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 to lớn charge the capacitors & inductors or transformers.Quý Khách đang xem: Inrush current là gì

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

Bạn đang xem: Inrush current là gì

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

Inrush current specifications

High inrush currents indicate more bít tất tay 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 wavekhung when the device is powered up – Image Credit

Limiting inrush currents

The two commonly used protection methods are, the passive sầu, where a resistive 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 & 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 lớn inefficiency caused by the high power dissipation and losses in the series resistor.


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 khổng lồ short circuit the device. The method ensures that thermistor is able to lớn cool down to lớn its initial resistance and be ready to protect against a subsequent surge in case of a power failure and resumption, or when one switches off an equipment & then turns it one immediately.

Active circuits

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

Xem thêm: Thầy Nguyễn Hoàng Khắc Hiếu, Học Online Cùng Giảng Viên Nguyễn Hoàng Khắc Hiếu

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