Granddad_Mike
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Low voltage warning on boot

Sat Feb 02, 2019 3:47 pm

I've just upgraded to a Pi 3 B+. On boot I'm getting a low voltage warning about halfway through, though the boot process continues until log in and then displays the low voltage warning, whereupon everything then hangs. I have tried two different PSU's, (One Apple, One Generic) requiring 100 - 240v AC and both supplying 5v and max power 2.4A. No accessories are connected except for a monitor. The house voltage is showing 230v but I don't know which are the power pin on the usb connection

pcmanbob
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Re: Low voltage warning on boot

Sun Feb 03, 2019 1:11 am

Hi.

I guess what you are actually using are battery chargers which often don't actually provide 5V at higher currents, this may be because a battery charger does not need to or that cable supplied with it may have small cores that can't handle the current.

Your best option is to use the official power supply which can supply 5V at 2.5A and has an 18 swg cable capable of handling the full current at 5V
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Imperf3kt
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Re: Low voltage warning on boot

Sun Feb 03, 2019 5:19 am

First, I would try a better usb cable.

While it is true most phone chargers are not particularly suited to be raspberry pi power supplies, they should at least do the job under light load.
Most usb cables you will find are only rated for about 0.5A, so even if your charger is rated to 'up to 2.4A', the cable will limit you to 0.5A and you'll get the low voltage warning. ⚡
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Re: Low voltage warning on boot

Sun Feb 03, 2019 7:11 am

Imperf3kt wrote:
Sun Feb 03, 2019 5:19 am
First, I would try a better usb cable.
While it is true most phone chargers are not particularly suited to be raspberry pi power supplies, they should at least do the job under light load.
The problem being that the boot process is anything but "light load" while lots of processes are starting up using all four cores. If you can't get passed that then (as the OP has demonstrated) having something that will work with a "light load" is pretty pointless :roll:

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Re: Low voltage warning on boot

Sun Feb 03, 2019 7:22 am


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Imperf3kt
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Re: Low voltage warning on boot

Sun Feb 03, 2019 7:25 am

PeterO wrote:
Sun Feb 03, 2019 7:11 am
Imperf3kt wrote:
Sun Feb 03, 2019 5:19 am
First, I would try a better usb cable.
While it is true most phone chargers are not particularly suited to be raspberry pi power supplies, they should at least do the job under light load.
The problem being that the boot process is anything but "light load" while lots of processes are starting up using all four cores. If you can't get passed that then (as the OP has demonstrated) having something that will work with a "light load" is pretty pointless :roll:

PeterO
That process shouldn't take any more than 1.3A, according to the power FAQ. My own measurements put it at much less (~0.7A)

A 2A phone charger should still be capable of 1.3A at 5v
I said "should", not "is", which is why the much cheaper option of trying a different cable first, is my recommendation. Then, if it still doesn't work, the power supply should be looked into.
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Re: Low voltage warning on boot

Sun Feb 03, 2019 1:22 pm

rpiMike wrote:
Sun Feb 03, 2019 7:22 am
Buy the official power supply:

https://www.raspberrypi.org/products/ra ... er-supply/

+1 simply the best !
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Re: Low voltage warning on boot

Sun Feb 03, 2019 3:50 pm

Imperf3kt wrote:
Sun Feb 03, 2019 7:25 am
PeterO wrote:
Sun Feb 03, 2019 7:11 am
Imperf3kt wrote:
Sun Feb 03, 2019 5:19 am
First, I would try a better usb cable.
While it is true most phone chargers are not particularly suited to be raspberry pi power supplies, they should at least do the job under light load.
The problem being that the boot process is anything but "light load" while lots of processes are starting up using all four cores. If you can't get passed that then (as the OP has demonstrated) having something that will work with a "light load" is pretty pointless :roll:

PeterO
That process shouldn't take any more than 1.3A, according to the power FAQ. My own measurements put it at much less (~0.7A)
And yet the majority of people with weak power supplies report problems (warnings) during the boot process.... I stand by what I wrote based on what people report.

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Re: Low voltage warning on boot

Sun Feb 03, 2019 7:41 pm

"majority"?
You know how many people have weak power supplies and have compared this to the number of people who reported power problems? Where did you source this data?



