Burning out a Sonoff SV with 12V

Hi all.

I am a beginner. Self-thought via youtube - dangerous, I know!

I’ve had an issue with 2 Sonoff SV’s burning out when connecting them to my alarm system.

  • Both were flashed with Tasmotoa.
  • Both had the two tiny resistors removed so it can be used as a dry contact relay
  • Both were set up and tested using a 5V power supply to the two dedicated power pins and worked fine

However, when I connect them to my alarm system things get smelly. The second time, I disconnected the alarm system from its battery, tested the battery with my Voltmeter (it tested 13.8V) and then connected the SV to the battery with nothing else connected. It immediately fried.

I understand that the SV can accept power safely from 5-24V DC. What am I doing wrong?

Appreciate any help, feel free to explain like I’m five.

Hendrik

Where exactly, here?

Yeah I have 2 of these running off 12v (an alarm and my Garage Door opener) Using the pins Tom indicates. Never an issue.

Hi Tom

That’s correct, I used the two pins at the bottom of your diagram where the green arrow is pointing to. The two inputs you crossed out, I bridged so I could use the two outputs at the right of the diagram to go to my switch.

From memory the ac input is supposed to be shorted for a dry contact switch?

O yes, sorry. That is what I mean by bridged. I shorted out the inputs on the left of the diagram. (So the circuit is completed from the -Output, through the relay, to the +Output)

Hi all.

Hoping anyone has some views to share?

Can you take a couple of high resolution photos, both sides, of the fried boards?


Thank you Tom. I hope these show sufficient detail

Looks like you may have applied the 12V DC with the incorrect polarity.

Awesome, thanks so much. That’s something I can not do next time.
Just to be clear, you’re saying I may have connected the live wire to the - on the board making the “flow go in the wrong direction”?

In the spirit of learning more, how could you tell?

12V DC positive from the alarm has to go to the + terminal on the SV. Negative From the alarm to the SV - terminal.

Sure, I accept that thanks. Since I removed the wires, what on the photo showed you that this was not the case?

The exploded tantalum smoothing capacitors across the DC input. These are polarised components. Hook them up backwards and they go ‘pop’.

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Cool. Thanks for the help! You are making me less likely to burn down my house or electrocute any of its inhabitants

I just did this same thing :pensive: I covered the board writings for the polarity with some soldered on screw terminals and so googled a pinout to try and remember which was which… The pinout I grabbed was wrong! … and I didn’t take the time to double check because I wrongly assumed that there would be some reverse polarity protection… I take it that once you make this mistake you just generated more e-waste… anyone know how to revive this or is it just dead? Supplying 12v correctly, I can see that the relay is receiving about 7v and the 3.3v pin is receiving 3.3v so the power supply part of the board is still working, but the chip is unresponsive… I tried re “tasmotizing” it, but no such luck. :frowning:

In the images supplied above it looks like the smps ic has also suffered some damage. May or may not be the same on yours. If you can do SMD soldering you could repair any blown pcb tracks, replace the input smoothing capacitors and the smps ic.

If you don’t know what you’re doing, don’t try it.
Without proper damage diagnosis, you can cause further damage.
I’d rather use another one, I wouldn’t trust the victim anymore. :slightly_smiling_face:

It’s non-functional now. How could it be worse?

I understand that it is non-functional. And if it could be put into operation somehow, I probably wouldn’t trust him anymore because I’m not sure if anything else is partially damaged.