Magic smoke
Magic smoke turns up in 2 fields: machine shop, electrotechnology. Here is what it means to the people in each one.
Magic smoke in machine shop
Joke that electrical parts run on smoke sealed inside them, because once it gets out they stop working.
Shop humor about electrical failure: motors, drives, boards and power supplies stop working the moment their magic smoke escapes, so the goal is to keep it in. It is used for any burnout, from a cooked spindle drive to a tool battery that shorted, and implies the part is done rather than repairable.
"Hooked the VFD up to the wrong voltage and let the magic smoke out, so that's a new drive on the order."
Magic smoke in electrotechnology
Joking name for the smoke a burnt-out component releases, as if the smoke were what made it work.
Magic smoke is shop humour built on a mock theory: every component is packed with smoke that makes it work, so it dies the moment the smoke gets out. Engineers and technicians say a board 'let the smoke out' after overvoltage, reverse polarity or a wiring error on first power-up. The phrase marks a failure as obvious and destructive rather than subtle.
"Wired the 24 V supply onto the 5 V input and the controller let its magic smoke out on the first try."