Just because lots of people report it, doesn't mean a lot of other people don't try replacing the USB cable first and not need to report at all. Several times someone has reported that the lightning bolt just keeps appearing, they have later returned to say that a better usb cable fixed their problems.

Why are you so adamant that the OP MUST buy a new power supply before trying the simple solution?

Also, if the USB cable is limiting things to 0.5A, then all those other people who reported issues might have had a decent enough power supply, but the limiting cable meant that they experienced problems.
If they fixed it by getting the official power supply, thats isn't proof that a better usb cable wouldn't have fixed the issue, it's just proof that they never tried a better usb cable.
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Re: Low voltage warning on boot

Sun Feb 03, 2019 8:02 pm

Imperf3kt wrote:
Sun Feb 03, 2019 7:41 pm
"majority"?
You know how many people have weak power supplies and have compared this to the number of people who reported power problems? Where did you source this data?
When you've been here for a few more years you'll understand ! :lol:
Why are you so adamant that the OP MUST buy a new power supply before trying the simple solution?

Where did I say that ? I pointed out the fallacy of your view that a solution that would work for a "light load" would in fact be workable when you have to get passed a period of non-light load during the boot.

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Re: Low voltage warning on boot

Sun Feb 03, 2019 8:09 pm

PeterO wrote:
Sun Feb 03, 2019 8:02 pm
I pointed out the fallacy of your view that a solution that would work for a "light load" would in fact be workable when you have to get passed a period of non-light load during the boot.
Yes, and I believe I pointed out that I have measured this current draw to be around 0.7A (at least, on a Pi3b)
If using a usb cable rated for 0.5A then it doesn't matter if the power supply can output 1A or a million amps, the cable won't allow more than about half an amp through at 5v, which means the person using it is still about 0.2A short of what is required, even though the power supply probably can deliver it.
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Re: Low voltage warning on boot

Sun Feb 03, 2019 9:16 pm

Imperf3kt wrote:
Sun Feb 03, 2019 8:09 pm
If using a usb cable rated for 0.5A then it doesn't matter if the power supply can output 1A or a million amps, the cable won't allow more than about half an amp through at 5v, which means the person using it is still about 0.2A short of what is required, even though the power supply probably can deliver it.
I'm afraid your understanding is imperfect again. :o :(

A poor cable will not limit the current at any particular value. It's just that the voltage drop along the cable is proportional to the current, and at some value of that current (usually somewhat higher than 0A5) the voltage loss on the cable will be sufficient to trigger the low voltage warning. At that stage there may be problems with USB, HDMI, or other 5V peripherals, but the Pi SOC itself runs at 3V3 and lower voltages, so it will not fail until the voltage drops further. So your cable rated at 0A5 will not magically stop working at 0A5.

The symptoms in this case (as described in the OP) will be the same whether it is the cable or the PSU at fault. So a better cable, if one is readily available, could be a solution. Or not. But it is not easy to tell whether any chosen cable is "better", except by trying it and finding either that it solves the problem (good) or it doesn't. If not, you still don't know where the problem lies.

You may find a code like "28AWG/1P+28AWG/2C" on the cable. Ignore the first half which relates to the one data pair ("1P"). The second half tells you the gauge of the two power cores ("2C"). A good cable will have an AWG of 22 or less (less=thicker). The official PSU has 18AWG wires. Of course, a long USB power cable will always drop more volts than an otherwise identical short one. If power problems occur with the shortest, thickest cable available, then the PSU is suspect.

An "official" PSU (known to supply enough voltage at full load current, and known to have an adequate cable) will solve both potential problems in one go.
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Re: Low voltage warning on boot

Sun Feb 03, 2019 9:24 pm

Imperf3kt wrote:
Sun Feb 03, 2019 8:09 pm
If using a usb cable rated for 0.5A then it doesn't matter if the power supply can output 1A or a million amps, the cable won't allow more than about half an amp through at 5v, which means the person using it is still about 0.2A short of what is required, even though the power supply probably can deliver it.
Please explain what you have written above.

If nothing changed since I attended a technical school then Ohm's law is still I = V / R, which implies that a 0.5A cable has a resistance of 10 Ohm if the voltage is 5V, with the official power supply providing 5.1V the cable must have a resistance of 2.04 Ohm to get 2.5A current.

