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Board for automatic light

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VTI_16V

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Hello. My friend broght me this board for light that turns on and off automatically... I found that fuse is gone. Nothing is burnt, caps are tested and fine, diodes also... But this thing is unclear to me. I didn't see whole light, but he tells me it is connecting directly to 220V. And on this board I really have markings L and N. But when you track path from L, after the fuse it goes directly to capacitor which is 16V? You can see on the board, cap is just after the fuse, it doesn't make sense to me and I don't want to plug it in... Does anybody know how is this possible, am I missing something obvious? Thanks in advance...
 

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The obvious thing you are missing is: what is the OTHER side of the cap connected to? If one side is connected to 2,000,000 volts and the other side is connected to 2,000,005 volts, there's no problem.

Without a schematic, that's about all I can tell you. Pictures are just about useless.
 

this looks like a typical offline regulator with a diode bridge with series Mylar brown Cap for 320Vdc to low voltage shunt e-cap as a capacitance transformer divider. Perhaps the dimmer pot was too bright and blew the fuse or there was a large line transient.
 

    VTI_16V

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By putting a capacitor in series with AC, it's a common method of dropping house voltage down to a low power level. Cheaper than the cost of a step-down transformer. Suitable if you need just a few tens of mA, and are aware it requires safeguards due to high voltage risk.

The board circuitry itself runs on low voltage. If you track down a bad component, wait before you plug it into the wall, and locate a proper point to apply low voltage to the circuitry which controls the light. After you get it working, then you can test it with mains AC power. Even then there's no being sure the capacitor is still good, nor that neighboring components are good.
 

    VTI_16V

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Your lack of understanding is promoted by the fact that circuit common ground (apparently DC+) is tight to L while the transformerless supply is connected to N. This choice is due to triac trigger requirements and shouldn't bother you further unless you intend to completely decode the schematic.

It's unlikely that a control circuit overcurrent made the fuse trip, more likely a lamp short was the reason. If the triac isn't blown (would be usually indicated by a A1-A2 short) replacing the fuse should put the circuit back to operation.
 

    VTI_16V

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Thanks everyone! Most of the time I work with 12V devices, so I never camed across this way of lovering 220V... Good to know! It was just like FvM said, only fuse was gone because of short in the lamp :).
 

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