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I'm shopping for SATA to USB converters. It looks like most need AC power cable as well as the actual converter.

I know that his one leeches the power off the host computer but also has two usb plugs. But it is also only for 2.5" drives, I need 3.5"

Now it looks like this cable is supplies only the converter, and you have to buy the power cable separate, correct?

Ideally I'd like a cable that only has one usb and no AC power cable. Is there any like that?

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up vote 8 down vote accepted

USB can provide 5V and 0.5A on a single connection, for a total of 2.5W. Consumer 3.5" hard drives draw around 7 or 8 watts while idle, with a little variation around that number depending on the specific drive. That suggests that a single USB port simply cannot power a SATA hard drive on its own, and all available 3.5" hard drive enclosures bear that out.

Some 2.5" drives, however, can be powered out of a single USB port.

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Not all 2.5" drives can be powered from a single USB port (or, more accurately, a single USB header); hence the two male USB connectors in the SATA to USB adapter pictured in the original question's first link. But the main problem is that 3.5" drives require 12V in addition to 5V. If you could power a standard 3.5" drive using only 5V, there would be an adapter cable for that. – rob May 19 '11 at 21:46
I admit that I can't assert that all 2.5" drives can be powered by single USB port, but certainly some can. I've got one. :) Good point though, I'll edit in a sneaky weasel word to hide my shame. – Lukasa May 19 '11 at 21:48
+1 for the weasel word :) – Mehrdad May 19 '11 at 21:49
@rob: I don't think it is due to the 2.5" drives' power requirements that requires the use of the double USB cable sometimes. It's because the current output from some computers' USB ports are lower than others. High performance 2.5" drives for servers, which are thicker, have a higher power requirement to laptop drives though. – paradroid May 19 '11 at 22:16
you can get dongles though that allow 2 USB ports to be power-combined into 5w. – djangofan May 20 '11 at 0:14

There are two issues at play: power consumption (in Watts) and the input voltages to the drive. As others have mentioned, the total power requirement for any consumer 3.5" desktop drive might be around 13W, which far exceeds what you can get out of a single USB port, which supplies 2.5W (5V * 0.5A).

The second issue is that 3.5" drives require both 12V and 5V DC power, whereas 2.5" drives only require 5V. USB 2.0 only supplies 5V.

You don't necessarily have to use an external AC adapter to supply the 12V for your 3.5" drive, but since USB only supplies 5V you need to get 12V from somewhere else, such as the following:

  1. an AC adapter that outputs 12V DC - this is what all the manufacturers do
  2. a desktop computer's power supply - you could wire your own 12V adapter that goes from the 12V DC IN jack on your SATA to USB connector, to the 12V and ground leads on one of the power supply's MOLEX or SATA power connectors (in a desktop computer). The obvious flaw with this approach is that it wouldn't be very convenient if you want to use the drive on a different computer, and would not work if you wanted to connect the drive to a laptop.
  3. a battery pack - portable and easy to wire up, but probably not a good idea because you could lose data if the battery died
  4. stepping up the voltage from several 5V USB headers - technically possible, but completely impractical on USB 2.0 or older ports (and still not very practical on USB 3.0). As others have noted, any random consumer 3.5" drive might require 13W to initially spin up the hard drive. A drive on my desk right now is labeled 12V, 0.5A (i.e., 6W). Assuming perfect power conversion efficiency, that means stepping up the voltage from at least 3 USB 2.0 headers (5V * 0.5A * 3 = 7.5W) or 2 USB 3.0 headers (5V * 0.9A * 2 = 9W). This particular drive also requires 5V, 0.85A--which could be taken partially from the leftover power supplied by the first few USB headers. Even if the labeled power consumption refers to the peak power requirements during spinup (rather than typical power requirements), we'll definitely need at least 5 USB 2.0 headers or 3 USB 3.0 headers to fully power the drive.

(NOTE: any modding or custom wiring is done at your own risk)

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That's not quite correct. It's possible to boost 5V to 12V using a boost regulator in the enclosure electronics. However, in supplying both 5 & 12V power to the hard drive it will draw more power than the USB can provide. It's really all down to what can be supplied, not the actual voltage as such. – Matt H May 19 '11 at 23:05
Yes, I'm aware that you can step voltages up and down, but figured it was outside the scope of the question since any such adapter would be impractical even with USB 3.0's 900mA * 5V. However, I thought you had to use an inverter, transformer, and rectifier to do it; I didn't know you could do direct DC-to-DC conversion--so thanks for that tip. – rob May 20 '11 at 0:58

It's not related to either AC or DC power. The drives themselves require DC regulated power. Some external drives use an AC adaptor, some DC. Inside the external usb enclosure are electronics that convert AC power to DC.

While it's true that 3.5" drives require a 12V line and USB supplies only 5V, this is not the main limiting factor. It's possible to boost 5V to 12V using a boost regulator circuit.

No, the problem is that the USB port cannot supply sufficient current to start spinning the larger drives and maintain that. And even with a 100% efficient 12V boost circuit it can't supply sufficient power to start the drive spinning and maintain it. It's obvious that a 2.5" disk being smaller will not require as much power to spin the smaller platters.

USB can only provide 500mA or 2.5W per port. Which is why you see 2.5" drives with 2 USB connectors. That can draw up to 5W. It may be possible to see a 2.5" drive work with 1 connector. That would mean that the USB port is supplying enough current. Not all computers USB ports are in spec and some may give slightly more than the 500mA needed.

Some drives like the new 3TB WD Cavier Green drives claim to have very low power consumption. However, it's near 13W in total to make it spin up. Most of that is needed on the 12V rail with only 2W on the 5V rail. While idle it'll use around 3-4W but when busy near 7W of power. Well beyond what USB can supply even with 2 connectors.

With 4 USB connectors we could in theory supply 10W of power. But, it's still not enough for the drive start up.

Now do you get the picture? it's just simpler to have an external power supply. Although I see that the eSata standard will soon supply drive power too as well as greater speed so I guess all these problems will go away.

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