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USB Powered Devices

Gareth Owen

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Maybe a question better aimed at a computer forum, however :


How come I can run my Sound Devices USBPre




off a single USB socket, driving two 48v phantom powered condenser mics without a glitch, but when I want to connect a small external Lacie drive




to the same computer (or any computer for that matter) I need to connect two USB cables - one for data and one to pull the extra power?


I would have thought the USBPre would pull way more power than the motor in an external hard drive...

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As Chaz mentioned above, the current draw is the issue here. A condenser mic has a very high impedance (nominally about 10k ohm), which means that the current draw is very low. A standard 2.5" laptop-sized hard drive has a nominal current draw of 600-800mA. The rated supply current of a USB port is normally 500mA. Plus, the initial start-up current to start the motor spinning at 5400rpm (or 4200 rpm for cheaper drives) will be much greater.


It is also worth noting that the total USB current draw will have a limit dependant on the computer's motherboard and power supply. I have just bought a powered USB hub to get round this problem as I have multiple external hard drives. (Check eBay for a USB hub supplied with a 5V power supply of at least 2A)

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You will often find that the Hard drive only requires a standard mini Usb cable, without the second USB plug. It is mainly required on older machines and USB1.1 . We replace all these 2 - 1 USB cables with standard ones with no issues.


Not necessarily saying this is the correct way to do things, however we have had no issues.

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If it's a 3.5" hard drive (comes with a mains power adaptor) you shouldn't need a 2nd USB for power.


If it's a 2.5" hard drive (laptop type, powered by the USB only), the best way to check is to open the caddy housing and look at the label on top of the drive. Somewhere on it, it will say "5v xxxA" (where xxx is the current rating)


Look at this example which shows "5V 0.6A"

I know when I connect an external 2.5" hard drive, my laptop reports that the USB power has been overloaded, but then because it has already started to spin, I just click on the little USB icon in the taskbar, and then 'Reset' and it works fine.


A higher power drive such as 0.8A may have more of a problem...


Either way, your computer is able to monitor the load on the USB ports, and will effectively turn it off if it exceeds the safe limit. So basically, just try it!

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