Safely substituting a power brick
October 21, 2008 1:32 PM   Subscribe

Low-power device, how closely do I need to match the power supply ratings?

I've got a WD MyBook external harddisk which I'm using for laptop backups and since my laptop disk died a few weeks ago, I'm being quite paranoid about it.

Now, the MyBook is only a few weeks old itself and the power supply has just died, annoyingly. It was rated at 12Vdc and 1.5A output.

I have a drawer full of power bricks, and the closest match I have is 12Vdc and 1.3A output, short by 200mA. It powers up and seems to work just fine, but will the extra .2A make a big difference if the disk gets busy busy?

I guess I mainly don't see the point of sending the whole kaboodle back to WD if I can use what I already have. I know the voltage needs to be an exact match (done) but the amperage rating is simply there as current draw - will the disk actually draw those 200mA do you think?

posted by gkhewitt to Computers & Internet (6 answers total) 1 user marked this as a favorite
Go to a Radio Shack and pick up a multi-voltage power supply. It'll come with all sorts of end plugs and is switchable to match your voltage.

As far as amperage ratings, bigger is better. It may run just fine with 1.3A available, but it could cause that power supply to run hot also. Better off just spending $15 or so to get something that will for sure handle it.

Remember, more amperage (at the same voltage) is okay. Less is not.
posted by chrisfromthelc at 1:47 PM on October 21, 2008

Current is drawn, and not pushed--that is, you can't give a device too much current without also giving it too much voltage. So, it's entirely possible that the original power supply was over-rated for the actual current draw from the disk.

I'd just try it and see what happens.
posted by Netzapper at 1:48 PM on October 21, 2008

Best answer: Drives typically draw the most power when spinning up. So if it spins up, it's likely going to be fine.

From what I can tell the drive itself is likely to draw 600-700ma on the 12V line and 400-500ma on the 5V line. (Some of the 12V will be converted to 5V internally by the enclosure's circuits, with some loss -- so figure 250-300ma load on the 12V line.) That makes about an amp, and that still leaves 300ma to drive the enclosure's bridge board. Again, looks OK to me.

That said, you can probably get a replacement power supply that matches the specifications exactly on eBay for less than you'd pay to ship the drive back to WD.
posted by kindall at 1:50 PM on October 21, 2008

The power supply should have a current rating equal to or higher than the load.
posted by Class Goat at 2:18 PM on October 21, 2008

My take is that the original brick incorporated some margin. I would be REALLY surprised if it needed that extra .2A.

As kindall said, spin up is where the highest current demands are and that supply will likely be more than adequate for getting the disk rotating.

I'd use what you have for a few months. I think you'll be just fine.
posted by FauxScot at 3:31 PM on October 21, 2008

Best answer: You're probably fine, but you are definitely going in the wrong direction. Ideally, you want a power supply that can supply more current (amps) than the device needs. If the device draws 1.5A and your power supply is rated for 1.3, it may overheat and fail dramatically. Of course, you're only a little shy, and the 1.5A was probably including some kind of tolerance for error -- it'll probably be ok.
posted by knave at 5:40 PM on October 21, 2008

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