Excuse my french, your statement above is complete balderdash. A cable rating of 0.5A (or 2.5A) means that the cable can be used, and is safe, with that current level and has nothing to do with the current that can be provided by the power supply.

A power supply rating of 5V 0.5A (=2.5W) or 5.1V 2.5A (12.75W) means that the manufacturer states that the power supply can deliver a constant voltage (tension) at a particular current (consumption). The current consumption depends on the load (internal resistance) of the device being powered.

Most power problems that come up here can be caused by:
1 Power supplies that are unable to provide a constant voltage at the published consumption
2 Insufficient dimensioned cables having a higher internal resistance causing a voltage drop
3 Contact resistance caused by under dimensioned USB connectors on the cable or power supply

Case 1 can be applied to phone chargers, USB standard is 5V so the manufacturer labels the charger as 5V, with a maximum current of 850mA. However, the phone being charged has a 3.7V LiPo battery that does not really suffer if the voltage drops below 5V, but a Raspberry does care and will have problems it the voltage drops below the acceptable level. When combined with a low quality cable and/or connectors the problem can deteriorate very quickly.

When you have used 2.5" USB powered HDDs you should have experienced already what happens when thin cables, or too long cables, are used to attach the disk to a computer, the disk tries to spin up but does not manage because the voltage supplied to the drive is not enough to get up to speed.

Imho every Raspberry Pi user must have access to at least one official Raspberry Pi power supply as this is the baseline when trying analyse this type of problems.

Note: I have seen the post by davidcotton but after writing several minutes I do not want to recycle the electrons consumed by this post.
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Re: Low voltage warning on boot

Sun Feb 03, 2019 9:57 pm

Ernst wrote:
Sun Feb 03, 2019 9:24 pm
If nothing changed since I attended a technical school then Ohm's law is still I = V / R, which implies that a 0.5A cable has a resistance of 10 Ohm if the voltage is 5V, with the official power supply providing 5.1V the cable must have a resistance of 2.04 Ohm to get 2.5A current.
The cable would need to have that resistance if it was a dead short circuit on the far end. It needs to be a much lower resistance to supply a Pi. Ideally you'd want to drop no more than about 0.25V at 2.5A on the cable, which would give a total (double the length of the cable, because both ways) cable resistance of 0.25/2.5 = 0.1 Ohm maximum acceptable.

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Re: Low voltage warning on boot

Sun Feb 03, 2019 10:19 pm

I appear to have incorrectly conveyed what I was trying to say, and I don't think I'm articulate enough to get it right, so it's not really worth arguing my point further. I'm only making a mess of the situation.

I was not saying you were wrong by the way, very much correct.


Anyway, thanks for the detailed explanation, a few things there I didn't know.

I will say that my official power supply is not giving me 5.1v, but rather 5.25v
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Re: Low voltage warning on boot

Mon Feb 04, 2019 2:01 pm

Thank you all for replying. I have to say that my post has generated a more "spirited" discussion than I would have liked. Lets be pleasant to each other. I did do 4 years of a 5 year electrical apprenticeship in the worlds most famous shipyard. Unfortunately its most famous product sank on its maiden voyage (no, Mr. DiCaprio wasn't a passenger). However, I took a different career path and most of that knowledge has faded, though some is still stuck in the back of my head. Lets take a look at some numbers, that I have come up with my trusty multimeter:

The Apple PSU is outputting 5.4V with a maximum rating of 2.4A. The USB cable core is 0.5mm (17AWG),1M long and is made of aluminium. (aluminum in the former colonies). No Volt resistance is 0.8 ohm.

According to a Voltage drop calculator (Good old mister Ohm) the following numbers arise for the 1M USB cable :
@5v 2.5A max, drop = 0.13V (2.57%) V at end 4.97 (if Copper Drop is 0.083V/1.63%/5.017V
@5.1v 2.5A max, drop =0.13V (2.57%0 V at end 4.97 ( " 0.083V/1.63%/5.017V
@5.4v 2.4A max, drop =0.13V (2.33%) V at end 5.27 This PSU is rated to 2.4A, the numbers for 2.5% are the same (If Copper Drop is 0.083V/1.48%/5.32V)

As stated originally I initially used a multi port adaptor outputting less than 5V. However, using the Apple PSU, which was outputting more than the advertised 5.1V of the Pi charger, I was still getting low voltage warnings using at least 6 different connectors. (Having been messing about with computers since 1984 I have boxes and boxes of cables/connectors/chargers etc etc) The only one that allowed the Pi to boot properly was a recent one, more or less the same external diameter of all the others but only 40cm long.

