What is Inrush Current?

Inrush current is the instantaneous high input đầu vào current drawn by a power supply or electrical equipment at turn-on. This arises due lớn the high initial currents required lớn charge the capacitors và inductors or transformers.Bạn vẫn xem: Inrush current là gì

The inrush current is also known as the switch–on surge, or the input đầu vào surge current.

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At turn-on, the discharged capacitors in power supplies offer low impedance that allows high currents to lớn flow into the circuit as they charge from zero to their maximum values. These currents can be as high as trăng tròn times the steady state currents. Even though it only lasts for about 10ms it takes between 30 and 40 cycles for the current to stabilize lớn the normal operating value. If not limited, the high currents can damage the equipment in addition lớn producing voltage dips in the supply line and causing malfunctioning of other equipment powered from the same supply.

Inrush current specifications

High inrush currents indicate more stress on the rectifier components and hence lower reliability. The inrush current is specified in terms of:

Average during a half cycle or peak: – where peak is about 40% larger than the averageVoltage range – 120V or 240VOperating temperature range in which the limiting technique is effective


Current waveform when the device is powered up – Image Credit

Limiting inrush currents

The two commonly used protection methods are, the passive sầu, where a resistive sầu current limiting device is connected in series with the supply, và the active sầu which uses an electronic circuit consisting of resistors, a switching device and a control circuit.Resistor in Series

For small wattage supplies, the resistor is connected in series with the power đầu vào line. However, the method is not suitable in larger power supplies due khổng lồ inefficiency caused by the high power dissipation and losses in the series resistor.

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Image Credit

NTC thermistors

Parallel electronic switch

Using an electronic switch or relay in parallel with the thermistors or resistor. The currently limiting device offers high resistance at startup after which the switch is turned on to short circuit the device. The method ensures that thermistor is able khổng lồ cool down to its initial resistance & be ready lớn protect against a subsequent surge in case of a power failure & resumption, or when one switches off an equipment và then turns it one immediately.

Active sầu circuits

An active circuit comprises of resistors, transistors, triacs or thyristors & a control circuit khổng lồ drive sầu the switching device. These are suitable for applications requiring hot restart capabilities.

Factors considered when designing for inrush current limiting.

Load capacitance valueSteady currentAmbient temperatureSupply voltageRequired current reduction of the inrush current

Soft start function, low inrush current power supplies