Note: this pick is the least-bad option from a weak batch. Most candidates were PC unboxings, hashtag-spam shorts, or a "0 subscribers" microgrid promo. This one is also tagged #Shorts, but the description suggests it actually explains a real circuit topology worth knowing about.

A capacitive transformerless power supply is a classic space-saving trick used inside cheap LED bulbs, smart plugs, and small appliances where a bulky mains transformer would blow the bill of materials. Instead of stepping down 220V AC with iron and copper, an X-rated capacitor acts as a reactive current limiter, dropping most of the voltage as reactance rather than heat. A bridge rectifier, bleeder resistor, and Zener clamp then produce a low-voltage DC rail suitable for driving a COB LED.

It's genuinely useful to understand — both because you'll find this topology in almost every teardown of a sub-$5 mains device, and because it's dangerously non-isolated. The DC output sits at mains potential, so anyone probing one of these on the bench needs to know why they must never touch the "low voltage" side while it's plugged in. Watching a build walkthrough helps demystify why the capacitor value sets the current, why the bleeder resistor matters for safety after unplugging, and where the Zener fits in.