MarkDaniels wrote: I only use Stontronics PSUs and have used tens of thousands of them over the past 10 years in industrial equipment which I manufacture. So, I am reasonably confident that the PSUs are up to the job, particularly when it is considered that one of those PSUs had already been running a Pi 3 continuously for nearly two weeks without problem.
You didn't say in the original post, I don't think you said, that you had a lot of known high quality power supplies and you'd tried them variously. All the time around here, the problem is, however, power, so that is the first thing to mention when someone posts with a problem like you posted about, right before implicating the sdcard. This doesn't mean that it is impossible for a pi to be bad, someone was complaining recently that parts were falling off their pi, that can't be good.
> If you think about it, the amount of current that
> goes into the flash drive is at least partly defined
> by the drive itself and not the source. If the
> flash drive negotiates a high current that it can't
> deal with and then draws that current for no reason,
> where is it going to go but into getting hot?
I am a qualified electrical engineer with many years experience in power engineering and electronics. I understand the heating effect of an electric current.
With all due respect, the current drawn by a device (assuming a fixed voltage source) is ENTIRELY defined by the device (load) so long as the source VOLTAGE remains constant.
I'm not arguing against Ohm's law, obviously if the source keeps the voltage constant, the load is defined by the load. I'm talking about which circuit is responsible for limiting the current, is it the $2000 laptop or the $2 USB flash device? The responsibility for limiting the current isn't a question of Ohm's law, it's a question of design. I shouldn't be afraid to plug *any* USB device into any host because part of the design should be that you can do that without fear. That was the design of the original RS-232, you could touch any pin to any other pin. (Now, of course, "RS-232" is highly risky with different voltage tolerances and all that.)
If I stick a fork into the electrical outlet I will create a direct short. Under the USB design philosophy, the fact that the fork doesn't limit the current is a problem of the fork. That's not how house wiring works, depending on the country, the house might have circuit breakers or fuses on the line or even in the plug box itself, the latter being similar to what one would think negotiating the maximum current available in usb means. And to current design, it might have ground fault and arc fault interrupters, all sorts of high tech designed to prevent house fires.
In many USB host circuits, it appears that the only physical limitation on the current that it will allow is its capability and some sort of polyfuse or fuses. The rest of it appears to be simply software based. For example, if you have five flash drives attached to a device and they are all low current, say that's 100ma, then depending on other software settings, the operating system will be happy or not by simply adding up what the loads *claim* to be.
Supposedly USB wasn't to allow any one device access to more than 500ma. This was a problem quickly because of wifi and mobile phone modems and people wanting to run their hard drives without a wall wart, just attach to USB. Because people have been able to often get away with this, it is clear that the average port isn't really limiting the current to some set amount based on the specs. There would surely be some high current limit but it is likely higher, and more scarily, it seems to be limited in some cases by what the power supply can physically put out, causing brown out rebooting and possibly damage to the host.
If, as you say, "a flash drive negotiates a high current" (presumably this means that it negotiates for the current LIMIT on the USB port to be raised) and actually draws current to the negotiated limit, it will get HOT, whether that current is being used for a "designed for purpose" or otherwise ("no reason"). That is the very nature of electronic devices. They do not, of themselves, perform WORK, hence all of the energy consumed ultimately is turned into heat energy.
We can use this by looking at it in reverse to show that clearly the flash drive wouldn't normally draw that level of current since it normally, even on constant writing, never gets *that* warm. So we might even conclude that because the flash drive would never need that much power, it isn't even negotiating a higher amount, it is just going nuts and loading things down without even asking. *And the host is letting it do that.*
Of course this isn't completely true, the host has its polyfuse, say, or it might have some other means, the newer pi have something that monitors at 600ma and 1.2 amps and supposedly shuts things off. But even 600ma is a lot into something that is supposedly to never need more than 100ma.
The next version of USB will fix the which side is up insertion problem, I hope it also addresses the need for more control of the loads that will be allowed on each port, including off into compliment hubs.