What is Inrush Current : Circuit & Limit Inrush Current Surges

Inrush current is nothing but an input current or locked rotor current and it will get increased as the device starts to age. To define it in a more simple form, after turning on any electrical instrument, a large current which exceeds the steady-state current value is called inrush current. It is caused due to many factors. When the power is turned on, the decoupling capacitors get charged by a large current flow. We can have a clear view of the inrush current by this diagram. It shows how the current waveform flows when the power is turned on. When power turned on, the current value reaches a peak current value which is larger than steady-state current value. When the steady-state value is stabilized, the current value decreases. As it is mentioned in the diagram the part before reaching steady-state value is inrush current. This article discusses an overview of what is an inrush current and its working.

Inrush Current Measurement

The meters which are having an inrush button can measure this current easily. Mostly electronic devices consist of inductive or capacitive components which will cause disproportionately high current when the device is switched on. It may be an FPGA or a drilling machine, inrush current is required for the devices. For the proper design of the circuitry, the correct determination or measurement of this current is required.


The R&S®HMC8015 power analyzer provides real-time signal processing and acquisition and also the dynamic signal range. This particular power analyzer will give accurate and fast results. The range of internal current measurements may vary from 15mA to 60A. The R&S®HMC8015 power analyzer provides additional sensor input for extended current measurements than the normal range. For displaying smaller current ranges the input is combined with external current sense resistor. For displaying larger currents, the input is combined with current probes

If we require maximum current, the numerical value gives the direct result. IP peak is the peak value that is displayed. The manual settings should be needed for the current range to meet the expected current using the Range Up button. Holding the Range Down bottom will make auto range mode activate. There is an inrush view for precise analysis of switch-on operation.

Circuit Diagram

The inrush current circuit diagram is shown below. The circuit that is used in limiting this type of current to protect the components and fuses is inrush current limiter. To limit this current mostly fixed resistors and negative temperature coefficient thermistors are used.

Inrush Limiter Circuit
Inrush Limiter Circuit
  • These negative temperature coefficient thermistors are larger than normal type thermistors and they are specially designed for power applications. These negative temperature coefficient thermistors initially provide higher resistance for preventing large current from flowing. These thermistors are normally disk-shaped and their power handling is proportional to the size and is disk-shaped with a radial lead each side.
  • Fixed resistors are another source to limit inrush current. One of the feedback regarding fixed resistors is their less efficiency, so they are preferred for lower power circuitry. The advantage of fixed resistors when compared with thermistors is its low cost

Inrush Current vs Starting Current

It is nothing but current transient that will flow in the first cycle for establishing a magnetic field on iron. After establishing the magnetic field, a current will be in steady-state starting current up to the time motor reaches its full speed. This current is a pure resistive current which is taken care of by DC resistance of the windings. This starting current is normally higher and the usage of star-delta starting relay the higher level can be reduced

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How to Limit Inrush Current Surges?

In various demanding temperature and power conditions, PTC thermistors provide reliable protection against short circuits and inrush current surges. These thermistors provide accurate measurement and control temperature.
When power supplies are turned on, current flow spikes may occur in electrical currents. These inappropriate currents may damage the power supply components and also other components that are receiving electrical current therefore measures are needed to protect the components from the issues.

Mostly one or two approaches are implemented to control these damages.

  • One is passive inrush current limiting, in this passive inrush current limiting protection device is used.
  • Another one is active inrush current limiting, in this active bypass circuit is used.

Which type of current limiting should be used is based on different variables like power rating, frequency, the operating temperature range, and also the system cost requirements.

Transformer Inrush Current

The maximum instantaneous current is drawn by the primary of the transformer while the secondary of the transformer is open circuit is called the transformer inrush current. Even though permanent fault is not caused by this current but it can cause unwanted switching in the circuit in the transformer.

A transformer will draw inrush current that exceeds saturation current at the point when the power is up. The magnetic property of the core is affected by the inrush current. Its magnitude depends on a particular point on the AC wave when the transformed is switched on. The AC wave when gone through the zero value, the current drawn will reach a high level and exceeds the saturation current

Capacitor Inrush Current

While filtering a power control circuit, a lot of capacitors are used. These capacitors can be of different types like aluminum polymer and tantalum. During normal operation ripple current will not cause any effects but at first, when the circuit is attached with battery capacitors will behave like a short circuit which consumes high inrush current exceeding ripple current

Toroidal Transformer Inrush Current

Toroidal transformers are made of ring-shaped cores for some low-frequency applications and these transformers are approximately 40% lighter when compared to E-I transformers. This toroidal technology is very much efficient when compared with the stacked core transformers due to its windings and core. Sometimes toroidal transformer inrush current may cause some damages when it is not properly managed. The inrush current also trip the circuit breakers may damage fuses and even it can make the complete transformer operation to fail.

How to Reduce Inrush Current of  DC Motor?

When DC motor is switched on, the instantaneous input current is drawn by the motor and the Inrush in DC motor is maximum. Limiting this current is important for limiting the damage of the DC motor. A DC motor uses stator for converting electrical energy into mechanical energy and the rotor which causes mechanical rotations.

Thus, this is all about an overview of inrush current, current limiter circuit, its working, and how to limit its surges. Here is a question for you, what is starting current?

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