Questions relating to undervoltage in B+

[tpylkko]

I know next to nohing about Linux operating systms or electronics, so bare with me, even if this is dumb.

I have a B+ and I have set up two micro sd cards, one with Openelec and another with Raspbian. (I didn't realize when I did this that with a NOOBS one could easily set these two up on the same card via a GUI).

Now, using Raspbian I frequently get low voltage warnigns, or this is how I interpret the fact that a rainbow square appears in the upper right corner, and the red led on the board goes out. Openelec never does this.... In Raspbian, this happens when I have hdmi plugged into a screen, a mcrosoft usb mouse, a 20 mA usb minikeyboard, and a usb wifi dongle. Interestingly, this does not happen with the same setup in Openelec. I am not overclocking, both of the sd cards are class 4 Kingston. The power suplly unit is from PiHut (marked 2A 5V) and claimed to be desgined to work well with B+. Now, I don't understand why this would be happening, and what is the significance of this.

Several threads on this site seem to claim that undervoltage is a serious problem. However, I am not sure if I understand why this would be so. Even thoguh the red led goes out for max one minute at a time, nothing significant appears to happen to the useability of the devices, i.e all the USB devices remain online, no crashes etc. I understand that card corruption may occur, but I have backup images of the cards, and in the case that the cards stop functioning (something that has not happened in the last two weeks that I have been experimenting with this) I have understood I could just reformat them and burn the images back on the cards. Apparently the cards can also be destroyed permanently, but micro sd cards are now relatively cheap (around 5€ for a 4G), so unless they start dying on a weekly basis, I'm not sure if I should care... Or does undervoltage cause other risks that I am not currently arare of?

At the end of the day, my question is, why does this occur on Raspbian but not on Openelec. Is there some setting in the config.txt or cmdline that I am not awre of. I tried setting max current output from config.txt to 1,2 amps just in case the wifi dongle was consuming too much, but this does not seem to have made much difference. What should I do? Should I even care?

[mahjongg]

The brownout detector of the PI that turns of the power LED consists of a detector that checks VCC (the 5V line), when VCC drops below (about) 4.64V it turns off the LED, and also signals to a GPIO, kernel software then generates "brownout messages", which other software (a daemon probably) uses to generate the brownout rainbow squares in the GUI.

Officially USB devices need 5V with 5% precision, but thee may be plenty that still work when the voltage drop much lower, specially USB 1 "one current unit" (100mA max) devices like (some) keyboards and mice may work with voltages as low as 4.0V. That doesn't mean the brownout detector is meaningless!

Reasons for not generating the rainbow square may be many, perhaps the current draw is not enough to make VCC drop below 4.65V, or the current draw is more regular without fast peaks, perhaps the kernel misses the feature to read out (or is interrupted by) the brownout detector and therefore doesn't generate messages, or the top-level software ignores those messages.

getting brownouts is a warning, a warning that something is amiss with the VCC, and that dips below 4.65V occur is what you can be sure off! Most often its not even the power supply, which may be a 3A 5V supply, often its the cable between the supply and the PI, which was designed as charger cable (chargers don't need 5V with 5% precision) not as a power supply cable, and have a minimum of copper wire in them (or they might even use aluminum, with crimped instead of soldered connections).

[tpylkko]

Yet, the worst thing that can occur is card corruption, is this correct? Or can something more serious such as permanent hardware damage occur?

If the PSU cable is the problem, then can one possibly get a better one than from a vendor which claims to have specifically designed their product for the use with RP? The cable is non-detachable btw. So, specifically, what product is better than this one?

Also, why would such warnings occur with one SD card but not the other, especially since they are both of same brand? For clarification: the red led never once goes off using the other sd card, while it constantly does so with the other one... why?

[FTrevorGowen]

Yet, the worst thing that can occur is card corruption, is this correct? Or can something more serious such as permanent hardware damage occur?
If the PSU cable is the problem, then can one possibly get a better one than from a vendor which claims to have specifically designed their product for the use with RP? The cable is non-detachable btw. So, specifically, what product is better than this one?
Also, why would such warnings occur with one SD card but not the other, especially since they are both of same brand? For clarification: the red led never once goes off using the other sd card, while it constantly does so with the other one... why?

W.r.t. to the SDHC card that does not appear to exhibit "brownout detection" - is that the one with Openelec on it? If so it's possible that it does not have the most up to date firmware on it (there were some changes to the firmware during the first week or two post "B+ release"). What versions (datestamp) of Openelec cv. Raspbian? (Raspbian is generally more up to date than the other O.S.'s)
Trev.

