Just adding a couple of asides:
The "3v LED" may actually be a "constant current" type - one with a current limiting chip built-in. Although most of those need a minimum of
4v before the current starts becoming constant. An "LED" with 12v across it is not always a fault condition.It's years since I taught electronics, but one of the most popular parts of lab sessions was, "How not to do it". Usually demonstrations (the Health and Safety Elf rules) - but things like reverse biased tantalum capacitors, running transistors well outside their SOR, fuses massively overloaded in volts or amps, unsnubbed triacs, etc, etc. I've always thought that it is as important to teach engineering students how things fail as well as how they succeed. Certainly some of the demonstrations very successfully dealt with the average student's sleepiness after a few pints in the SU, lunch time.
-- Sue