This then begs the question, given the numbers above, are the tolerances built into Pi 3 B+ too tight? or is it that there some with production faults? If the latter, then the one arriving some time today should be okay. If the former, and giving the numbers above and those on this forum complaining along similar lines , then it begs the question is this particular Pi of marketable quality? Oh, and please, lets remember my first sentence

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Re: Low voltage warning on boot

Mon Feb 04, 2019 4:31 pm

Granddad_Mike wrote:
Mon Feb 04, 2019 2:01 pm
Thank you all for replying. I have to say that my post has generated a more "spirited" discussion than I would have liked. Lets be pleasant to each other. I did do 4 years of a 5 year electrical apprenticeship in the worlds most famous shipyard. Unfortunately its most famous product sank on its maiden voyage (no, Mr. DiCaprio wasn't a passenger). However, I took a different career path and most of that knowledge has faded, though some is still stuck in the back of my head. Lets take a look at some numbers, that I have come up with my trusty multimeter:

The Apple PSU is outputting 5.4V with a maximum rating of 2.4A. The USB cable core is 0.5mm (17AWG),1M long and is made of aluminium. (aluminum in the former colonies). No Volt resistance is 0.8 ohm.

According to a Voltage drop calculator (Good old mister Ohm) the following numbers arise for the 1M USB cable :
@5v 2.5A max, drop = 0.13V (2.57%) V at end 4.97 (if Copper Drop is 0.083V/1.63%/5.017V
@5.1v 2.5A max, drop =0.13V (2.57%0 V at end 4.97 ( " 0.083V/1.63%/5.017V
@5.4v 2.4A max, drop =0.13V (2.33%) V at end 5.27 This PSU is rated to 2.4A, the numbers for 2.5% are the same (If Copper Drop is 0.083V/1.48%/5.32V)

As stated originally I initially used a multi port adaptor outputting less than 5V. However, using the Apple PSU, which was outputting more than the advertised 5.1V of the Pi charger, I was still getting low voltage warnings using at least 6 different connectors. (Having been messing about with computers since 1984 I have boxes and boxes of cables/connectors/chargers etc etc) The only one that allowed the Pi to boot properly was a recent one, more or less the same external diameter of all the others but only 40cm long.

This then begs the question, given the numbers above, are the tolerances built into Pi 3 B+ too tight? or is it that there some with production faults? If the latter, then the one arriving some time today should be okay. If the former, and giving the numbers above and those on this forum complaining along similar lines , then it begs the question is this particular Pi of marketable quality? Oh, and please, lets remember my first sentence
FWIW, you may find this (old) Apple Charger (and it's clone) data of interest:
http://www.cpmspectrepi.uk/raspberry_pi ... rger_A1399
I've measured the "loading curves" for quite a few (USB) power supplies & chargers since the "first Pi":
http://www.cpmspectrepi.uk/raspberry_pi ... plies.html
http://www.cpmspectrepi.uk/raspberry_pi ... 2FChargers
Trev.
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my 2xP3A+, P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

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Re: Low voltage warning on boot

Mon Feb 04, 2019 5:43 pm

Granddad_Mike wrote:
Mon Feb 04, 2019 2:01 pm
The Apple PSU is outputting 5.4V with a maximum rating of 2.4A. The USB cable core is 0.5mm (17AWG),1M long and is made of aluminium. (aluminum in the former colonies). No Volt resistance is 0.8 ohm.
I have never seen a micro USB cable with 17 AWG power wires, so I'm dubious of that spec. Make and model of cable?

5.4V is too high for the Pi computers. Recommended range is 5V +/- 5% (4.75-5.25), with voltages slightly above 5V preferred.

Another problem is the micro USB connector on the cable, which won't be rated for 2+ amps either. In addition to the cable losses you will also have connector losses. Standard micro USB cables just aren't good for much more than 0.5A without significant voltage loss, and one with an aluminum core is probable a pretty crappy cable.

