LinuxQuestions.org
Visit Jeremy's Blog.
Go Back   LinuxQuestions.org > Forums > Linux Forums > Linux - Distributions > Slackware > Slackware - ARM
User Name
Password
Slackware - ARM This forum is for the discussion of Slackware ARM.

Notices


Reply
  Search this Thread
Old 03-19-2021, 12:04 PM   #1
business_kid
LQ Guru
 
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, RPi OS, Mint & Android
Posts: 12,531

Rep: Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559
USB Hubs question


Can someone briefly explain what goes wrong it a powered usb hub sends regulated 5V power back into the Pi 4?

/Background.
It's actually quite hard to get a separately powered usb hub here in the EU. Amazon are a washout, Europe seem to be poorly supplied also. Things from the UK have specious charges from Ebay & Amazon "in case we have to pay duty," … which they definitely do not to Ireland. Maybe if I was multilingual, I could choose better in Europe. Sadly, no.

I did eventually get this one because my sdd is draining the main Pi 4 PSU, voltage low warnings are onscreen and consequently keyboard/mouse perform poorly. If you find a closer, neater, better, cheaper one I really don't want the url, so please don't post links now. Don't you know that last request will send everyone opening a search tab ?
 
Old 03-19-2021, 12:09 PM   #2
Emerson
LQ Sage
 
Registered: Nov 2004
Location: Saint Amant, Acadiana
Distribution: Gentoo ~arch
Posts: 7,039

Rep: Reputation: Disabled
I'd use a separate PS for SSD instead of messing with powered hub. Don't know about you, but my drawer has plenty of parts, connectors, power supplies and whatnot to accomplish this without spending a penny.
 
Old 03-19-2021, 12:25 PM   #3
Ser Olmy
Senior Member
 
Registered: Jan 2012
Distribution: Slackware
Posts: 2,909

Rep: Reputation: Disabled
Quote:
Originally Posted by business_kid View Post
Can someone briefly explain what goes wrong it a powered usb hub sends regulated 5V power back into the Pi 4?
I don't know about the Pi 4 specifically, but I can tell you that in some cases, the slight difference in voltage between a powered USB hub and the +5V SB line on a PC PSU can cause enough current to flow to trigger the short-circuit protection on the PSU, preventing the PC from powering on.

(I put a piece of sticky tape on the +5V connector of the USB A plug to avoid this, and then everything worked just fine.)
 
1 members found this post helpful.
Old 03-19-2021, 01:06 PM   #4
michaelk
Moderator
 
Registered: Aug 2002
Posts: 21,181

Rep: Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945
The input cable power +5 VDC line should be isolated from the power supply so current should never flow back into the host. I would expect any powered hub sold to be isolated but these who knows about quality. I have read on the Pi forums where this has happened but I have never used a hub on my Pi. I have also read awhile ago that not all USB 3 hubs work with the Pi. I have also read of USB 3 devices interfering with wireless and causing power messages.

How many and what type of USB devices are connected to your Pi? The max current for USB is 1.2 amps that is shared between the ports. The min recommend power supply current rating for the Pi 4 is 3.0 amps. If you have a sufficient rated power supply the surge current of the SSD at start up might be exceeding its capability.
 
1 members found this post helpful.
Old 03-19-2021, 02:02 PM   #5
louigi600
Member
 
Registered: Dec 2013
Location: Italy
Distribution: Slackware
Posts: 619
Blog Entries: 19

Rep: Reputation: 76
I would see it in another way: if the external power supply for the powered hub is out of tolerance I would not be happy to power any device from it even if the RPi is protected from back current.
The RPi4 itself can only drive a max of 1200mA to all the usb ports which is enough for one full power usb3 device and not much more.

I only see one safe option here: make sure the external power supply for the hub is within tolerance (4.45-5.25 V) ... measured with a good multimeter (and maybe still guard the RPi from back current).
Any other option is either dangerous for the RPi or the usb devices connected to the hub or both.

