NOTHING, repeat, nothing you work with in these lessons and projects should ever, ever, get even warm, let alone hot.
RULE 1: If something gets warm or smokes or starts emitting a burnt or chemical smell disconnect all power immediately!
RULE 2: If something gets warm or smokes or smells bad assume YOUR CIRCUIT HAS SOMETHING WRONG. Don't try again before knowing for certain what caused the problem. You will only cause another part to become distressed.
RULE 3: Be extremely careful around rechargeable batteries. All can start fires under the right circumstances. Some, especially those that include lithium (Lithium Polymer and Lithium Ion), can explode, releasing copious quantities of smoke, flame, and chemicals. General safety rules:
- Never leave charging batteries unattended.
- Only charge with the appropriate charger set to the batteries characteristics.
- Place charging batteries in a flame-retardant envelope while charging.
- Always have a CO2 or chemical fire extinguisher near by.
- Always wear safety goggles when working with rechargeable batteries.
- Never connect a rechargeable battery to a circuit that hasn't first been successfully tested with a bench power supply and found to work correctly. Bench power supplies have circuitry that disconnects power if too much current is being drawn. A battery does not.
RULE 4: Be very wary around batteries of all sorts. Even the common AA alkaline cell can pop open its casing, melt plastic, and even start a fire if shorted long enough.