BG273 - Showing how a benchtop Variac (variable autotransformer) is made. These are super useful on the bench. This along with an isolation transformer and a dim bulb setup can keep you safe and be effective at working on vintage electronic equipment. Especially helpful when working on tube gear such as amplifiers, preamps, radios, stereos, etc.
I acknowledge the DC isn't pure DC and you could enhance this with a full wave rectifier and a CRC or similar circuit. Just showing what he has been building.