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Came across these small Zwave controllers here .These are small enough to fit inside the electrical switch housing in the wall. Just wondering how is this device so small as the AC->DC transformer is usually pretty big. Are they using some special kind of smaller transformers that are there?

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It's probably a switched-mode power supply which tend to need smaller transformers. If you pull apart a small AC adapter / wall wart you'll often find it contains a lot of empty space and the AC pins take up a substational amount of the total size.

Here's an example photo taken from Ken Shirriff's blog - although maybe it's a fraction smaller than it should be because it's a page describing an unsafe model:

enter image description here

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thanks for the link. I need to incorporate a 220v AC to 5v DC with minimal footprint in my project. This is to power a microcontroller. Any ideas on how to get started? –  Ankit Mar 9 at 13:13
    
Designing one that is safe from scratch is fairly complex and lots of things could go badly. If this is a one-off (or small qty) I'd probably just find a suitable adapter and pull it apart, still being careful about insulation etc. For a commercial product there are so many approval and safety implications you'd want a professional to do it, even for large companies it's one reason they often use wall warts instead of trying to embed the AC supply to save them having to worry about it. –  PeterJ Mar 9 at 13:24

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