You need a good quality cable designed for higher current charging. The charging cables included with smart phones or tablets usually have fatter power wires and higher current rated connectors. Of course Apple products use proprietary connectors, so you may not have a good micro USB charging cable. Also, as you have already discovered, shorter is better.

And if all of that isn't discouraging enough, charging phone (or tablet) batteries does not require precise voltage control (most phones only need a little over 4.2V to charge). Because of that, chargers often have poor voltage regulation and will not maintain a stable 5V under load. I have a 5V/2.4A charger that drops well below 5V at only a 1A load. It charges my phone fine, but it's a lousy Pi PSU.

So to answer your question...
... given the numbers above, are the tolerances built into Pi 3 B+ too tight?
The simple answer is no, but you need to be using a proper power supply.

The more complex answer is, too tight for what?

The 3B+ is the most powerful Raspberry Pi computer available at this time, so it also consumes the most power. Which means you are more likely to run into power problems with that model. It's not a fault, it's just the reality that, all else being equal, a more powerful computer will use more power. The official Raspberry Pi Universal Power Supply for the older Pi 3B still works fine on the newer 3B+ model.

The problem with using phone chargers, especially those with separate USB cables, is that you don't know if they'll work without a proper load test. Even if your 3B+ booted and seemed to run fine with your existing arrangement, you could run into problems later when you start doing something heavier on your Pi, or connect a power hungry HAT or USB HDD. That uncertainty makes the more experienced users here wary of them. The official PSU is a proven performer that is usually very affordable (depends on where you live and who you buy from).

Now here's the disclaimer part:
Your Pi may very well be faulty, but it's unlikely, as defective Pi computers are uncommon. It's more likely your charger and cable are the issue. The charger may be experiencing transient voltage drops with sudden load changes. These may be too brief to measure with a DVM, but the Pi's low voltage warning will still react to transients, and if the voltage dips enough, boot will fail.

Not all chargers have crap regulation. I have a couple of older RAVPower chargers that work great as Pi power supplies. Unfortunately that model is no longer in production. And even if they were, the official PSU is half the price I paid for those chargers (so I'd still recommend it).
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Re: Low voltage warning on boot

Mon Feb 04, 2019 6:59 pm

Trev,
Thanks a bunch - now I have to go and teach my poor old brain about slope resistance.

HawaaiianPi, here's my disclaimer. I don't know a great deal about these subjects, in particular large current loads at DC low voltages. However, when I say too tight, I mean that the system should, surely, be able to tolerate different qualities and capabilities of cables, connectors, extensions and PSUs. For instance when is a PSU an actual PSU as opposed to a charger? My Apple PSU, capable of providing current up to 2.4A would, I suggest, actually be a PSU as opposed to my Apple and other power devices which, limited to 500ma, would most certainly not be a PSU but simply a charger.

Be that as it may, the appropriate Pi 3 B+ PSU in the UK is quite inexpensive and will be delivered tomorrow

I will keep my eye on the various fora on this site and hear what all the others who have had complaints similar to mine have to say

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Re: Low voltage warning on boot

Mon Feb 04, 2019 7:10 pm

Granddad_Mike wrote:
Mon Feb 04, 2019 6:59 pm
... when is a PSU an actual PSU as opposed to a charger?
Like many things, it probably depends on who's talking.
But the way I would use the terms, what you connect to your phone is a charger, because its primary purpose is to charge the battery - the phone uses its battery as its power supply.

The charger doesn't have to be clever, because it only has to provide more power than the phone is using, at a voltage that the phone's circuitry can use to supply current to the battery.

A Pi doesn't have that built-in cleverness, so it relies on the PSU to maintain the voltage at the current it wants.

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Re: Low voltage warning on boot

Tue Feb 05, 2019 10:07 pm

Granddad_Mike wrote:
Mon Feb 04, 2019 6:59 pm
...
I mean that the system should, surely, be able to tolerate different qualities and capabilities of cables, connectors, extensions and PSUs.
Computers don't work that way. They need reliable, stable power. How is your Pi supposed to deal with a faulty power supply? That's not a realistic expectation for any computer, much less one that only costs $35.

