Anyone know if a usb hub and it's own power supply is isolated from the computers 5v rail?
Or does that 5v get distributed to everything on the rail?
In short... supplying a clean steady 5v supply via a filtered switching regulator to a usb hub... my concern is with how the psu will cope?
A USB port on a computer is isolated/regulated. The USB spec tells you how much current you can draw (which I don't remember... maybe about 1/2 an amp... and it depends on if it's USB 1, 2, or 3... I'm sure it's in Wikipedia, if you want to look it up.)
There are powed hubs like [u]this[/u]. A powered hub has it's own separate power supply. With a powered hub, each USB port can supply the current specified in the USB spec.
An un-powered (or self-powered) hub gets its power from the USB port it's plugged into. Of course in that case, the total power (current) cannot cannot exceed what the one main port can supply.
Yes...
So how tolerant is my psu going to be if I feed a second 5v (externally) source to the usb rail?
If it's a powered hub you have to assume they already thought of that and put something inside the hub to make sure it works....
I'd be careful saying "isolated" - USB ground is computer ground (usually mains
ground too), and will be shared in the hub too as USB signalling is not transformer
isolated (in contrast to ethernet). A powered hub just replaces +5V with a beefier
supply to avoid multiple 0.5A USB devices from overloading the computer's USB port.
The hub power supply may be implemented as isolated in order to allow for slightly different
mains grounds between computer and hub, but once you connect a USB cable between
computer and hub they are sharing ground. But a hub supply probably doesn't have to
be isolated - I wouldn't make such an assumption.
However there are USB hubs available which are fully isolated and use opto-isolated
data signals to allow it to work - they are meant for industrial environments where
large mains-ground discrepancies occur between equipment on multi-phase high current
power networks.