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I am making a smart switch for toggling appliances with a 10 A relay using esp8266 as my MCU. I am using an isolated 3 W (5 V, 600 mA) SMPS. I have seen in some specifications about 2.5 kV surge protection. I want to know how can I achieve that. By following some other schematic I kept a 300 Vrms MOV and a 1 A fuse in series. Will that be enough? How can I properly select values for given surge specification (1 or 2.5 kV)?

My second question is, sometimes my device is getting damaged, movistor itself is blasted, probably because of exposure to constant high voltage. Is there a way to protect the device against constant high voltage (300 Vrms)?

3rd question, will the design change if I used a non-isolated power supply using buck converter?

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  • \$\begingroup\$ The more impedance before your MOV, the better. If your load only is 3 W, consider a high voltage capable resistor in series. How much is enough? Only a (pre)compliance test can tell. How come it gets exposed to 300 V AC? \$\endgroup\$
    – winny
    Commented Sep 7, 2021 at 6:38
  • \$\begingroup\$ There are surges and these are surges. You need to specify which standard surge specification you wish to adhere to. Clue: EN61000-4-5 is indirect lightning and it has various sub-categories. In other words, define the threat. \$\endgroup\$
    – Andy aka
    Commented Sep 7, 2021 at 7:11

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