Supercapacitor for HDD

Hi,

I am designing an external HDD case, and my goal is to make it be able to be powered from USB. The problem is that most 2.5 inch HDD have spin up current around 0.9 or 1A, which is higher than standard 500mA for a USB port. In my case, current is limited to 700mA, but it's still not enough. I can hear the HDD attempting to spin up, but no luck. I am wondering, would it help to put some supercapacitor or other capacitive solution that would be able to release enough energy for the HDD to spin up? Spin up time is up to 4ms, according to the datasheet. Thank you in advance for any help!

Andy

It's worth a try. However you may find that your USB port only provides 100 mA, until it has negotiated (enumerated) with a device. Not until then do you get the usual 500 mA.

https://electronics.stackexchange.com/questions/32867/supply-power-to-usb-with-external-power-supply

A value of 1F might be sufficient to give the hard drive a boost for half a second.

First charge the capacitor through a 50 ohm resistor. This will prevent current from going over 100 mA. It will take about 3 minutes to charge 1F to 5V.

Then connect the hd simultaneously to the capacitor and the USB port. If you're lucky, and there are no glitches, the hd will be recognized by the computer.

This will be an accomplishment. It may be more practical to power the hd externally. However the questioner in the above link said this was unsuccessful.

I have an external USB drive that came with two cables. One cable draws power from one USB port. The other transfers data to/from another USB port.

Your power requirements are 5Wp for 5ms or 25 watt-milliseconds.[mJ] but the capacitance required is for a 1% drop in energy stored at 5V ... Using 1/2CV² for 1% drop.

I will let you calculate supercap F value. Compare batteries with supercap if expensive, using 4AA's operating at 5v. Chemistry, cost and energy capacity, lifetime, ESR will guide your choice.