What is inrush current and why does it need to be limited?
Inrush current is the temporary surge of current that occurs when electrical equipment is first powered on. It can be 10-100 times higher than normal operating current and can damage components, trip circuit breakers, or cause voltage drops. NTC thermistors limit this surge by providing high initial resistance that decreases as they heat up. See also our use the Voltage Regulation Calculator.
How do I calculate the minimum resistance for my NTC thermistor?
The minimum resistance is calculated using Ohm's law: R = Peak Voltage / Maximum Allowable Inrush Current. For AC circuits, use the peak voltage (Vrms × √2) rather than RMS voltage for accurate calculations.
What factors affect the energy rating requirement for inrush current limiters?
The energy rating depends on the peak voltage and capacitance in your circuit. It's calculated as E = ½ × C × V². Higher voltages and larger capacitances require thermistors with higher energy ratings to handle the surge without damage.
How do I determine the steady-state current in my circuit?
Steady-state current is calculated as: I = (Power × 100) / (Voltage × Efficiency%). This represents the normal operating current after the inrush period, which the thermistor must be able to handle continuously.
Can I use the same NTC thermistor for different voltage applications?
No, NTC thermistors are voltage-specific. The resistance and energy rating requirements change significantly with voltage. Always recalculate the requirements when changing supply voltage or load conditions.
What happens if I choose an NTC thermistor with too low resistance?
A thermistor with resistance lower than the calculated minimum will not adequately limit inrush current, potentially allowing damaging current spikes that can harm sensitive components or trip protective devices.
How does temperature affect NTC thermistor performance?
NTC thermistors have negative temperature coefficients, meaning their resistance decreases as temperature increases. This self-heating effect is what makes them effective for inrush limiting - they start with high resistance when cold and drop to low resistance when heated by the initial current. You might also find our Buck Converter Calculator useful.