Strange USB PSU issue and a merry dance

[Harry Bloomfield Esq]

USB wallwart, supplied power to a RPi, which subsequently fed USB power to my weather stations display. RPi logs data and provides a web site. Display had batteries, for independence when not on USB supply.

The web site went off air, then days later the display went blank. Power LED on RPi remained lit throughout, so I made a wrong assumption that the USB PSU was OK.

I checked the display on another USB PSU, that worked, pulled the USB leads out and checked those. I began to think the internal PCB connections for the USB might have failed and was about to pull that apart, but thought to plug the RPi into another PSU first - which worked.

I've not yet got around to checking out the obviously faulty PSU yet.

[John Rumm]

Yup it can be frustrating trying to fault find when the thing that you had assumed was beyond question turns out to be the problem all along!

[Brian Gaff]

Are you aware that some of these require a certain load to work properly? Apple ones go into pulse mode when current drain is low. I'd imagine that this is deliberate to protect batteries on charge? Brian

[Harry Bloomfield Esq]

The PSU + RPi + display combined load has worked fine for many years.

[Theo]

I think the pulsing is the way they save power by detecting if there's a load. If they sense there's no load, they go to sleep and turn off the output. Every so often they wake up and turn on the load to the full 5V, then they wait a little while and measure the current. If there's insufficient current they go back to sleep, while if they sense current being drawn they keep the output on.

Once a phone is fully charged it'll stop taking power from the PSU, and so perhaps the PSU detects it's in 'no load' and goes into this sleep behaviour.

Theo

[Harry Bloomfield Esq]

I'm not so convinced that is correct...

Old desktop PSU's would pulse in the same way, if they had no load. Add a load whilst they were pulsing, and they would not then power up. The only way was to remove mains power, wait a few seconds, then reapply the mains.

[John Walliker]

A USB power supply will never pulse on and off with no load unless it is broken.

John

[Harry Bloomfield Esq]

Just to make clear - My USB PSU didn't seem to be pulsing it's output, the power LED on my RPi was fully and continuously lit.

I have not yet investigated the output of the failed PSU.