The SSD should not be power thirsty, it's the spinners that are power thirsty.

If you are ok to bend the rules (and void your RPi warranty) you can try stealing current from +5V GPIO pins with a jump wire ... I've done that on one of my RPi4 and I can get 2 external 2.5" HDD (spinners)to work ... if you are going to go the same route I advise you use a power supply that can reliably deliver at minimum 3.5A ... and possibly also relieve the USB-C power port from having to handle all that current (to my understanding it can handle no more then 3A).

Last edited by louigi600; 03-19-2021 at 02:11 PM.
 
1 members found this post helpful.
Old 03-19-2021, 02:28 PM   #6
business_kid
LQ Guru
 
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, RPi OS, Mint & Android
Posts: 12,531

Original Poster
Rep: Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559
Quote:
Originally Posted by Ser Olmy
I don't know about the Pi 4 specifically, but I can tell you that in some cases, the slight difference in voltage between a powered USB hub and the +5V SB line on a PC PSU can cause enough current to flow to trigger the short-circuit protection on the PSU, preventing the PC from powering on.
Thanks for the consequences. There must have been a pretty significant voltage difference for that to happen. It conceivably could happen with an Analogue power supply, or poorly desighed smpsu as if one was excessively high, the other might regulate itself off. It Sounds more like one psu was not plugged in! PSUs typically drop up to 0.5V under load, so power sharing might not be 50/50, but it's rarely catastrophohic. I don't know if the usb hub powers it's feed, as many sellers don't know themselves. I've never got an answer on ebay to a tech question like that.

The devices on the usb on my Pi 4 are:
  1. USB SSD sold for usb-3.0(=500 mA) but rated on the drive as 1A(which they don't declare).
  2. Amazon cheapo keyboard/mouse wifi unit <500mA and probably much less.
  3. Ralink2.4G/5.0G wifi, 100-200mA at a guess.

No gpio is in use, and 1 video only. Clock is standard.
 
Old 03-19-2021, 02:46 PM   #7
michaelk
Moderator
 
Registered: Aug 2002
Posts: 21,181

Rep: Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945Reputation: 3945
I just read where the low voltage warning is set at 4.63V +/5%.

Back powering would not be a good thing and it could damage the Pi.

What is the power rating on the Pi's power supply?

Is the drive plugged into a USB 3 or USB 2 port. If connected to a 3 try a 2.
 
Old 03-19-2021, 02:47 PM   #8
business_kid
LQ Guru
 
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, RPi OS, Mint & Android
Posts: 12,531

Original Poster
Rep: Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559
Quote:
Originally Posted by louigi600 View Post
I would see it in another way: if the external power supply for the powered hub is out of tolerance I would not be happy to power any device from it even if the RPi is protected from back current.
The RPi4 itself can only drive a max of 1200mA to all the usb ports which is enough for one full power usb3 device and not much more.

I only see one safe option here: make sure the external power supply for the hub is within tolerance (4.45-5.25 V) ... measured with a good multimeter (and maybe still guard the RPi from back current).
Any other option is either dangerous for the RPi or the usb devices connected to the hub or both.

The SSD should not be power thirsty, it's the spinners that are power thirsty.

If you are ok to bend the rules (and void your RPi warranty) you can try stealing current from +5V GPIO pins with a jump wire ... I've done that on one of my RPi4 and I can get 2 external 2.5" HDD (spinners)to work ... if you are going to go the same route I advise you use a power supply that can reliably deliver at minimum 3.5A ... and possibly also relieve the USB-C power port from having to handle all that current (to my understanding it can handle no more then 3A).
Thx for reply. We cross posted, so let me address your points: I actually retain a good multimeter from my days in Electronics. I would consider 4.45V very low, as tracks across the board drop voltage. 4.8-5.2V is more like it. Power supplies under load are what matter. My Razpi Main supply may well be low, as the UK uses 240V RMS mains and Ireland uses 220V RMS. Anything but garbage power supplies should regulate that out. The '220V' is usually nearer 230V anyhow.

