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Minimum Resistance Required
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Steady State Current
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Energy Rating Required
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Power Dissipation
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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.
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.
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.
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.
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.
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.
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.