Inrush Current Limiter Calculator

Use these calculators to support the selection and assessment of inrush current limiters for electrical and electronic circuits.



Inrush current is the initial surge of current that can occur when equipment is switched on. By calculating resistance, steady state current and total energy, these tools can help you understand key circuit requirements before selecting a suitable inrush current limiter.

1. Calculating Minimum Resistance

Calculate the minimum resistance required to limit inrush current to a specified maximum value.



This calculator uses the AC input voltage and maximum allowable inrush current to estimate the minimum resistance needed. The voltage peak is calculated from the AC voltage before determining the resistance value.

Example Calculation

Given an AC voltage of 230V and a maximum allowable inrush current of 5A:

Given:

Voltage VAC = 230V
Maximum allowable inrush current = 5A

Calculation:

Voltage peak = 230 × √2

Voltage peak = 325.22V

Resistance = 325.22 / 5
Resistance = 65.04Ω

Therefore, the minimum resistance required is approximately 65.04Ω.

2. Calculating Steady State Current

Calculate the steady state current based on power, voltage and efficiency.



This calculator helps determine the expected operating current once the initial inrush event has passed. It can be useful when comparing circuit requirements against the current rating of a selected inrush current limiter.

Example Calculation

Given a power value of 500W, voltage of 220V and efficiency of 90%:

Given:

Power = 500W
Voltage = 220V
Efficiency = 90%

Calculation:

Efficiency as a decimal = 90 / 100 = 0.9

Current = (500 / 220) × 0.9
Current = 2.05A

Therefore, the steady state current is approximately 2.05A.

3. Calculating Total Energy

Calculate the energy rating required based on voltage and capacitance.



This calculator estimates the energy stored in the circuit, which can help when assessing the required energy capability of an inrush current limiter.

Example Calculation

Given a voltage of 230V and capacitance of 50µF:

Given:

Voltage = 230V

Capacitance = 50µF

Calculation:

Capacitance in farads = 50 / 1,000,000
Capacitance = 0.00005F

Energy = ½ × 0.00005 × 230²
Energy = 1.3225J

Therefore, the total energy is approximately 1.32J.

The results provided by this calculator are intended as a guide only. While care has been taken to ensure accuracy, Charcroft Electronics does not accept responsibility for any errors or omissions. All results should be independently verified before being used in design, specification or application.