Inrush Current
I_inrush = Vpeak / Zsource
Quick answer
This calculator solves I_inrush = V_peak / Z_source for Inrush Current. With Peak Voltage 12 V, Source Impedance 0.47 kΩ, the result is 25.53 mA. It solves in 3 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
When a power supply or transformer is first energized, the input filter capacitors are fully discharged and appear as a short circuit, causing a large inrush current limited only by the source impedance and wiring resistance. The worst case occurs when the AC voltage is at its peak at the instant of switch-on. Inrush current can be 10-100 times the steady-state current and may trip circuit breakers, blow fuses, or damage connectors and relay contacts. NTC thermistors (inrush current limiters) are commonly used to reduce the inrush to safe levels.
Variables
- I_inrush — Peak inrush current (A)
- V_peak — Peak mains voltage at switch-on (V)
- Z_source — Total impedance including wiring, NTC, and source (Ω)
Practical Notes
For a 230 V mains supply, Vpeak ≈ 325 V. With only 0.5 Ω of wiring and source impedance, the inrush can reach 650 A. An NTC thermistor of 5 Ω cold resistance limits inrush to about 65 A. Active inrush limiters use a series MOSFET with controlled gate drive to gradually charge the input capacitors. IEC 62368 and other safety standards specify maximum allowable inrush currents.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| I | Inrush Current | A | pA, nA, µA, mA, A, kA |
| Vpeak | Peak Voltage | V | pV, nV, µV, mV, V, kV |
| Z | Source Impedance | Ω | mΩ, Ω, kΩ, MΩ |
Step by step
- 1. What you know: Peak Voltage 12 V, Source Impedance 0.47 kΩ
- 2. The relation: I_inrush = V_peak / Z_source
- 3. Substitute: Vpeak = 12 V, Z = 0.47 kΩ
- 4. Result: Inrush Current = 25.53 mA
Related quantities from the same inputs
- P = V × I = 0.3064 W
Worked examples
Minimum range
Given: Peak Voltage 0.33 V, Source Impedance 1 Ω
Result: Inrush Current = 0.33 A
Low range
Given: Peak Voltage 3.3 V, Source Impedance 10 Ω
Result: Inrush Current = 0.33 A
Typical
Given: Peak Voltage 12 V, Source Impedance 0.47 kΩ
Result: Inrush Current = 25.53 mA
High range
Given: Peak Voltage 0.23 kV, Source Impedance 4.7 kΩ
Result: Inrush Current = 48.94 mA
Maximum range
Given: Peak Voltage 2.3 kV, Source Impedance 47 kΩ
Result: Inrush Current = 48.94 mA
Solving for each variable
Solve for Inrush Current
Peak Voltage 0.33 V, Source Impedance 1 Ω → 0.33 A
Solve for Peak Voltage
Inrush Current 2 mA, Source Impedance 1 Ω → 2 mV
Solve for Source Impedance
Inrush Current 2 mA, Peak Voltage 0.33 V → 0.165 kΩ
How Inrush Current changes with Peak Voltage
| Peak Voltage (input) | Inrush Current (result) |
|---|---|
| 1.2 V | 2.553 mA |
| 3 V | 6.383 mA |
| 6 V | 12.77 mA |
| 9 V | 19.15 mA |
| 12 V | 25.53 mA |
| 18 V | 38.3 mA |
| 24 V | 51.06 mA |
| 48 V | 102.1 mA |
| 84 V | 0.1787 A |
| 120 V | 0.2553 A |
The same result in other units
| Unit | Value |
|---|---|
| µA | 25530 |
| mA | 25.53 |
| A | 0.02553 |
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<p><a href="https://vbatpower.com/formulas/inrush-current/">Inrush Current</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Inrush Current?
Use I_inrush = V_peak / Z_source. With Peak Voltage 12 V, Source Impedance 0.47 kΩ, Inrush Current works out to 25.53 mA.
What do i need to calculate inrush?
Rearrange for Inrush Current: with Peak Voltage 3.3 V, Source Impedance 10 Ω, Inrush Current is 0.33 A.
What is the formula for Inrush Current?
The formula is I_inrush = V_peak / Z_source. When a power supply or transformer is first energized, the input filter capacitors are fully discharged and appear as a short circuit, causing a large inrush current limited only by the source impedance and wiring resistance.
How do you find Peak Voltage when you know the other values?
Rearrange for Peak Voltage: with Inrush Current 2 mA, Source Impedance 1 Ω, Peak Voltage is 2 mV.
How do you find Source Impedance when you know the other values?
Rearrange for Source Impedance: with Inrush Current 2 mA, Peak Voltage 0.33 V, Source Impedance is 0.165 kΩ.
What units does Inrush Current use?
Inrush Current is shown in µA, mA, A. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
Related Concepts
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