USB Power Injector: Circuit Diagram for External Drives

Description

A portable USB hard drive represents a convenient method for data backups, however, what if your USB ports are unable to provide sufficient electrical current to operate the drive? A modified version of the Silicon Chip USB Power Injector presents a suitable solution. For some time now, the author has utilized a portable USB hard drive for data backup at the workplace. As with most such drives, it’s powered directly from the USB port, therefore, it doesn’t necessitate an external power adapter.

Project's Picture:

In fact, the device is powered from two USB ports, since one port lacks the capacity to supply adequate current. This is accomplished using a specialized USB cable that’s supplied with the drive. It features two connectors on one end, forming a “Y” configuration (see photograph). One connector is wired for both power and data transmission while the other connector contains solely the power supply connections. During operation, the two connectors are plugged into adjacent USB ports, ensuring that power for the drive originates from both ports simultaneously.

USB Cable:

An external USB hard drive is generally powered by connecting two connectors at one end of a specific USB cable into adjacent USB ports on the computer. This facilitates the acquisition of power from both ports. According to the USB specification, USB ports are rated to supply up to 500mA at 5V DC, and two connected in parallel should therefore be quite capable of powering a portable USB hard drive – at least in theory.

Complete Project:

Unfortunately, in my case, this didn’t quite materialize as anticipated. Despite the USB drive functioning adequately with several work computers, it proved to be a failure on my home machine. Instead, when plugged into the front-panel USB ports, the drive repeatedly emitted a distinctive chirping sound as it unsuccessfully attempted to initiate operation. During this process, Windows XP did recognize that a device had been connected, however, it couldn’t identify the device and certainly didn’t acknowledge the drive.

Plugging the drive into the rear-panel ports yielded precisely the same outcome. The problem was not limited to this particular drive either. A newly-acquired Maxtor OneTouch4 Mini drive also failed to power up correctly on my home computer, despite operating flawlessly on several work computers.

Circuit diagram:

The revised USB Power Injector consists essentially of a switch and a 5V regulator. The Vbus supply from USB socket CON1 activates transistor Q1, which subsequently activates power MOSFET Q2. This then feeds a 6V DC regulated supply from an external plug pack to regulator REG1, which in turn delivers 5V to USB socket CON2.

5V Regulator

A 5V regulator, often denoted as REG1, is a crucial component within the USB Power Injector circuit. This integrated circuit (IC) is responsible for stabilizing and maintaining a consistent 5V DC output voltage. Typically, a common choice is the LM7805, a three-terminal voltage regulator. This IC accepts a higher input voltage (often from the USB port – around 5V) and precisely regulates it down to a stable 5V, ensuring that the USB hard drive receives a reliable power supply. Its ability to manage voltage fluctuations is vital for the smooth and dependable operation of the drive.

Transistor Q1

Transistor Q1, often a MOSFET like the IRF1737, performs the initial switching function within the USB Power Injector circuit. As a field-effect transistor, Q1 acts as a controlled switch, enabling or disabling current flow between the USB port and the next stage of the circuit. When activated by the Vbus signal from USB socket CON1, Q1 conducts current, effectively powering on the power MOSFET Q2. Its fast switching speed and low on-resistance are important for efficient power delivery.

Power MOSFET Q2

Power MOSFET Q2, frequently an IRF2572, plays a key role as a power switch within the USB Power Injector circuit. As a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), Q2 provides a high-current, low-resistance path for power delivery. When activated by the signal from transistor Q1, Q2 conducts substantial current to supply the necessary power to the external plug pack. MOSFETs are favored for power switching due to their high current handling capability and fast switching speed.