geffers
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Back Voltage from USB Hub

Wed Sep 11, 2019 7:24 pm

Folks,

I recall in the early Pi days they mentioned 'back voltage' from USB hubs was an issue and that R-Pi recommended hubs had no 'back voltage'.

I recently purchased a 4 port hub from Pi Hut, it can work as a passive hub as well as powered, I recently discovered that if you power this hub and then plug it into one of the Pi USB ports the Pi powers up.

Is this 'back voltage' still an issue? I did initially intend to power the Pi as well as the hub and am now wondering what might have happened had I connected the power to the Pi before plugging the hub into the Pi's USB ports.

Geffers

Andyroo

Re: Back Voltage from USB Hub

Wed Sep 11, 2019 8:43 pm

I would never trust a hub as a power unit.

You can get little hardware dongles that drop the 5v (The Unexpected Maker had one on YouTube this week) or cables that have no power but data only.

A quick scan of a couple of the limited schematics do not show any USB data ports (except for the 4s USB-C) so I have no way of telling if there are any diodes for protection (and my eyes are not good enough to trace the lines).

geffers
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Re: Back Voltage from USB Hub

Thu Sep 12, 2019 8:47 am

It was never my intention to use it to power the Pi. I was very surprised when the Pi booted up as soon as I connected the hub to the Pi.

Initially I was impressed with this device as it had both the conventional 'A' connector plus a micro USB so could be used on Zero without adapters.

Not so sure now.

Geffers

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Burngate
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Re: Back Voltage from USB Hub

Thu Sep 12, 2019 10:06 am

The B+ had a AP2553 acting as a reverse-current blocking device.
Apparently the 4B does not.

One of the dangers is that two power supplies, as you have here, with slightly different voltages can fight each other, with unknown consequences.

Would it be possible to open the hub and cut the 5v trace to the input socket?

hippy
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Re: Back Voltage from USB Hub

Thu Sep 12, 2019 11:15 am

geffers wrote:
Thu Sep 12, 2019 8:47 am
I was very surprised when the Pi booted up as soon as I connected the hub to the Pi.
You don't mention which model of Pi you have so it is hard to tell if you should have been surprised or not.

trejan
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Re: Back Voltage from USB Hub

Thu Sep 12, 2019 11:28 am

Burngate wrote:
Thu Sep 12, 2019 10:06 am
The B+ had a AP2553 acting as a reverse-current blocking device.
Apparently the 4B does not.
4B has an AP2552 which is an active low enable version of the AP2553.

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BogoMax
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Re: Back Voltage from USB Hub

Thu Sep 12, 2019 4:51 pm

As I am interested in acquiring a powered USB hub I have followed this thread, but I'm still not clear on whether the Pi Hut accessory is not back-powering the Pi.
trejan's post seems to suggest that there is active protection against back powering yet the OP saw his Pi power up regardless (accepting that the Pi model wasn't stated)
I am using an HDMI-VGA convertor which needs all the power the Pi3B+ can supply via USB.
A powered hub to allow active peripherals such as wireless/kbd/mouse and an external USB HDD would be a boon.
Is there a safe hub compatible with a Pi3B+?
Did I mention that I love my Raspberry Pi(s) :D. Amazing power from a tiny device.

ProDigit
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Re: Back Voltage from USB Hub

Thu Sep 12, 2019 6:15 pm

On a positive note, you could use one of the hub's USB ports and power a pi with that. As long as the powered hub gets more than 2,5A at 5V (1A at 12V) on the input, you can save cables there. (Meaning using data connection + power connection to the hub, for a more stable power).

The problems usually with powering 2 power supplies, is a ground lift issue of one PSU, can cause the other to blow up or fail. So using only 1 supply is important.

As far as USB hub power issue,
When using cables that transmit just data, no power, theres still a common ground.

