0

USB Mini Hub with Power Switch
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

USB Mini Hub with Power Switch

by monore on Sun Jun 05, 2022 9:32 pm

Looking at the USB Mini Hub with Power Switch Product ID: 2998 (https://www.adafruit.com/product/2998). The documentation states:

There is a 1.35mm DC port for an external power supply but its connected directly to the 5V line so if you need to externally power, we recommend prying open the case and cutting the red 5V line from the USB connection.


The keyword here is "Recommend". Why is this recommended? I feel that this should either work or not work, when I read "recommend" I feel that if I plug an external power supply without cutting the red wire I may or may not break something.... (what is it!!!???

Can someone from Adafruit please be specific and affirm wheathear you absolutely should or should not connect an external power supply to that hub without modifications to the hub?

If you must cut the red wire, can you please explain why is this necessary?

Thanks.
Last edited by monore on Sun Jun 05, 2022 9:37 pm, edited 2 times in total.

monore
 
Posts: 14
Joined: Mon Sep 27, 2021 1:54 pm

Re: USB Mini Hub with Power Switch

by monore on Sun Jun 05, 2022 9:34 pm

oh one more question. I would like to use something like the following to power the hub:

https://www.amazon.com/CableDeconn-1-35 ... B01F1VT2X8

The idea here is that by powering the hub from another USB port, the hub can get the needed extra juice to power other usb plugged to it. Is this a good idea?

monore
 
Posts: 14
Joined: Mon Sep 27, 2021 1:54 pm

Re: USB Mini Hub with Power Switch

by adafruit_support_mike on Wed Jun 08, 2022 5:41 pm

monore wrote:The keyword here is "Recommend". Why is this recommended?

Older versions of USB forbid downstream devices from sending power back to the hub. USB-OTG changed that by letting devices be both USB hubs and USB devices.

Because of that, USB-OTG devices don't have any isolation between their own supply and their USB ports.

It's generally a bad idea to put power supplies in parallel. The voltages are never exactly the same, and the one with higher voltage will try to push current backwards through the one with lower voltage.

That's a problem because all power supplies can be modeled as a perfect voltage source in series with a resistor. The resistor represents the power supply's tendency to lose voltage when the current load increases. We want power supplies to maintain voltage across a wide range of loads, so we want the series resistance to be as low as possible.

Putting two power sources in parallel ends up being equivalent to putting two low-value resistors between two different voltages. Even a small voltage mismatch allows a large amount of current to flow. Most power supplies aren't designed to handle back-power, and will probably be damaged by it.

The OTG USB hub is mostly used with the RasPi Zero, which is an exercise in designing down to the lowest price point. Its power system doesn't have things like protection from back-power coming through the USB hub while the Zero gets power from its micro-B jack.

That's a recipe for people killing Zeros as fast as they can plug the hubs in, which we don't recommend.. or refund or replace.

adafruit_support_mike
 
Posts: 64984
Joined: Thu Feb 11, 2010 2:51 pm

Re: USB Mini Hub with Power Switch

by monore on Wed Jun 08, 2022 5:57 pm

Thank you, this is very helpful. Two quick more question if you don't mind:

1) How many amps can the USB hub supply to its connected devices without assistance of an external power source? Can it provide all the amps the Raspberry Pi 4 can deliver (I believe is 1.2 amps)?

2) If the USB hub can't supply the 1.2 amps to its connected devices washout the assistance of an external power source, I assume that using a USB power jack (to provide extra amps) such as https://www.amazon.com/CableDeconn-1-35 ... B01F1VT2X8 would be perfectly fine on a Raspberry Pi 4? After all, the power is taken directly from the Pi USB which should provide the exact voltage so that would mean perfect harmony correct?

Thanks for your feedback.

monore
 
Posts: 14
Joined: Mon Sep 27, 2021 1:54 pm

Re: USB Mini Hub with Power Switch

by adafruit_support_mike on Thu Jun 09, 2022 7:12 pm

monore wrote:How many amps can the USB hub supply to its connected devices without assistance of an external power source? Can it provide all the amps the Raspberry Pi 4 can deliver (I believe is 1.2 amps)?

We don't have a spec for the hub's ports, so I wouldn't expect more than the USB-2 requirement of 500mA. It may be able to do 500mA per port, but I can't guarantee that.

Given the simple construction, I'd expect it to be limited by the USB port it's plugged into.

monore wrote:If the USB hub can't supply the 1.2 amps to its connected devices washout the assistance of an external power source, I assume that using a USB power jack (to provide extra amps) such as https://www.amazon.com/CableDeconn-1-35 ... B01F1VT2X8 would be perfectly fine on a Raspberry Pi 4? After all, the power is taken directly from the Pi USB which should provide the exact voltage so that would mean perfect harmony correct?

The Pi4's USB host is limited to a total of 1.2A from all the ports combined, with no per-port limits. That means the hub will also have an upper limit of 1.2A for all ports combined.

Given the Pi4's specs, taking power from another port wouldn't do anything useful. You'd just be splitting the 1.2A limit across two ports.

adafruit_support_mike
 
Posts: 64984
Joined: Thu Feb 11, 2010 2:51 pm

Re: USB Mini Hub with Power Switch

by monore on Thu Jun 23, 2022 5:06 pm

So just to recap. What would happen if I connect a 5V power supply to the 'USB Mini Hub with Power Switch' without doing any modification to it (no cutting hub cables) and given that the hub is connected to a Raspberry Pi 4.

Do I or do I not have to worry about it? No guessing here, I would need the answer to be 100% correct :)

Sorry, we have like 40 of this things and I prefer not to mess with them if I don't absolutely need to.

monore
 
Posts: 14
Joined: Mon Sep 27, 2021 1:54 pm

Re: USB Mini Hub with Power Switch

by monore on Thu Jun 23, 2022 5:54 pm

oh, almost forgot, what is the polarization on this? Is it center positive? Will something like this work:

https://www.amazon.com/Adapter-Power-Su ... C92&sr=1-4

monore
 
Posts: 14
Joined: Mon Sep 27, 2021 1:54 pm

Re: USB Mini Hub with Power Switch

by adafruit_support_mike on Fri Jun 24, 2022 7:14 pm

Our policy WRT connecting an external power supply to those hubs is "we *really* don't recommend doing that".

Asking for technical suggestions on how to make it work runs directly against that policy. Anything beyond, "seriously.. don't" is officially Not Our Problem.

adafruit_support_mike
 
Posts: 64984
Joined: Thu Feb 11, 2010 2:51 pm

Please be positive and constructive with your questions and comments.