I have a computer and a Powered USB hub. Specifically a Targus ACH82EUZ 7 port hub.

I have several devices on the powered USB hub (all powered from the hub) that I do not need on all day every day, so I thought I would put the USB hub on a timeswitch (or whatever) to save on the old global warming.

So far so good.

The problem seems to be when I switch off the powered USB hub. What happens then is that the hub turns into a bus powered hub, and all the devices attached to the hub then draw power over the bus which causes the computer USB port, well, let's just say gets hot.

So I thought, OK, make up a cable with the 5V disconnected. Ground and the difference lines should be enough for the computer to see the USB hub, but alas the computer doesn't see anything.

So then I thought, maybe the computer needs to see some current drain. So I put a 50R across 5V and gnd, to simulate a 100mA drain. The computer still doesn't see anything.

By now I was thinking maybe I've destroyed the USB port on the computer, but connect a normal cable and everything work OK.

I see there are things called USB isolators available. Would one of those allow me to switch off a powered USB hub (and all the attached devices) and not cause any current drain from my computer ? or are these devices for something else ?

The reverse problem does not occur, i.e. switching off the computer but leaving the Powered Hub switched on. It seems there is a diode blocking the 5V from the hub going upstream. I did try adding a diode to block the 5V going from computer to hub, but again, the computer fails to see the hub.

Any help/advice greatfully received.

 

One of these might work.

https://www.ebay.co.uk/itm/USB-Cabl...cat=0&_nkw=switched+usb+cable&_from=R40&rt=nc

At this price, you won’t need a bank loan.

 

Yeah, but that won't work will it.

I've effectively done that by disconnecting the 5V line. With the 5V switched off, the PC does not see the powered USB hub, even when the USB hub is powered.

 

The problem seems to be when I switch off the powered USB hub. What happens then is that the hub turns into a bus powered hub, and all the devices attached to the hub then draw power over the bus which causes the computer USB port, well, let's just say gets hot.

Have you considered unplugging the USB cable between the computer and the hub and disconnecting power from the hub?

With the data lines still connected between the hub and the hub not powered, it becomes, as you discovered, an unpowered hub and draws power from the USB host.

 

Unfortunately, the computer is in a remote location. Physically unplugging the USB cable is not an option.

(Later edit)

OK, I guess I could switch the USB 5V line through a relay, and make the switch on/off of the USB hub a two stage process. It just seems a bit overkill - but if there is no other way ...

 

If you are using a USB port add-in board in the computer, chances are it is a badly-made piece of junk that does not conform to USB standards.

A USB 2 port should limit the current to an individual port to 500 mA maximum, under any conditions. You are not entitled to higher current even as a transient beyond a few milliseconds. Add-in boards that I have seen are uniformly non-compliant. The best ones I've seen use self-resetting "fuse" type devices for current limiting. They are sloppy, slow and allow current well above what should be allowed, but they will at least interrupt current that is more than twice their rating sometime in same hour. Ten-times overload gets shut down quite fast. I've seen boards designed to have such parts but with the parts replaced with "zero ohm resistors" - jumper wires, presumably because some managerial idiot decided it would be good to save twenty cents per board. With these the current is interrupted when something burns up. If you are lucky, it is the add-in board and not the motherboard or power supply. I have yet to see a board that actually properly manages current limiting with active circuitry.

Many hubs are even worse.

 

Ha ! Ha !

Found the problem. It is down to the hub and the way it has been designed.

The hub chip is a 3.3V job, and it gets it's power from a voltage divider hanging off the 5V line from the computer USB. It does NOT use the DC in jack. The DC in jack just seems to go directly to the 7 hub ports.

So, with no computer 5V power, the hub chip is not powered, and nothing works.

I wonder why they did it like that ?