Pi 4 Boots up correctly on 2nd attempt

The chances are that it’s “universal” input, i.e. good for anything between about 100V and 250V, so the local mains voltage is very unlikely to be a factor unless the particular specimen is very faulty.

Even if it wasn’t, the likelihood of the sort of SMPS that you’ll be using on a Pi propagating a variation in its input voltage across to its output is very small, and would almost certainly have been noticeable before – if it can’t do something that simple, it will have other ‘issues’ that would affect its day-to-day use, too.

Generally anything that wants to comply with the normal standards will have been tested at both 98Vac and 264Vac during its development.

If the power supplied to the computer is an issue, it’s more likely to be caused by the cable between PSU and computer being too long and/or thin, so the 5V (or whatever) launched into the cable is significantly less at the destination.

Or a failure in the output voltage regulation or output current limit of the PSU, which is not the same thing as it being affected by voltage variations at its input.

Or a failure in the output voltage regulation or output current limit of the PSU, which is not the same thing as it being affected by voltage variations at its input.

Yes. I’ve come across three PSUs that could no longer supply their rated maximum current. It’s a reminder that measuring the output voltage with no load does not tell the whole story.

The reason I mentioned temperature a while back was exactly that: exactly how much current a PSU can deliver (and/or the voltage at which it delivers it) not infrequently varies with temperature. Also of course the amount a Pi (plus whatever else is attached and using current) consumes at what voltage may also vary with temperature. If you’re anywhere near the limits, a degree or less can take you one side of it or the other.

Yes. I’ve come across three PSUs that could no longer supply their rated maximum current.

Funny, I ran into this exact same problem just recently.

I have a Peltier effect dehumidifier in the kitchen. The thing with those Peltier things is they take a rather large amount of power. In order to make a large heatsink cold to condense water on it, it takes 9V at around 2.5A.
So it came with a 2.5A power brick that can output 9.35V at the required 2.5A.

After almost exactly four years of near-continuous operation (except in the middle of winter when it was too cold in the kitchen so the thing would only ice up…), the power brick packed up. With no load, it output a good 9.35V. When the heat pump was brought in, the voltage would nosedive.

The power brick can still output 9.35V, it was running my little oscilloscope yesterday. But it’s rated 2.5A? No chance.
I got a 3A rated supply from Amazon, so it can run the dehumidifier with some overhead. I can’t help but think always running something at full capacity isn’t the best idea.

not infrequently varies with temperature

I’d have thought that most external temperature differences would be negligible compared with the internal heating of the device while it is in use.

It’s a reminder that measuring the output voltage with no load does not tell the whole story.

DVM measured vs. Avo 8¹ measured.

¹ They seem to have ceased manufacturing in 2008, after a mere 8 decades.

I’d have thought that most external temperature differences would be negligible compared with the internal heating of the device while it is in use.

It’s very small, but it only takes small differences like that to take one above or below a threshold. It’s a demonstrable effect.

Oh, it’s not booted properly at power on this morning, and needed a restart again. :-(
The room temperature was 18C, a bit lower than than the usual 20+C. FWIW, the CPU temperature initially was 42C according to CPUClock. I have still not changed any settings or configuration. It looks like I’ll have to swap my PSU with my Wife’s mobile phone charger which gives 5V and up to 3A.

It’s failed again with the mobile phone charger, and needed a restart. CPU temperature is now 50C and room temperature 20C.

DVM measured vs. Avo 8 measured.

In my student days I once connected all the Avo meters in the electronics lab in series on a current range, put a direct current through them, and was astonished to see major differences in the readings.

It is notoriously difficult to make a reliable, very low, resistance, which is what you need (usually!) for measuring current. What you actually measure, or sense (in the case of a power supply with a current limit), is the voltage across the resistance. The resistance tends to go high as a result of losing some contact at one of the interfaces between different materials.

It’s failed again with the mobile phone charger

What cable(s) have you got between the PSU and the computer?

I still haven’t found the file config.ro5 or config.ros. So where is it?

Have a look while you are running Linux – CONFIG.TXT (with the RISC OS instructions in it) will have been renamed to CONFIG.RO5 and CONFIG.LIN will have been copied over (or renamed to) CONFIG.TXT (with the Linux instructions in it).

Not on my R-Pi 4. I have looked while in Linux, and there is config.lin and config.txt, but no other config files.

Problem solved?

A week or two back I decided to take the case off to investigate whether any electrical connections were broken, in particular, the seating of the SD card. I pulled it out and pushed it in a few times as I have heard enough corrosion can build up on data connections resulting in lack of data transmission. Since then my R Pi 4 has booted correctly.

Comments?

Don’t lick the SD cards before use.

Oh! The problem returns: My PiAno Boots correctly only after a Restart, but sometimes from power on (without a restart). :-(