[ShiftPlusOne]

No permanent damage will occur. You may start seeing reduced network traffic, malfunctioning USB devices and the pi locking up. On the other hand, you might not notice any problems at all. Basically, if something doesn't work, fix your power supply before asking for help.

[tpylkko]

W.r.t. to the SDHC card that does not appear to exhibit "brownout detection" - is that the one with Openelec on it? If so it's possible that it does not have the most up to date firmware on it (there were some changes to the firmware during the first week or two post "B+ release"). What versions (datestamp) of Openelec cv. Raspbian? (Raspbian is generally more up to date than the other O.S.'s)
Trev.

Yes it is the one with Openelec. The image is from late september, and AFAIK the B+ was released in July, so probably not, although I'd have to check the firmware version and it's possible update. Something like this is what I was originally suspecting. Given that with the exact same setup with identical peripherials, sd card and psu, one card gives undervoltage warnings and the other nothing, even no knowing much about electronics and linux OS's one can naturally suspect that either the difference is in the card physically, or in some content that is on the cards.

Otherwise suggestions to get a new PSU or cable sound good, except that it seems that there is no way to know in advance which PSU might be good or not. People seem to give advice more based on presumptions and racism ("don't buy Chinese cheap) than on some objective verifiable citeria.

[mahjongg]

don't buy cheap Chinese stuff is a remark simply based on experience.

one card gives undervoltage warnings and the other nothing,

if you mean by nothing that not even the PWR LED is affected, then there can be just two reasons for that:

• No brownouts are happening,
  or
• The brownouts are too short to be visible, and there is no software in place that looks at the brownout detector.

Some software may tax the CPU/GPU more than other software, and using the CPU/GPU more, means drawing more current.

there are posts that document what happens with the current consumption when running various softwares.

[FTrevorGowen]

[
...
Otherwise suggestions to get a new PSU or cable sound good, except that it seems that there is no way to know in advance which PSU might be good or not. People seem to give advice more based on presumptions and racism ("don't buy Chinese cheap) than on some objective verifiable citeria.

I suspect it depends on where you find the suggestions or advice. FWIW, any advice I give is based upon experience and measurements (the latter can be found in my webpages**). On that basis I believe the main confusion lies in the fact that product labels for power supplies and ('phone) chargers contain the "same information" but the interpretation is different ie. a "true power supply" labelled 5V 1A should provide 5V at 1A (but not, necessarily, at 1.1A), whereas a charger would not be expected to do so, since a (fully) discharged battery requires a lower charging voltage to avoid excess heating of said battery. For that same reason, relatively "high resistance" connecting cables are an "advantage" for a charger.
Trev.
** starting here:
http://www.cpmspectrepi.webspace.virgin ... plies.html

[tpylkko]

[
...
Otherwise suggestions to get a new PSU or cable sound good, except that it seems that there is no way to know in advance which PSU might be good or not. People seem to give advice more based on presumptions and racism ("don't buy Chinese cheap) than on some objective verifiable citeria.

I suspect it depends on where you find the suggestions or advice. FWIW, any advice I give is based upon experience and measurements (the latter can be found in my webpages**). On that basis I believe the main confusion lies in the fact that product labels for power supplies and ('phone) chargers contain the "same information" but the interpretation is different ie. a "true power supply" labelled 5V 1A should provide 5V at 1A (but not, necessarily, at 1.1A), whereas a charger would not be expected to do so, since a (fully) discharged battery requires a lower charging voltage to avoid excess heating of said battery. For that same reason, relatively "high resistance" connecting cables are an "advantage" for a charger.
Trev.
** starting here:
http://www.cpmspectrepi.webspace.virgin ... plies.html

So, the unit I am using, if I understood you correctly, is a true power supply, and not a charger. It is marked at 5V 2A, so it should according to you, the voltage could drop to below 5 at over 2000 mA. How likely is it, however, that such a current is being drawn by the device, when it has only two USB devices connected, HDMI and is idle in Raspbian when the brownout warnings occur? I think that is extremely unlikely. Unfortunately, however, I do not know whether the cord is "thick" or not. I don't even know in what units or dimension that could be measured in. But since it is not a detachable USB-cable, if it is truely at fault, then the only choice seems to be to blindly buy another one and pray.