I would just power the pi from the hub, using 2 cables, and make sure the hub has a sufficiently high rated PSU.

ejolson
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Re: Back Voltage from USB Hub

Thu Sep 12, 2019 6:27 pm

trejan wrote:
Thu Sep 12, 2019 11:28 am
Burngate wrote:
Thu Sep 12, 2019 10:06 am
The B+ had a AP2553 acting as a reverse-current blocking device.
Apparently the 4B does not.
4B has an AP2552 which is an active low enable version of the AP2553.
How does this work? Does it prevent problems when the Pi is running from it's own power supply?

geffers
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Re: Back Voltage from USB Hub

Tue Sep 17, 2019 11:50 pm

Burngate wrote:
Thu Sep 12, 2019 10:06 am

Would it be possible to open the hub and cut the 5v trace to the input socket?
I'll use it just as a passive hub.

Geffers

geffers
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Re: Back Voltage from USB Hub

Tue Sep 17, 2019 11:50 pm

hippy wrote:
Thu Sep 12, 2019 11:15 am
geffers wrote:
Thu Sep 12, 2019 8:47 am
I was very surprised when the Pi booted up as soon as I connected the hub to the Pi.
You don't mention which model of Pi you have so it is hard to tell if you should have been surprised or not.
Only two USB ports so a very early one.

Geffers

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Re: Back Voltage from USB Hub

Tue Sep 17, 2019 11:55 pm

BogoMax wrote:
Thu Sep 12, 2019 4:51 pm
As I am interested in acquiring a powered USB hub I have followed this thread, but I'm still not clear on whether the Pi Hut accessory is not back-powering the Pi.
It was definitely back powering, the hub had a PSU plugged in, I connected the hub's input plug to the Pi's USB socket, no PSU was connected to the Pi but it powered up.

Geffers

geffers
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Re: Back Voltage from USB Hub

Tue Sep 17, 2019 11:59 pm

ProDigit wrote:
Thu Sep 12, 2019 6:15 pm
On a positive note, you could use one of the hub's USB ports and power a pi with that. As long as the powered hub gets more than 2,5A at 5V (1A at 12V) on the input, you can save cables there. (Meaning using data connection + power connection to the hub, for a more stable power).

The problems usually with powering 2 power supplies, is a ground lift issue of one PSU, can cause the other to blow up or fail. So using only 1 supply is important.

As far as USB hub power issue,
When using cables that transmit just data, no power, theres still a common ground.

I would just power the pi from the hub, using 2 cables, and make sure the hub has a sufficiently high rated PSU.
I didnt have to supply power to the Pi, merely plugging a powered hub into the USB socket of the Pi powered it up.

Geffers

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Re: Back Voltage from USB Hub

Wed Sep 18, 2019 12:27 am

geffers wrote:
Tue Sep 17, 2019 11:59 pm
I didnt have to supply power to the Pi, merely plugging a powered hub into the USB socket of the Pi powered it up.
Did you connect the hub using one of the output ports (in which case, the behavior is expected for the Pi you've indicated), or was it by connecting the hub's uplink connection? The former is not back powering.

In any case, the very early Pis could be back powered, as can the Pi0 and Pi0W.

geffers
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Re: Back Voltage from USB Hub

Wed Sep 18, 2019 9:15 am

W. H. Heydt wrote:
Wed Sep 18, 2019 12:27 am
geffers wrote:
Tue Sep 17, 2019 11:59 pm
I didnt have to supply power to the Pi, merely plugging a powered hub into the USB socket of the Pi powered it up.
Did you connect the hub using one of the output ports (in which case, the behavior is expected for the Pi you've indicated), or was it by connecting the hub's uplink connection? The former is not back powering.

In any case, the very early Pis could be back powered, as can the Pi0 and Pi0W.
It was very early Pi - only two USB ports.

Hub connected in normal way, Pi USB socket to hub giving 4 sockets available from hub. Moment PSU plugged into HUB the Pi powered up.

Geffers

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Re: Back Voltage from USB Hub

Mon Sep 23, 2019 8:55 am

BogoMax wrote:
Thu Sep 12, 2019 4:51 pm
As I am interested in acquiring a powered USB hub I have followed this thread, but I'm still not clear on whether the Pi Hut accessory is not back-powering the Pi.
The PiHut hub definitely back powered the Pi. No connection was made to the microUSB power input socket on the Pi.