I'm perfectly fine with ripping up my warranty, if I still have any. Familiarity breeds contempt of everything inexpensive. I don't take engineer's specs too seriously because I've had to listen to the convoluted drivel and the dodgy math engineers reason through while writing specs, and I confess to doing my share also.

Last edited by business_kid; 03-19-2021 at 02:49 PM.
 
Old 03-19-2021, 02:56 PM   #9
Ser Olmy
Senior Member
 
Registered: Jan 2012
Distribution: Slackware
Posts: 2,909

Rep: Reputation: Disabled
Quote:
Originally Posted by business_kid View Post
Thanks for the consequences. There must have been a pretty significant voltage difference for that to happen.
Around 0.08 V in this case.

A year or so later I had to replace the filter cap on the +5VSB circuit, so it's entirely possible that the component was marginal even back then. I haven't removed the tape to see if the issue might now be resolved.
 
Old 03-19-2021, 03:33 PM   #10
louigi600
Member
 
Registered: Dec 2013
Location: Italy
Distribution: Slackware
Posts: 619
Blog Entries: 19

Rep: Reputation: 76
Quote:
Originally Posted by business_kid View Post
Thx for reply. We cross posted, so let me address your points: I actually retain a good multimeter from my days in Electronics. I would consider 4.45V very low, as tracks across the board drop voltage. 4.8-5.2V is more like it. Power supplies under load are what matter. My Razpi Main supply may well be low, as the UK uses 240V RMS mains and Ireland uses 220V RMS. Anything but garbage power supplies should regulate that out. The '220V' is usually nearer 230V anyhow.

I'm perfectly fine with ripping up my warranty, if I still have any. Familiarity breeds contempt of everything inexpensive. I don't take engineer's specs too seriously because I've had to listen to the convoluted drivel and the dodgy math engineers reason through while writing specs, and I confess to doing my share also.
I got the numbers after searching for usb3 voltage tolerance ... it was not out of my own considerations ... but I will say that I'm not too worried about too low as long as the usb devices can still operate, it's the too high that is dangerous.
If the numbers I found are not correct use whatever are the correct tolerances as guide. Agreed on starting at the very bottom will probably drag the voltage down below tolerance as you draw power.

But I do find comfort in 4.62 +/- 5% which would put the very lowest at damn close to 4.4v.
 
Old 03-20-2021, 07:09 AM   #11
business_kid
LQ Guru
 
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, RPi OS, Mint & Android
Posts: 12,531

Original Poster
Rep: Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559
I can't give you numbers, I'm afraid for an unknown situation.

I'm not exercised about the usb voltages as much as the chips. The earliest form of logic was ttl which needed 4.8-5.2 or it misbehaved. With a 5V supply, it seems the RazPi wants 5V which is certainly regulated down in sensitive areas. Power consumption in principally capacitive circuits like modern ICs is calculated by the formula
Code:
W=CVF
where
  • W = watts
  • C = capacitance
  • V = Voltage
  • F = frequency

So it's apparent from the formula that the less voltage the less watts, heat, etc. Every regulator, however needs an 'overhead.' You might not be able to deliver a regulated 3.3V without an insufficient overhead above 3.3V. Now Raspberry specified a 5V supply for their Pi 4. As frequency increases, voltage must reduce. But I take the approach of using what's specified as far as possible, although if I would expect up half a volt either way to be tolerated. My Pi is displaying a 'low voltage' warning sometimes, so I gather it's borderline. My experienced guess is that the cheapo supply is a regulated transformer o/p and doesn't have the overhead if the mains supply is low. Next stage is to ditch both supplies and buy a 5A switch mode one for both devices.

Last edited by business_kid; 03-20-2021 at 07:10 AM.
 
