Why Plugging In a Charger Can Spark

The moment you plug in a power supply, a fuse can blow or the plug can spark. The reason is inrush current: a discharged bulk capacitor behind the rectifier pulls a huge current at first, almost like a short. Many supplies put an NTC thermistor at the input. NTC means its resistance goes down as its temperature goes up. The part shown is rated 5Ω at 25°C. Cold, its resistance limits the inrush current. As current flows, it heats itself and its resistance drops, so it wastes less power in normal operation. The catch: switch off and right back on, and the thermistor is still hot, so its resistance is low and it may not limit the inrush properly. An inrush NTC works best when it's cold. Don't open a power supply: the capacitor can stay charged after unplugging. #Electronics #PowerSupply #Thermistor #Shorts