How NTC thermistors stop destructive inrush current spikes in high-capacity inverters

When powering up massive high-capacity backup systems like the EcoFlow DELTA Pro Ultra or modular Power Kits, the internal electronics contain large banks of bulk capacitors. The exact split-second these empty capacitors are connected to voltage, they act momentarily like an absolute short circuit, drawing a massive, violent surge of electrical current known as "inrush current."

Without protection, this microsecond spike can weld mechanical relays shut or blow internal fuses.To tame this electrical shock, engineers integrate Negative Temperature Coefficient (NTC) thermistors into the power input stage. An NTC thermistor is a specialized semiconductor resistor whose electrical resistance is inversely proportional to its temperature. When the system is cold and first turned on, the thermistor possesses high internal resistance, safely choking off the dangerous inrush current spike.

As the electrical current continues to flow normally, internal self-heating causes the temperature of the thermistor to rise rapidly. Its resistance drops instantly to near-zero, allowing ultra-efficient power flow during normal operation without generating thermal waste.