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I am looking for a power supply adapter for my USB Hub so it can support at least 2 bus-powered external hard drives. The hub's rating is 5 volts, 2 amperes.

I would like to know if it would be OK to buy a power supply adapter rated at less than 2A. I've been looking for power supply adapters and the ones that do support 2A are more expensive (more expensive than the USB hub itself) compared to those that support less than 2A.

Will a power supply adapter that supports less than 2A (~1-1.5A) work fine to support two external hard drives?

The external hard drives are both bus-powered Western Digital My Passport Essentials 250 GB. The OS is Windows XP SP3.

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what size drive? 2.5" (laptop form factor) or 3.5" (desktop form factor)? being bus powered i'm assuming they'd be 2.5" (which have lower power req's than desktop drives). –  quack quixote Mar 14 '10 at 10:21
    
quack: The drives are 2.5". –  galacticninja Mar 14 '10 at 10:55

1 Answer 1

up vote 1 down vote accepted

As long as the voltage and polarity are correct, you should be fine.

If you do have problems, staggering the power-up of the drives by waiting between connecting them should help.

Current draw is additive, and the maximum power a device can draw over one USB port is 500 mA (or 0.5 amps) Two drives should be fine with a one amp supply (0.5 + 0.5 = 1 A).

Note: Some manufacturers violate the USB spec on their devices, and they draw more current then they should when they are powering up (this commonly causes problems on laptops). However, it is very unlikely that an accidental overload of a few hundred mA will damage anything, it would just cause the drives to not enumerate over USB correctly. If it's a problem, staggering the power-up of the drives will probably fix the issue.

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