[aTao]

But since it is not a detachable USB-cable, if it is truely at fault, then the only choice seems to be to blindly buy another one and pray.

Or....
You could cut the cable near the PSU and splice in a thicker/shorter one.

[FTrevorGowen]

[
...
Otherwise suggestions to get a new PSU or cable sound good, except that it seems that there is no way to know in advance which PSU might be good or not. People seem to give advice more based on presumptions and racism ("don't buy Chinese cheap) than on some objective verifiable citeria.

I suspect it depends on where you find the suggestions or advice. FWIW, any advice I give is based upon experience and measurements (the latter can be found in my webpages**). On that basis I believe the main confusion lies in the fact that product labels for power supplies and ('phone) chargers contain the "same information" but the interpretation is different ie. a "true power supply" labelled 5V 1A should provide 5V at 1A (but not, necessarily, at 1.1A), whereas a charger would not be expected to do so, since a (fully) discharged battery requires a lower charging voltage to avoid excess heating of said battery. For that same reason, relatively "high resistance" connecting cables are an "advantage" for a charger.
Trev.
** starting here:
http://www.cpmspectrepi.webspace.virgin ... plies.html

So, the unit I am using, if I understood you correctly, is a true power supply, and not a charger. It is marked at 5V 2A ...

You misunderstand me - what I am saying is that a label stating "5V 2A" may be misleading (and often is) since that, on its own, tells you nothing about what the device was designed to do. However if the label also included the words "power supply" rather than "travel adapter" or "charger" then one would expect (true) power supply behaviour. Many years ago, when "switched-mode" power supplies were in their infancy, the physical (and cost) differences were more obvious and it was easy to tell a (car or motorcycle) battery charger from a power supply of similar voltage/current/power rating. Often additional data such as percentage "line" or "load" regulation was quoted in the specs. This is probably still true for "laboratory (bench) PSU's", but not for those devices dubbed "wall-warts".
Trev.

[blachanc]

If the PSU cable is the problem, then can one possibly get a better one than from a vendor which claims to have specifically designed their product for the use with RP? The cable is non-detachable btw. So, specifically, what product is better than this one?

Also, why would such warnings occur with one SD card but not the other, especially since they are both of same brand? For clarification: the red led never once goes off using the other sd card, while it constantly does so with the other one... why?

This cable solved my power stability issues:

monoprice 28/24AWG micro usb GOOD

I f you want my full story regarding voltage drop refer to this thread to understand what I mean: voltage drop per USB cable type

Hope this help.

Ben

[tpylkko]

I'm not sure that I can rely on myself enough to go through with the operation of splicing the cable etc. But perhaps I will try another PSU and the cable mentioned here first, then see what I can do with the existing one. Does anyone have any good links to any good prayers or magic to help?

[gkreidl]

I'm not sure that I can rely on myself enough to go through with the operation of splicing the cable etc. But perhaps I will try another PSU and the cable mentioned here first, then see what I can do with the existing one. Does anyone have any good links to any good prayers or magic to help?

This thread covers a similar question and provides a solution:
http://www.raspberrypi.org/forums/viewt ... 28&t=88427

[Joe Schmoe]

BTW (and FWIW), the best solution I can recommend to this problem is to follow these guidelines:

• 1) Use a powered USB hub with a good power supply - and power the Pi from the hub.
  2) Use the best cable you can scrounge up (Note: I'm going to try to buy one of the cables recommended by Ben)
  3) Plug as few of your USB peripherals into the Pi directly as possible. I.e., favor plugging them into the hub instead of into the Pi. My B+ was unstable w.r.t. power until I moved the keyboard/mouse from the Pi to the hub. It's kind of a shame, given that one of the main selling points of the B+ is the extra (4 total) USB ports, but alas, you can't really use all of them because of the power issues.

Notes:

• 1) WiFi dongles seem to be the worst in terms of sucking power. GIven that the OP's core question seems to be "Why should I care?", let me say that the two areas where I've had problems relating to power drops is the WiFi (keeps dropping out) and with over-clocking. BTW, the OP asked whether or not any permanent damage can result from under-voltage - and was, of course, told "No, that's can't happen." Well, for suitable values of "Permanent", the answer (IME) is "Yes" (a recent adventure with over-clocking, which left the machine unusable for about 8 hours). Believe me, when you think you've broken it, 8 hours can seem like "permanent".
  2) BTW, I've heard that most of the electronics of the Pi operate at voltages below the 5v standard and that the 5V is really only needed for the USB peripherals. This would imply that SD card corruption should not occur as a result of under-voltage, but yet such does not seem to hold in the real world...

