Faulty power supply = Bricked core

Bugger… just bricked my core

Not sure how… it was working fine, running off my new Power supply card that was pumping out 5.12v DC when measured with a multimeter. the power supply card is pretty much the same as the one on the relay shield… just the default values for the AP1509 buck regulator

it had been running for about 30mins, when i decided to flash new firmware, half way through the update it heated up really quick and died. now its dead as a doornail on the USB… the regulator just gets hot, and when i supply it with 5v it pulls the voltage down to 3.6v hmmm

i have a digital scope/analyzer with me (MSO-19 if anyone knows how to use it), but need to work out how to use it to measure the power supply output correctly…

Any idea’s?

Ok so i have hooked up the scope to the psu

it appears to be working really well without any load, outputting 5.2v and fairly stable. however with load it changes to a sine wave. Unfortunately Im very limited with the tools i have for testing, i have a DMM with a blown fuse for the current measurement, and a 7w light bulb for a dummy load

no load, probe on x10 and so voltage displayed needs to be multiplied by 10

load ~1.5A, probe on x10 and so voltage displayed needs to be multiplied by 10

next step is to “repair” the fuse and check the current with the DMM :slight_smile:

edit: DMM current meter is still not working properly, but i did manage to work out the power supply board does work well up to 1.3A then the voltage drops down and starts oscillating as seen above, by using 3 different torch bulbs i could work that out. Now im not sure why the core bricked, maybe the noise had an effect on the regulator and started doing something funny

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This noise may have made the LDO on the core oscillate and overheat.

These parts have a thermal shutdown, but sometimes it doesn’t work to save itself.

If you are lucky, all that has failed will be the LDO - you could test this by trying to power the core with 3.3V via the 0.1" header pins.

Given that you may have a failed LDO on board, I would do this very carefully measuring the current consumption and preferably using a current limiting bench supply (set to something like 450mA to make sure it doesn’t trip on any transients.)

It may also be that the flash has got borked, and if it will take power via the 3.3V rain, but still doesn’t boot properly, you may be able to reprogram it via JTAG (but I have no idea if you have a JTAG shield or programmer…)

the STM32 gets hot as… so im not holding my breath. i wont power the core up again to see until i am home and can trouble shoot it better. its interesting you mention the flash, as it died when i was doing a flash OTA when it died. when i get home i will try powering it up from 3.3v and see how it goes.

with the power supply testing, i have made a makeshift wire wound variable resistor with a strand of stainless wire wrapped around a pen, it gave me 10ohms resistance to play with. so my thoughts are V/R=I so 5/10 = 0.5A all the way up until the PSU goes into current limit mode.

so then i did some testing to see what happens as i increase the current, at 0.5A the output is a good clean output with a little blip at 150khz as expected as that’s the switching frequency of the buck regulator.

now as it approaches the 1A mark, where i measured 5ohms on the variable resistor i start to see a blip show at 120hz and it grows in amplitude as the current goes up, till the pen started melting then i had to stop… but this number looks strange to me as the current limit frequency should be 10khz

the other thing is, I’m still puzzled as to how the core could draw that much current on its own, unless i have a short somewhere… hmmm

anyone know who designed the power supply on the relay shield? and if they found anything like this during testing?