Monday, April 3, 2017

Badly Behaving USB 3.0 Hub?

I just ran into an interesting issue with my desktop computer:

I have a 7-port USB 3.0 hub attached. It has been part of my system for a short while and have had no issues until recently.

I usually just plug in USB 3.0 thumb drives into the hub, so I haven't had a need to use the +5V/2A adaptor that comes with the hub. But recently, I have been plugging in a portable USB-powered hard disk into it and I decided I had better use the adaptor to make sure all current requirements are met.

Well, I started having some weird keyboard issues this morning, and I decided to shutdown (turn off) my system for a short bit and then reboot. But when the system shutdown, the drive LED was still lit!

I didn't want to do a hard shutdown, for fear that something being written to the hard disk would get corrupted (although, I didn't hear any typical hard disk noises).

Then I happened to look at the USB hub with the power plug going into it. I removed the plug and ... the drive LED went out! It turns out that the USB Hub was feeding the external +5V from the adaptor back into the motherboard! YIKES!

So - I opened the hub and identified the trace that was bringing +5V from motherboard via the main USB plug of the hub and ... just cut it.

Now, I will _always_ have to have the +5V adaptor plugged into the hub - it will no longer power by itself when plugged into the motherboard USB. But that's okay, because I planned to use the +5V adaptor anyway, and now the USB hub power does not feed back into the system. Annnnd it shuts down properly now.

But ... I wonder who is at fault? Is the motherboard supposed to block +5V coming back in from a hub? Or is the hub supposed to keep it's +5V from feeding back into the motherboard?

I could install a 1N4007 diode as a blocker between the +5V from the hub and the +5V from the desktop, so it won't feed back into the motherboard, but ... I worry about the 0.7V drop across the diode. Would the voltage inside the hub be only +4.3V when connected with the AC adaptor? Maybe. 

I need to do a few experiments to confirm that but not today. I have work to do and everything is working well enough. For now.

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