The problem isn't the Raspberry Pi computer, the problem is that phone chargers often don't output their rated voltage at their rated current, and that standard micro USB 2.0 cables aren't designed to deliver much more than 500mA without voltage loss. If you have a charger that can maintain a stable 5V at its rated output current, and you have a good quality, fast charge rated micro USB cable, your Pi 3B+ will work fine.

If this all sounds a bit too complicated, it is, but the quality of someone else's phone charger or micro USB cable is not something the Raspberry Pi Foundation has any control over. That's why they sell their own power supply. They first released a 2A PSU when the quad core Pi 2B came out, and when the 3B was introduced they bumped it up to 2.5A output. That 2.5A version for the Pi 3B still works fine on the newer 3B+ model.
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Re: Low voltage warning on boot

Tue Feb 05, 2019 10:30 pm

Granddad_Mike wrote:
Mon Feb 04, 2019 6:59 pm
I mean that the system should, surely, be able to tolerate different qualities and capabilities of cables, connectors, extensions and PSUs.
Your current RPi is proof that it doesn't.
Give it insufficient power it will complain. Its"hard coded" in the RPi.
Give it sufficient power and it will perform as expected...
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Re: Low voltage warning on boot

Wed Feb 06, 2019 11:52 am


when is a PSU an actual PSU as opposed to a charger?
Simple answer,

Any device that has a battery within it ( phone, tablet ) then that will have a charger, only needs to supply a voltage greater than the battery voltage at sufficient current to charge the battery, voltage can vary across a wide range.

Any device that does not have a battery in it ( broadband router, network switch , pi ) will have a power supply, it needs to supply voltage with a much smaller variation under all loads as there is no battery to make up the short fall in voltage.
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Re: Low voltage warning on boot

Wed Feb 06, 2019 6:26 pm

Granddad_Mike wrote:
Mon Feb 04, 2019 6:59 pm
... the system should, surely, be able to tolerate different qualities and capabilities of cables, connectors, extensions and PSUs.
I've been thinking about this since you wrote it, and I've come to the conclusion that it does tolerate these things, provided they, together, meet the minimum spec. of 5v ±0.25v at 2A5 or there-abouts.

So does your car - It'll run quite happily on fuel that meets the minimum spec. it was designed for, but would not work very well on best-quality vodka.
Just because you can fill up at the off-licence doesn't mean it's a good idea.

Similarly, just because you can plug your phone-charger into your Pi doesn't mean it's a good idea.
Nor would it be a good idea to plug your phone into your ev charging-point, supposing you could get the connectors to mate.

Everything has a minimum spec. The problem occurs because phone chargers look like PSUs.
Ducks quack, but not everything that quacks goes well with orange.

Milliways
Posts: 365
Joined: Fri Apr 25, 2014 12:18 am
Location: Sydney, Australia

Re: Low voltage warning on boot

Thu Feb 07, 2019 1:34 am

Granddad_Mike wrote:
Sat Feb 02, 2019 3:47 pm
I've just upgraded to a Pi 3 B+. On boot I'm getting a low voltage warning about halfway through, though the boot process continues until log in and then displays the low voltage warning, whereupon everything then hangs. I have tried two different PSU's, (One Apple, One Generic) requiring 100 - 240v AC and both supplying 5v and max power 2.4A. No accessories are connected except for a monitor. The house voltage is showing 230v but I don't know which are the power pin on the usb connection
You appear to have got lots of discussion. A few facts:-

1. You can run a Pi3B+ from an Apple 1A supply; I regularly do this, but do not recommend the practice. It is OK if just using the Pi without demanding peripherals. Apple did once produce a 12W supply, this should be OK, but these were uncommon - the latest models seem to be rated at 10W. All the Apple chargers I have tested (attempt to) supply 5V, and will do so unless overloaded.
2. No one can say what a Generic charger will actually do. Many are designed to provide high charging currents at voltages as low as 3.6V. Many will supply 1A and work with a lightly loaded Pi.

You real problem is probably unrelated to power supply. The "low voltage warning" on the Pi3B+ is NOT hard wired as in earlier models, and only gives reliable results with up-to-date firmware.

I recommend you put your SD Card back in the original Pi, perform an update/upgrade to get the latest firmware then try in the Pi3B+.

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