Implementation of automatic AC line voltage selector, App note from Littelfuse. Link here (PDF)
In some cases, appliances and equipment are able to operate when supplies by two different levels of AC line voltage to their main terminals (10V or 240V). This is why, it is very common that appliances and equipment have mechanical selectors or switches as an option for selecting the level of voltage needed. Nevertheless, it is also very common that these types of equipment can suffer extensive damage caused for not putting the selector in the right position. To prevent these kind of problems, thyristors can be used as a solution for making automatic voltage selectors in order to avoid possibilities of equipment damage due to over or low voltages AC line supplies to them.
from Dangerous Prototypes https://ift.tt/3gGC5lB