Old 03-20-2021, 02:15 PM   #12
TenTenths
Senior Member
 
Registered: Aug 2011
Location: Dublin
Distribution: Centos 5 / 6 / 7
Posts: 3,159

Rep: Reputation: 1359Reputation: 1359Reputation: 1359Reputation: 1359Reputation: 1359Reputation: 1359Reputation: 1359Reputation: 1359Reputation: 1359Reputation: 1359
Quote:
Originally Posted by business_kid View Post
Things from the UK have specious charges from Ebay & Amazon "in case we have to pay duty," which they definitely do not to Ireland.
At the moment Duty and Irish VAT have to be applied as anything from the UK is (post Brexit) now being exported to the EU.

So yes, they definitely DO apply to Ireland.

How it works (with Amazon anyway) is that they take the UK VAT OFF the price.

If the item is UNDER something like 25 then Irish VAT isn't applied. For example, a BluRay I purchased the other day is listed at 7, and I paid 5.83 for it.

If it's over the threshold but under something like 150 then Irish VAT is applied.

If it's over 150 then Irish VAT and customs duty is applied. Amazon apply this as an estimated fee, and if the charges are MORE than the fee they charge then Amazon will absorb the extra. If the charges are LESS than the fee they charge they will refund the difference.

I've had 3 refunds from Amazon recently, biggest one of them being 7 refunded on a 27" BenQ monitor.
 
Old 03-21-2021, 05:41 AM   #13
business_kid
LQ Guru
 
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, RPi OS, Mint & Android
Posts: 12,531

Original Poster
Rep: Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559
Perhaps we have a different account type but I'm getting ~20% charge on the price from Amazon, and vat isn't broken down. It doesn't encourage sales!

Ebay is much worse, with their 'Global Shipping charges.' My son has been through this with both companies as he's a regular online buyer, and he has got through on the phone to zombies repeating set lines from their answer books, or help pages. He is sharp enough to cut through all that and point out where he's being swindled. They just transfer him to other zombies…

Neither Ebay or Amazon have refunded €0.01.

Anyhow, if you're from Dublin, you'll know that there's the Anglo-Irish Free Trade Agreement going back to Gareth Fitzgerald & Maggie Thatcher (or John Major?)

Last edited by business_kid; 03-21-2021 at 05:45 AM.
 
Old 03-21-2021, 02:13 PM   #14
fatmac
Senior Member
 
Registered: Sep 2011
Location: Upper Hale, Surrey/Hants Border, UK
Posts: 4,072

Rep: Reputation: Disabled
If you aren't using the official power supply, do so, I have no problems powering & booting from an external 320GB HDD, with keyboard & mouse attached, works fine on both my RPi4B/4GB & my RPi400.

Edit: You might like to try The Pi Hut, it's where I get all my RPi gear.
https://thepihut.com/collections/raspberry-pi

Last edited by fatmac; 03-21-2021 at 02:16 PM.
 
Old 03-21-2021, 02:49 PM   #15
business_kid
LQ Guru
 
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, RPi OS, Mint & Android
Posts: 12,531

Original Poster
Rep: Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559Reputation: 1559
I have the official psu, and had forgotten the Pi Hut after Brexit. But if their P&P hasn't gone through the roof, business could indeed be done in future.
 
  


Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off



Similar Threads
Thread Thread Starter Forum Replies Last Post
Compatibility of Wireless USB Hubs (not the type that runs over network) spudgunner Linux - Hardware 2 06-19-2010 02:54 PM
change read/write permissions for USB hubs -- have I done it? redfoxbluefox Linux - Newbie 0 06-16-2009 05:28 PM
usb wifi and usb hubs ryan944299 Linux - Wireless Networking 1 01-11-2007 12:59 PM
usb hubs and linux dop Linux - Hardware 4 06-05-2005 04:41 PM
usb hubs and external hard drives buffed317 Linux - Hardware 1 07-20-2004 01:44 PM

LinuxQuestions.org > Forums > Linux Forums > Linux - Distributions > Slackware > Slackware - ARM

All times are GMT -5. The time now is 02:15 PM.

Main Menu
Advertisement
My LQ
Write for LQ
LinuxQuestions.org is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
Syndicate
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
Open Source Consulting | Domain Registration