P.S. There is more I could write on this topic, but this is enough for now…

[gkreidl]

@Joe Schmoe:

Basically you are telling people that the B+ doesn't do what it was designed for to do.

I bought my new B+ for one reason only: to get rid of the need to use a powered hub. And it does exactly what I was looking for: booting the root FS from a 2.5 USB HD connected directly to it. But I took great care to find the right power supply (no "USB loader") without the need to add one of those crappy USB cables. And of course I used the high power USB setting in config.txt.

BTW, I also tested it with a Ralink WiFi adapter and it worked without any problem (but normally I use wired ethernet).

[Joe Schmoe]

Basically you are telling people that the B+ doesn't do what it was designed for to do.

Yes. That has been my experience.

[tpylkko]

I tried reducing USB devices, but in my case that did not have any significant effect. However, I wrote and read large amounts of data to test the performance of a usb stick and the sd card (I know that this is causing wear and tear on them). When performing the I/O-events on the stick no warnings, when doing it on the card, however, brownout warning present almost the entire time. Now, should I take this to mean that the voltage drop is caused by the microsd card reader? It appears that when I move the Raspbian root parition away from the card that i get barely any warnings...

I have had two other items that I bought from The Pi Hut that were not up to par, and I had to return one item. I guess they quality is not a high priority for them. I no longer use them, and suggest that other don't either. I will try the USB cables and PSU mentioned here. Thank you all for the input.

[B.Goode]

I have had two other items that I bought from The Pi Hut that were not up to par, and I had to return one item. I guess they quality is not a high priority for them. I no longer use them, and suggest that other don't either.

I can't comment on the experiences you feel you have had, but I have to say that I have purchased from The Pi Hut on multiple occasions and have always had prompt, courteous service and good-value items that were exactly as described. I have no association with them other than as a satisfied customer, and will not hesitate to recommend them or use them again.

If you have had cause to return an item and had a refund or replacement it seems to me that is evidence that they do care. If you have had an item you are not happy with but have not reported to them it seems unfair to 'guess' at their priorities.

[mahjongg]

If the PSU cable is the problem, then can one possibly get a better one than from a vendor which claims to have specifically designed their product for the use with RP? The cable is non-detachable btw. So, specifically, what product is better than this one?

Also, why would such warnings occur with one SD card but not the other, especially since they are both of same brand? For clarification: the red led never once goes off using the other sd card, while it constantly does so with the other one... why?

This cable solved my power stability issues:

monoprice 28/24AWG micro usb GOOD

I f you want my full story regarding voltage drop refer to this thread to understand what I mean: voltage drop per USB cable type

Hope this help.

Ben

AWG means American Wire Gauge, and is an indication of the thickness of the copper wires inside the cable, the lower the AWG number the thicker the wires are.
https://en.wikipedia.org/wiki/American_ ... wire_sizes
I don't know what the dual AWG numbers mean, but 28 means 0.321mm and 24 means 0.511mm, so perhaps it means the wires are between 0.3 and 0.5mm thick, or it may mean the wire is 26AWG with a tolerance of +- 2 AWG points. If the MicroUSB cable is 50 cm long, the total length of the copper wire will be 100cm or 1 meter long.
if the cable is "worst case", that is at the thinnest specified, which may well be the default, then its 0.321mm thick, and has a resistance of 212.9 milliohm per meter, or 0.21 ohm per meter. And if one amp runs through it, the cable losses will be U=V*R = 1 A x 0.21 Ohm = 0.21 Volt. As 5% of 5V is the maximum allowed tolerance the maximum allowed drop is just 0.25 Volt! So IF the power supply is exactly on target and outputs 5.00 V an 28 AWG cable is just barely good enough for 1A!

I suspect cables that do not mention their AWG to be FAR worse, I've seen MicroUSB cables with wires as thin as a hair, that is about 0.1mm, or in terms of AWG 38, which has a whopping 2.164 Ohm per meter, and so loses 2.164 Volt, IF you manage to run 1A through it!

Ive even seen cables with hair thin aluminum wires, which have an even higher resistance than copper wires (about 1.7 times as much).

That is why you should never underestimate the importance of good cables!

you might think of the cable as an afterthought, but a mediocre cable can totally screw up the performance of even a good power supply.

