PCIe USB 3.0 card w/ aux power (why?)

[swaaye]

Why does a PCIe x1 USB 3.0 card need supplemental power from a HDD/floppy power cable? Apparently USB 3.0 only specifies 900ma 5v so what gives? Doesn't PCIe allow up to 75W power draw?

 

[Npl]

Why does a PCIe x1 USB 3.0 card need supplemental power from a HDD/floppy power cable? Apparently USB 3.0 only specifies 900ma 5v so what gives? Doesn't PCIe allow up to 75W power draw?

PCIe x1 only guarantees 10W or something like that. still more than enough for sure.
I can imagine that its more efficient and cheaper to use a direct 5V feed, since PCIe only provides 12V and 3.3V, no need for power conversion that way.

 

[swaaye]

Yeah I was guessing that it was due to it being cheaper to make the card with the plug instead of have onboard voltage conversions and stuff. I've been running one of these cards without the aux power plugged in and it seems to work fine. I'll let you know if it explodes.

 

[Grall]

What kind of board is it? *curious!*

 

[swaaye]

It be this stumpy little thing.

http://www.rosewill.com/products/1551/productDetail.htm

 

[pcchen]

PCIe x1 provides 25W, but only 10W for low profile cards. Since a high-powered USB 3.0 device may draw as much as 900mA, it needs 4.5W. Two ports would be 9W. If you add the power requirement for the host controller chip, it's fairly close and may be problematic if the motherboard failed to provide a stable 10W power supply. That's probably why it has an external power connector.

If you use it on a normal motherboard (instead of a low profile system), it'd be able to provide 25W and you should be fine without the external power.