pic of hard drive and enclosure in piecesI recently purchased an external USB hard drive enclosure for a 2.5″ SATA disk drive.

It came with a USB cable with two ‘heads’, presumably because the hard drive might draw more current than a single USB port is specified to provide (500mA or 2.5W @ 5V).

In the past, I’ve done measurements on 2.5″ hard drives, and found that they draw much less than 2.5 W even at spin-up (the drive is an Hitachi 5400 RPM model). At idle, the drive consumes  about 400 mW and when spun-down, about 100 mW. At spin-up the power consumption surges to about 900 mW, then drops back to 400 mW.

Surely then, this external enclosure could power this 2.5″ hard drive from a single USB port? Not so. It turns out that the circuitry inside the enclosure draws a significant amount of power. About 900 mW to be exact. I went on a search to figure out why…

The enclosure is based around a Sunplus SATAlink SPIF215A single-chip USB->SATA controller. Looking at it’s datasheet, it’s supposed to draw a maximum of about 310mW from 3.3V and 1.8V rails, provided by a EMP5523 dual-output linear regulator.

There are also 3 (read it, three) bright blue LEDs to show you that the enclosure is powered up. Two of the LEDs had 100 ohm current-limiting resistors, and the third a 1 Kohm resistor. The two LEDs with 100 ohm resistors consume ~120mW of power alone.

A single USB port on my desktop machine was unable to provide enough power to spin up the hard drive until I unsoldered the two brightest LEDs leaving a single not-quite-so-bright LED to tell me when my hard drive enclosure is on. The hard drive now spins up.

What’s the lesson here? Poorly designed electronic devices consume a crazy amount of power. Low-efficiency linear regulators waste power too. When I’m using this external drive on my Laptop, I want it to consume as little power as possible so that my battery lasts as long as possible!

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