It was a very early Pi (only two USB sockets), the hub's fixed data input cable was connected to one of the two Pi USB sockets, when power was supplied to the hub via a microUSB socket on the hub the Pi booted up.

Geffers

geffers
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Re: Back Voltage from USB Hub

Mon Sep 23, 2019 9:09 am

I'm a wee bit puzzled with this back power issue.

In the early days of Desktops, powered hubs were the norm otherwise, if used in passive mode the 4 (or 8) port hub could only use a maximum of 500mA being the current output of the single port the hub is plugged in to.

I never had even heard of the term 'back voltage' until the Pi was launched so why was it a problem for the Pi but not for conventional computers? Are later Pi devices protected, if so which one's? Is the Zero protected?

Reason I ask is that over the years I have acquired numerous models for various projects, I have four that have only got two USB sockets.

Geffers

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Re: Back Voltage from USB Hub

Mon Sep 23, 2019 9:11 am

I kind or skimmed through this thread but want to mention something I think is relevant. If it has already been mentioned my apologies.
The Pi Zero was designed to be able to be back powered though the USB Data port. It's so USB gadget mode works with just the one USB cable and no need for a separate power supply. I have no idea what other models may support this feature though. Probably some of the A models maybe?

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Re: Back Voltage from USB Hub

Mon Sep 23, 2019 9:53 am

R-Pi B Rev. 1.0 had a 140mA polyfuse feeding each USB socket from the 5v rail after the input polyfuse.
R-Pi B Rev. 2.0 & 2.1 had the USB sockets fed directly from the 5v rail after the input polyfuse.
For each of these, back-powering is possible.
Also, this generation is recognisable by having only two USB sockets, and a separate video socket.

R-Pi B+ and later have a chip that prevents back-powering.

R-Pi A (original version) had no protection from back-powering, and the SoC's USB directly connected to the socket, so could be used as a gadget, just like the Zero.
I have no information regarding either the A+ or the 3A+, but believe they can be used in gadget mode, and can be back-powered by a hub.
I never had even heard of the term 'back voltage' until the Pi was launched so why was it a problem for the Pi but not for conventional computers?
It's not seen as a problem, because the great unwashed use either a desktop or a laptop which take a great deal of power compared to the amount a powered hub could supply, and probably have protection built-in because there's plenty of room on board, and the cost is small compared to the rest of the circuitry.

The only other thing they use is a phone or tablet, which they want to use as a gadget hanging on their laptop.

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Re: Back Voltage from USB Hub

Mon Sep 23, 2019 9:58 am

geffers wrote:
Mon Sep 23, 2019 9:09 am
I'm a wee bit puzzled with this back power issue.

In the early days of Desktops, powered hubs were the norm otherwise, if used in passive mode the 4 (or 8) port hub could only use a maximum of 500mA being the current output of the single port the hub is plugged in to.

I never had even heard of the term 'back voltage' until the Pi was launched so why was it a problem for the Pi but not for conventional computers? Are later Pi devices protected, if so which one's? Is the Zero protected?

Reason I ask is that over the years I have acquired numerous models for various projects, I have four that have only got two USB sockets.

Geffers
I'm not certain that it wasn't a "problem" for "conventional computers". Having contributed to previous threads about this (and elsewhere before acquiring my first Pi from the "second batch" in 2012) and checked out quite a few devices I already had at that time (see: http://www.cpmspectrepi.uk/raspberry_pi ... esChk.html ),I recall that in at least one thread it was stated that, according to the specifications for a powered USB 2.0 hub it should not "back-power" (ie. have a protection diode or equivalent built-in). At that time, AIUI, USB 2.0 was still a specification defined by a "consortium" of USB (chip) manufacturers rather that an international (ISO) standard (and I'm not sure if it's a "standard" now - or rather no one here has referenced such AFAIK).
Trev.
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, A+ and a B2) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

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