Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on. Alternating-current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. Power Converters also often have inrush currents much higher than their steady-state currents, due to the charging current of the input capacitance.

A discharged or partially charged capacitor appears as a short circuit to the source when the source voltage is higher than the potential of the capacitor. A fully discharged capacitor will take approximately 5 RC time cycles to fully charge; during the charging portion of the cycle, instantaneous current can exceed load current by a substantial multiple. Instantaneous current declines to load current as the capacitor reaches full charge.

In the case of charging a capacitor from a linear DC voltage, like that from a battery, the capacitor will still appear as a short circuit; it will draw current from the source limited only by the internal resistance of the source and ESR of the capacitor. The charging current will be continuous and decline exponentially to the load current. For open circuit, the capacitor will be charged to the DC voltage.

Safeguarding against the filter capacitor’s charging period’s initial current inrush flow is crucial for the performance of the device. Temporarily introducing a high resistance between the input power and rectifier can increase the resistance of the powerup, leading to reducing the inrush current. Using an inrush current limiter for this purpose helps, as it can provide the initial resistance needed.

A cost effective way of reducing the inrush current is to place a resistor in series. While choosing the resistor, the designer should be aware of the inrush handling capability of the resistor. Wirewound resistors can be designed for reliable high inrush current handling capability and repeatable performance. The capability of the resistor can be far in excess of its normal rated load and circuits can be optimised in discussion with the resistor manufacturer. Send us details of your inrush current problems for optimised solutions.