Powered Hub w/ Raspberry Pi

ntewinkel  wrote:

 

1. If I supply external power to the hub, does it feed back to the Raspberry Pi it's plugged into?

1b. If so, is that a bad thing? Does it interfere with the main power supply to the Pi?

 

 

 

 

 

 

 

 

 

ntewinkel  wrote:

 

Hello!

 

I'm trying to sort out some suspected power issues with my Raspberry Pi by using a powered USB hub.

 

I have a couple of questions...

 

1. If I supply external power to the hub, does it feed back to the Raspberry Pi it's plugged into?

1b. If so, is that a bad thing? Does it interfere with the main power supply to the Pi?

 

The reason I ask is because on Adafruit they recommend "There is a 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." - I'm assuming that's to avoid the 5v going back to the host.

ps on that - if cutting is recommended, would a diode (spanning that cut) be a good idea to allow it to be used as a non-powered hub elsewhere?

 

2. Can a USB hub be powered through one of its ports? ie, It's a 4 port hub - Can I plug a powered usb cable into one of those ports to provide power to the rest of the things plugged into the hub?

 

And also sort of related:

3. Have you seen any powered USB hubs that use a micro-usb (or mini) port to accept power? That would make it easy to power it with a standard tablet charger or another dedicated Raspberry Pi charger.

 

Related notes:

I intend to use the hub to power a WiPi WiFi adapter and a generic sort of wireless keyboard (likely WiFi, maybe Bluetooth)

 

Thanks!

-Nico

I can't figure out how to quote you and then type a few sentences of my own not-marked-as-quote. My text keeps being marked as a quote and I don't want to put words in your mouth. Clicking "quote" again to try to go unquote" inserts a nested quote of your message again. as you can see.

So 1: Yes. Once the PI has booted it can use the power from the powered hub. 1b: Many people say to avoid this situation. Sometimes bad things happen when you design a system like that, but with two five volt power supplies things are usually OK. One of the things to watch out for is that if your pi requires 1A and you're using a 1.5A supply for that, it should work. Then if your USB devices off the hub are using 1.5A and you're using a 2A power supply for that, you would think you should be ok... but couple those together, and unexpected currents might flow between the two supplies. In effect you could be running the everything off the 2A powersupply, which might then shut down: "overheating", leaving the 1.5A powersupply to handle everything and immediately shut down to "overcurrent".

(I count the pi as "1A" which is "high". The  foundation recommends even more, but that's because they are expecting you to power USB devices from the pi, that in your case are hanging off the powered hub. )

An option would be to have the pi being powered by the hub as well. Just use a cable to plug it into one of the slave ports as well. You're 100% certain creating a "ground loop" in that case, but I expect this situation to be better than with multiple powersupplies.....

2) The USB standard promotes that HUB manufacturers "regulate" the power going to each port. Then the computer can monitor the power devices are using, and cut off devices that are overloading the system. In practice, this is not always done and there is simply one 5V busline that goes to all ports, both the upstream, the power input and the downstream ports. Thus: yes, often a hub can be powered from a downstream device.

3) You can buy SUPER cheap usb hubs on ebay. They cost next to nothing, and if you buy a "high speed" one, you can be assured that it is not, so it will become even cheaper when you complain about that. You can use that as a "power distribution" point. One of those small coaxial plugs for power in, and 4x USB out.

I ran a couple of tests today.

I found a USB hub with a 5v wall adapter that uses the same 1.7mm plug as the cheap one I had on my desk.

First with a cheap USB hub...

And then with the higher quality USB hub (which is also a card reader). Note that the green power lamp is indeed on...

So that answers question 1: For cheap USB hubs, yes the power could be backfeeding. For better quality USB hubs, it doesn't.

ps, another test with the higher quality hub shows that power from the host does still go through to power the ports if the hub is not plugged in with the adapter. Good design.

Another test, to answer question #2, and also addresses #3.

Looks like all the 5v and gnd lines are just connected to each other.

This means you can power these inexpensive little hubs using cheap and plentiful chargers/adapters.

But for this situation, it seems like a good idea to follow the Adafruit advice to disconnect the 5v line to the host.

Interesting find:

This page shows a list of powered hubs that work well with Raspberry Pi:

https://elinux.org/RPi_Powered_USB_Hubs

And it answers 1b:

"...if the HUB tries to feed the Raspberry Pi through their interconnect cable, due to the 100 mA limiting fuse in the Raspberry Pi, the Raspberry Pi will be partially powered which may cause problems (unwanted writes to the SD card)."

For hubs that backfeed power, they recommend powering the hub and the Pi from a single power bar so they can be turned on simultaneously.

With the PI3 the raspberry simply does not get any power from the hub unless it is powered up from its own power connector.

Once the raspberry pi is booted up, it enables a power switch between its USB connector and its own 5V. This is a bidirectional switch and it is strong enough (not a 0.1A fuse) to power the pi flawlessly. (I power my hub from a 15A@5V  PC powersupply.)

That switch in the PI does have some smarts: I understand you can tell it to limit the current to a certain value. Maybe 100mA for small USB devices. But it seems that it does not measure the current flowing the other way.

I can't figure out how to quote you and then type a few sentences of my own not-marked-as-quote

I've had some annoying side affects as well, and have resorted to typing what I want, then going back to where I want to copy and paste the sentence, then highlight and hit quote.

I've done it here.

Mark

An option would be to have the pi being powered by the hub as well. Just use a cable to plug it into one of the slave ports as well. You're 100% certain creating a "ground loop" in that case, but I expect this situation to be better than with multiple powersupplies.....

Multiple power supplies are going to try to fight each other, and as usual the bigest will win.

So yes the slave port feeding the Pi is better than multiple power, but you do run the risk of excessive voltage drop in the cheap units.

For downstream USB devices, having the power coming from another source should be fine BUT the ground needs to be there to stop them 'floating apart' and causing issues.

A diode in the positive line will certainly stop it backfeeding the host (Pi), however you'd need to use a Schottky diode and even then you'll drop 0.2v which could be enough to cause issues.

So you may want to use a USB to USB with a switch in place of the diode, so you can isolate it if necessary.

Mark

genial