[SOLVED] Why do some PCIe USB 3.0 header cards require a power supply connection?

Sep 23, 2019
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I'm currently searching for a PCIe card that will add an extra USB 3.0 header to my motherboard, since it only has one header, and my case has two cables coming off of the front panel. I've been looking, and it seems like some cards have a SATA/molex power port on them, and some don't. What is the reason for this? I'd like to keep cabling to a bare minimum, and don't need extra USB 3.0 ports, so I'd prefer if it didn't need power if at all possible.

This is the one I'm currently looking at, even though it has issues with driver installation, since it doesn't need a power supply. Will this work for what I'm trying to do?
https://www.amazon.com/PCI-Ports-Header-Express-Dual/dp/B07GSQR6D4

More info: The motherboard I have is the Gigabyte Aorus Pro Z390, and my case is the CoolerMaster H500M.
 

Barty1884

Titan
Moderator
I could be wrong, but I've only ever seen SATA or Molex on a PCIe add in card that's adding both header(s) internally, and ports externally.

That would suggest a limitation based on number of ports/headers - and that's likely why.
In theory it doesn't make much sense, as even add in cards with 2x headers & 4 ports, you should only be looking at nominally more than the max theoretical power draw via any given port (4.5W). Every header can drive two ports, so in the worst case, 8 ports. 8x4.5W = 36W. PCIe can provide 75W.... In practice though, it's going to be a power draw consideration.
 

Barty1884

Titan
Moderator
I could be wrong, but I've only ever seen SATA or Molex on a PCIe add in card that's adding both header(s) internally, and ports externally.

That would suggest a limitation based on number of ports/headers - and that's likely why.
In theory it doesn't make much sense, as even add in cards with 2x headers & 4 ports, you should only be looking at nominally more than the max theoretical power draw via any given port (4.5W). Every header can drive two ports, so in the worst case, 8 ports. 8x4.5W = 36W. PCIe can provide 75W.... In practice though, it's going to be a power draw consideration.
 
Sep 23, 2019
3
0
10
0
I could be wrong, but I've only ever seen SATA or Molex on a PCIe add in card that's adding both header(s) internally, and ports externally.

That would suggest a limitation based on number of ports/headers - and that's likely why.
In theory it doesn't make much sense, as even add in cards with 2x headers & 4 ports, you should only be looking at nominally more than the max theoretical power draw via any given port (4.5W). Every header can drive two ports, so in the worst case, 8 ports. 8x4.5W = 36W. PCIe can provide 75W.... In practice though, it's going to be a power draw consideration.
I kinda figured it had to do with the presence (or lack) of external 3.0 ports. Thanks for the response! I'll just end up going with one that doesn't have external ports and save myself the hassle of running a power cable to a card that has them.
 

Barty1884

Titan
Moderator
Sep 23, 2019
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10
0
That's not a bad suggestion, and definitely was something I looked into when I started down this road. I may go for the adapter cable for a bit, and if I truly need access to 4 front-panel USB 3.0 ports, I'll pick up a PCIe card. Thanks again.
 
Because a PCIe x1 slot can only supply up to 25w. This is sufficient for 4 ports that will not be used for charging.

A high-power USB 3.0 device can draw up to 900mA or 4.5w. However a USB battery charging port is specified to supply up to 5A or 25w per port.

In practice, many devices such as phones or tablets or external HDD nowadays will draw 2A or more with data transfer enabled, so are kind of bending the rules. If you use these on a card without an internal power connection, then it is suggested to use only one device at a time, or else a powered external USB 3.0 hub.

Only the PCIe x16 slot can supply 75w.
 
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