And yes, a B+ can let 1.2A through to an external USB device, but if you use a bad cable that drops 10% of the voltage then its obvious your USB device might have problems with that!

[blachanc]

AWG means American Wire Gauge, and is an indication of the thickness of the copper wires inside the cable, the lower the AWG number the thicker the wires are.
https://en.wikipedia.org/wiki/American_ ... wire_sizes
I don't know what the dual AWG numbers mean, but 28 means 0.321mm and 24 means 0.511mm,

28/24AWG means:
28 AWG (thiner wires) for the data lines (irrelevant for powering PI) and 24AWG (thicker wires) for the power/ground lines.
A big portion of the "good" USB cable available are 28/28AWG, but this is not good enough with a B+ with large load on the 4 USB ports.

you can do the voltage drop calculation yourself (this is a calculation that does take in account the loss in connectors):

voltage drop calculator (ONLINE)

all calculation are done with three feet wire (around a meter):
5V/2A = with 28 AWG = 0.78 V drop. PI supplied at 4.22V
5V/2A = with 24 AWG = 0.31 V drop PI supplied at 4.69V
5.25V/2A = with 24 AWG = 0.78 V drop PI supplied at 4.47V

5V/1A = with 28 AWG = 0.39 V drop. PI supplied at 4.61V
5V/1A = with 24 AWG = 0.15 V drop PI supplied at 4.85V
5.25V/1A = with 24 AWG = 0.39 V drop PI supplied at 4.86V

as a comparison, breadboard wire:
5V 2A = with 22AWG = .19 V drop

Ben

[tpylkko]

So, I did this: Moved all the content on the previously mentioned problematic Kingston card to another card (Sandisk). So, this new card now contains the exact same setup. No brownout warnings anymore. This now makes me suspect that there is something physically wrong with that one particular Kingston card. I don't know how likely this is (that one card does this, while another card of same brand not), nor what exactly is causing this.

Thanks alot for all the information. I learned much new stuff, like about AWG.

[aTao]

AWG means American Wire Gauge, and is an indication of the thickness of the copper wires inside the cable, the lower the AWG number the thicker the wires are.
https://en.wikipedia.org/wiki/American_ ... wire_sizes
I don't know what the dual AWG numbers mean, but 28 means 0.321mm and 24 means 0.511mm,

28/24AWG means:
28 AWG (thiner wires) for the data lines (irrelevant for powering PI) and 24AWG (thicker wires) for the power/ground lines.
A big portion of the "good" USB cable available are 28/28AWG, but this is not good enough with a B+ with large load on the 4 USB ports.

you can do the voltage drop calculation yourself (this is a calculation that does take in account the loss in connectors):

voltage drop calculator (ONLINE)

all calculation are done with three feet wire (around a meter):
5V/2A = with 28 AWG = 0.78 V drop. PI supplied at 4.22V
5V/2A = with 24 AWG = 0.31 V drop PI supplied at 4.69V
5.25V/2A = with 24 AWG = 0.78 V drop PI supplied at 4.47V

5V/1A = with 28 AWG = 0.39 V drop. PI supplied at 4.61V
5V/1A = with 24 AWG = 0.15 V drop PI supplied at 4.85V
5.25V/1A = with 24 AWG = 0.39 V drop PI supplied at 4.86V

as a comparison, breadboard wire:
5V 2A = with 22AWG = .19 V drop

Ben

A point to remember... the current has to make 2 trips, there and back so the effective wire length is twice that of the cable...

I would guess that a USB cable specified with "dual AWG" means that the signal conductors are the thinner gauge and the power ones thicker (just a guess)

[tpylkko]

I finally found the final solution to this.

Previously I reported that one sd-card was the culprit. This turned out to be semi-true in the sense that using it caused brownout warnigns constantly. However, while the second card did not, I later noticed that many other cards also did, usually to a lesser degree. So, I invested in a ModMyPi PSU which claims to be specifically designed to give out 5.25V. I tried this with a 50 cm Samsung usb cord (probably really thin). Did not help. Then bought cheap Chinese noname 18 AWG 35 cm cord. After that, I have never ever seen the warnings again.

It's probably the case that the cord on the PiHut PSU is too long a thin. AFAIK it was desgined before the B+ even came out, so maybe it was never tested thoroughly on that platform.

[gkreidl]

Best use a PSU that doesn't need a separate USB A to USB micro cable at all, but has the power cable directly connected to it. They are usually much thicker than all these crappy USB cables.