Tomorrow I will add some pictures and diagrams to this case…
As announced, some pictures and diagrams of the necropsy:
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Signs of blaze and vaporisation of electronic components are obvious...
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Even on the solder side of the PCB are signs of damage, as we realise in the upper shell.
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The component side of the PCB with MOSFET module removed.The black residue is soot...
Please note: Components covered with KAPTON tape (yellow) remained pristine!
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This is the main issue: Resistor R1 at MOSFET module is totally vaporised. Even the KAPTON foil is burnt…
What parts were damaged and where are they located in the circuit diagram?
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(Remark * : The over-temperature switch is with Version 2 of Shelly Dimmer 2 only!)
Components marked in yellow are faulty:
- Both PowerMOSFETS (Q1, Q2, type OSG65R22J. N-Channel MOSFET, VDS 650V.
- Shunt resistor R1, 0,1Ω
- IC SM7035P, Switching regulator
- and another small unidentified component at the solder side of the PCB
The Dimmer 2 doesn't work with external power supply (12V DC) - this indicates other faulty components in the logic part! Maybe the ESP8266 is damaged, because an inrush current of about 0,5 A was recognised during the test with the external power supply.
Another interesting question is how the Dimmer 2 is powered. Examining the above circuit diagram leaves us at first glance a bit helpless. But pointing out the relevant circuit paths gives us a deeper insight:
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Current for power supply is flowing from terminal L via the intrinsic diode of MOSFET Q2 via the resistor R2 through the switching regulator circuitry and via fusible resistor RF and diode D14 to terminal N. In the case, Shelly Dimmer 2 is operating without N-potential, current path ends via diode D2 and the load resistance at N-potential.