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jbeale
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surprising result after applying +12V to +5Vin

Mon Aug 20, 2018 2:05 am

I did a dumb thing today to a RPi 3B, applying +12V directly from a 14 Ah battery to the +5V / GND GPIO pins (and in fact it was through a good soldered connection, not just spring contact wires). Unsurprisingly there was small jet of flame from the PCB, as this battery can probably deliver 50 A in a short burst. https://photos.app.goo.gl/PD5EG7X9pQoN9CQa6

The board is (literally) toast, and bad-smelling toast at that. But quite surprisingly, the SD card the RPi was running from at the time, still works when I tried it in a different Pi. Likewise the USB Audio adapter I had plugged in at the time also still works. I'm puzzled by how that's even possible, as I would expect putting a high-current +12V source directly on the +5V rail would blow every "silicon fuse" much faster than any physical fuse or pcb trace melting. At any rate, congratulations to the PCB designers who managed this feat!

W. H. Heydt
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Re: surprising result after applying +12V to +5Vin

Mon Aug 20, 2018 4:02 am

Neither SD interface, nor the audio output run off the 5v "rail". There are circuits on the board that convert to other voltages from the 5v supply. Those converters probably blew up rather can pass the overvoltage to anything connected to them.

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jbeale
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Re: surprising result after applying +12V to +5Vin

Mon Aug 20, 2018 4:20 am

I was not speaking of the onboard audio output, but a third-party external USB-audio adapter which I was using for a mic input. Quite a flimsy and cheap one too, I think it was under $5 from ebay. The USB port provides +5V output, so I did not expect it to be much different than the board's own +5V power input. Apparently the USB +5V has more robust protection than I expected.

Yes, it is true the SD card uses a lower voltage but I did not expect the onboard DC-DC converter to be "fail-safe" with such an overload as +12V, although I am certainly happy that it did turn out to be.

Heater
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Re: surprising result after applying +12V to +5Vin

Mon Aug 20, 2018 8:04 am

I had a surprising result after connecting a Pi 3 to a PSU that was accidentally set to 12v.

I realized my mistake very quickly as smoke started coming out and turned it off. On inspection I found a protection diode had unsoldered itself and was now standing upright on one pin. ("tombstoned" as they say in the PCB business)

The surprise was that when I set the PSU back to 5v and plugged in in again the Pi worked! That protection diode had done a good job.

Sadly I was not so lucky when I made the same mistake again a few days later.

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jbeale
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Re: surprising result after applying +12V to +5Vin

Mon Aug 20, 2018 4:07 pm

That's a neat story (at least the first part). Seems like your Pi had a real mechanical circuit breaker!
Yes, my downfall was soldering on the same kind of generic 5.5/2.1 mm DC power jack as I use for +12V, and then being in a hurry and careless about connections.

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HawaiianPi
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Re: surprising result after applying +12V to +5Vin

Wed Aug 22, 2018 12:08 am

Heater wrote:
Mon Aug 20, 2018 8:04 am
Sadly I was not so lucky when I made the same mistake again a few days later.
Doncha hate it when that happens?

"Whoops, I got lucky this time! I'll be careful never to do that again."

Then a few days later...
doh.jpg
doh.jpg (10.85 KiB) Viewed 168 times
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?

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