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What is Inrush Current?

Inrush current is the instantaneous high 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 and inductors or transformers.

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

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 lớn 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 và 40 cycles for the current lớn stabilize to the normal operating value. If not limited, the high currents can damage the equipment in addition khổng lồ 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 áp lực on the rectifier components và 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


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Current waveform when the device is powered up – Image Credit

Limiting inrush currents

The two commonly used protection methods are, the passive, 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 to lớn inefficiency caused by the high power dissipation & losses in the series resistor.

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Image Credit

NTC thermistors

The method uses negative sầu temperature coefficient (NTC) resistor connected in series with the power đầu vào line.At ambient temperature the NTC device exhibits a high resistance, when the power is turned-on, the high resistance limits the amount of inrush current flowing inkhổng lồ the circuit. As the current flows, the thermistor temperature increases thereby reducing the resistance significantly. This stabilizes at less than one ohm, và is able khổng lồ allow the steady state current to flow into lớn the circuit.

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 lớn cool down khổng lồ its initial resistance and be ready lớn protect against a subsequent surge in case of a power failure & resumption, or when one switches off an equipment & then turns it one immediately.

Active sầu circuits

An active sầu 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.

The choice of the protection method depends on frequency of inrush currents, cost, equipment power cấp độ, reliability expected and performance. The NTC thermistor is widely used as the limiting device và preferred for its simple design & low cost as compared lớn the active sầu circuit, however, it has some drawbacks which make it unsuitable for use in extreme weather conditions or for sensitive applications.

Factors considered when designing for inrush current limiting.

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

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