Electronics > Beginners
How do USB Powered hubs decide to share loads between hosts and PSU?
Musclor:
Hi, my first post here, been lurking for a while - thank you so much for this wonderful forum! :-+
I hope this is not a stupid question, but having bought a bunch of little USB volt/amp meters and hooking them up to my 4 Port USB 3.0 Hub (ST4300USB3EU from startech, technically a kvm without the video), i have discovered major discrepancies between how each 5v supply i used to power the hub behaved. Or is it the other way around?
Examples:
3 machines connected to the hub.
3 devices connected on the other side: 1 mouse (0.05a), a fancy led keyboard (0.3a) and an artificial load for fun (0.65a).
First test, no power in. To my surprise, even the hosts not benefiting from the devices shared the load. So that's 1a (total of the above) + 0.1 (to power the hub itself) /3 hosts = ~0.33a per host, well at least in theory, where my macbook only contributed 0.1, the kali linux laptop provided most of the load and the windows nuc on par with the mac.
So first question: what kind of magic is this? I thought hubs as per specs were supposed to feed from hosts up to 0.5a then use the external, here we have clearly some 'favorite' hosts pushing a lot more A to the devices.
Then the fun started. I'll spare you the details (took me 3 hours to test everything) but i plugged in the 5v dc adapter of the powered up a series of PSUs, with wildly different results despite using the same (20awg) cable.
For example, Using a helper with 2x pd and 2x usb 2.4a ports, plugged in one of the usb a i get 5.1v, 0.5a , 2.6w fed to the hub. The load on the machines varied based on how much was fed of course, proportionally to the above based results (with the kali linux laptop topping at exactly 0.5a, i imagine that's the limit allowed by the standard).
But plugging in a wonderful raspi3b+ 5.1v adapter from the mains gave me 5.2v , 0.6a for a total of 3.2w average.
It got really wild when various ports from another Helper branded (single pd port this time) gave me different , but consistent (tested 10x times each) readings: 5.2v for the QC3+ port with 0.67a, while the other port was giving me 5.0v and 0.4a.
For fun, I tried portable chargers, and even the cheap lame crap chargers they give you in pound stores. Ironically, some of the cheapest stuff did very well , while the 'official' mac 80+watts adapters spewed out 1.6w at most.
While the result per brand/adapter and even ports were all over the place, they were consistent - meaning i didn't mess up my readings.
So, my stupid question:
WHY? :-DD
... why does the hub pull 'variable' amount of voltage/amperage from each PSU?
.... why do some fare WAY bettter than others, to the point where the osx adapters coundl't even power up the keyboard and the mouse kept disconnecting?
... is it the hub that decides? Or is it just some form of 'quality' factor associated to each charger?
.. and finally... why doens't the hub take ALL the power from the PSU when we provide it? Why only take up to 3.5w total (ish) and then split the load between host? And how is the split decided and by whom?
Thank you so much and i hope i'm not asking something too stupid :)
soldar:
>> 3 machines connected to the hub
I do not understand this. It is a hub which can only be connected to one machine/host.
grifftech:
--- Quote from: soldar on May 08, 2019, 10:55:06 pm --->> 3 machines connected to the hub
I do not understand this. It is a hub which can only be connected to one machine/host.
--- End quote ---
OP said it was a KVM switch with out the video, 3 computers "connected" only one communicating at a time.
soldar:
--- Quote from: grifftech on May 09, 2019, 02:06:58 pm --- OP said it was a KVM switch with out the video, 3 computers "connected" only one communicating at a time.
--- End quote ---
I know the op said that but I looked it up and is is a hub, not a KVM switch. That's why I am asking.
Brumby:
I'm also interested in how 3 machines are connected to one hub.
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