the beginning, devices used to follow USB 1.1 specification. It had
limited data rates of about 12 Mbits/sec. This was later upgraded to USB
2.0, and thereafter to USB 3.0. The data rates increased up to 480 Mbit/sec.
Both these USB specifications can provide Direct Current (DC) to the
peripheral devices.
Technical Nitty-Gritty
Under USB definitions, a unit load is defined as 100 mA in USB 2.0.
This unit load was defined as 150 mA in USB 3.0. You can draw a maximum
of 5 unit loads (that is 500 mA) from a USB 2.0 port. In a USB 3.0 port,
you can draw 6 unit loads.
The devices connected through USB port can be classified into two types
– low power devices and high power devices. Low power devices are those
that draw at the most 1 unit of load with a minimum operating voltage of
4.4 V in USB 2.0. The minimum operating voltage in case of a USB 3.0
device is 4 V. The high-power devices draw the maximum number of unit
loads as permissible by the USB 2.0 and USB 3.0 standards. By default
all devices are configured to be low-powered. However, the device’s
software may request higher power as long as the power is available on
the bus.
Non-Standard Devices
There are some USB devices that require more power than available
from a single USB port. External hard and optical disc drives and
devices with motors and lamps are examples of such power-hungry devices.
To run such devices, an external power supply might be required. A
dual-input USB cable may also be used. In the latter case, one input is
used for power and data transfer, whereas, the other input is solely
used for power. Some of the non-standard USB devices bypass the
participating USB network and draw power from the host interface. These
devices are termed ‘USB decorations’. USB-powered keyboard light, fans,
mug coolers, heaters, battery chargers are typical examples of USB
decorations. These devices, in the absence of digital circuitry, cannot
be seen as standard-complaint USB devices.
Powered USB
Let us now learn a bit about powered USB. Powered USB uses standard
USB signaling with additional power lines. A powered USB uses four
additional pins to supply up to 6 A power at a voltage of 5 V, 12 V, or
24 V. The wires and contacts on the USB portion are sturdy enough to
withstand the higher current. The power is enough to operate stationary
barcode scanners, printers, pin pads etc. This modification of USB is
however proprietary and has been developed by IBM, NCR, and FCI/Berg.
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