Some manufacturers make low power AC-DC modules for small appliances, IoT devices and smart meters. MeanWell may come to mind (IRM-05-3.3), but one of the most cost-competitive modules is the Hi-Link HLK-PM03.

Here I will have a closer look at how the module is built. There are of course other teardowns out there, but not necessarily of the 3.3V variant, and you never know if you're going to find something others could not or had not seen.

Unpotting

So what's inside the HLK-PM03? With a silicone potting compound that readily unmoulds from the enclosure, the module was definitely inviting further investigation. 

The assembly came out after gently pulling on the pins. Adhesion of the silicone compound to the electronic components is generally poor, and there is no primer / adhesion promoter on the surfaces.

Quoting https://acc-silicones.com/products/primers:
"Silicone adhesives and some 2-part rubbers have built in adhesion promoters - with these products the use of a primer will only be required for use with substrates that are exceptionally difficult to bond to. Most 2-part silicone rubbers will not have any adhesive qualities and the use of a primer will be essential if adhesion is required."

I'm not seeing a lot of adhesion here, but the surface texture of the injection moulded plastic housing is reproduced flawlessly :) Slides right out of small gaps:

The inner sides of the black housing have a satin texture added to better retain the silicone block. It seems like good practice, but only early models will tell the tale whether the mould texture was added later in production.
There's a surprising amount of gas bubbles in the compound. In the picture they're mostly seen around the secondary side where they don't matter much, but they are also found around components on the primary side. While possibly acceptable for normal operating conditions, the porosity becomes a concern for high voltage transients. Silicone compounds break down at around 20 kV/mm, but then again there are no zones (except for the transformer) where the design would rely on high dielectric strength.

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Unfortunately I will not finish this article on account of the hackaday.io having deleted a good portion of the text, lost graphics and giving error 413 when I save the draft.

hackaday.io is no hacker's "platform". If you can put up with a text editor that loses your work over and over again, that's your choice. I won't condemn your masochism, but I won't share it.

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