[kwlug-disc] USB 2.0 hubs, and power ...

[Andrew Kohlsmith (mailing lists account)]

> On Oct 29, 2019, at 12:56 PM, Benjamin Tompkins <bjtompkins at gmail.com> wrote:
> You may be better off skipping the hub and looking for an extension cable and putting the joint where the bend is.  
> As for the passive hub, USB 2.0 specifies a max amperage of 500mA.  It should provide that to a single port if that's all that is in use.

Yes, USB 2.0 says max 500mA per port, but the quality of the cable is important here as well, as cheap cable tends to use higher gauge (smaller diameter) wire which increases the resistance in the cable. If the camera draws a significant portion of that 500mA you can lose voltage along the cable (V=IR) and end up underpowering the camera.

e.g. let’s say the camera draws 300mA at 5V (150mW), but the USB cable is cheap and you have a couple ohms of resistance in the cable.  V=0.3*2 meaning your camera is only seeing 4.4V. This has the additional side effect of the camera drawing more current for the same 150mW power, if it powers up at all.

A real-world, albeit nerdy, example: I have a dev board arrangement that I use for one of my current projects which allows me to surreptitiously work while on flights: https://imgur.com/a/KvQGYKO <https://imgur.com/a/KvQGYKO>. The two dev boards, ethernet switch, data logger and combo USB3 hub+ethernet draw a total of 0.8A at 5V (this is 4W; the box gets quite warm after several hours). This is fine for USB3, but with a 6’ extension cable the voltage at the box falls to 4.65V and the ethernet switch I originally chose was browning out and basically did not work. Since I wanted to keep this box hidden in my carry-on under the seat in front of me, I found another switch which was happier with this reduced input voltage.

The voltage drop across that extension cable is easily measured by comparing the voltage at the box with and without it: 5.1V without, 4.65V with, for 450mV of voltage drop across the extension cable. R=V/I tells us that the resistance in the extension cable is just over half an ohm, and since two conductors are at work here (the one carrying +5V and the return line), that means the 6’ cable offers about 0.3 ohms of resistance in each of the power conductors. This is a good quality extension cable, but it’s just a little too long for this specific application. Or rather, too long for that specific ethernet switch with everything else connected.

It’s almost impossible to know beforehand if a cable is good quality or not, especially online. My best suggestion is to find a cable with the physical properties you need and hope that the electrical properties are good as well.

-A.

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