What Is Inrush Current and How Is It Used in Reliability Testing?

CONTENTS


1. What Is Electric Current?
2. What Is Inrush Current?
3. Types of Reliability Tests Involving Inrush Current
4. How to Calculate Inrush Current (With Circuit Example)
5. What to Do When Inrush Current Tests Fail
6. Final Thoughts

 

Hello, and welcome to the blog.

Today, I’d like to share something especially useful for engineers and professionals working

in the field of electronics and electrical systems — inrush current.

 

 

In daily operations, unexpected inrush currents can cause component failures or trigger protection

devices during power-up. That’s why understanding this concept is essential for both circuit design

and product validation.

 

 

In this post, we’ll cover:

 

 

• A brief refresher on what current is
• What inrush current means and when it occurs
• Types of reliability tests that involve inrush current
• How to calculate inrush current
• A simple circuit example
• And finally, how to reduce inrush current if a test fails

 

 

Let’s get started.

 

 

 

---

 

 

 

1. What Is Electric Current?

Electric current refers to the flow of electric charge — more specifically, the movement of electrons

through a conductor. It is driven by a voltage difference and is measured in amperes (A).

There are two main types of current:

 

 

• Direct Current (DC): Flows in one constant direction
• Alternating Current (AC): Changes direction periodically

 

 

In circuit design, accurately predicting and controlling current levels is critical to ensure safe and

stable operation.

 

 

 

---

 

 

 

2. What Is Inrush Current?

 

Inrush current is the initial surge of current that flows when electrical equipment is first powered on.

It can be several times higher than the steady-state operating current, even if only for a few

milliseconds.

Inrush current typically occurs in the following scenarios:

 

 

• When large capacitors begin charging at power-up
• When a transformer magnetizes its core
• When power is applied to circuits with initially low resistance paths

 

 

This brief but intense current spike can cause damage to components, trip fuses, or disrupt power

supplies. Therefore, predicting and managing inrush current is vital during both the design and

testing stages.

 

 

 

---

 

 

 

3. Types of Reliability Tests Involving Inrush Current

To assess how products behave under inrush conditions, various reliability tests are conducted. Here

are a few commonly used ones:

 

• Cold Start Test
  Measures the initial current when a device is powered on from a completely off state

 

 

• Inrush Current Tolerance Test
  Evaluates how much inrush current a component can withstand during multiple power cycles

 

 

• Switching Endurance Test
  Tests how components handle repeated on/off power switching

 

 

• Fuse Hold Test
  Determines whether the inrush current stays within the safe limits of a fuse

 

 

• Power Cycling Test
  Simulates stress caused by repeated power-up and shutdown over time

 

 

These tests help engineers understand how reliably a product can perform in real-world usage.

 

 

 

---

 

 

 

4. How to Calculate Inrush Current (With Circuit Example)

 

Before conducting a test, engineers often calculate the expected inrush current to set test parameters.

A simple calculation formula is:

Iinrush=VinRequivalentI_{inrush} = \frac{V_{in}}{R_{equivalent}}Iinrush​=Requivalent​Vin​​

Where:

 

For example, in a basic circuit with a capacitor:

 

 

 

 

When the power is applied, the capacitor begins charging through the resistor.

At this moment, the inrush current spikes due to the low initial resistance across the capacitor.

If the resistor’s value is small, the inrush current becomes significantly higher.

 

 

 

---

 

 

 

5. What to Do When Inrush Current Tests Fail

 

If a product fails a reliability test due to excessive inrush current, here are a few proven design

strategies to mitigate the issue:

 

Mitigation	How It Helps
NTC Thermistor	Starts with high R, self-heats to low R
Soft-Start IC / PWM	Ramps Vout or duty-cycle gradually
Staggered Relay / Timer	Powers sub-circuits in sequence
Surge-Suppressor (MOV / TVS)	Absorbs voltage spikes that accompany surges
Adjust R or Split Loads	Raises ReqR_{\text{eq}} or divides the peak

 

 

Selecting the best solution depends on the root cause of the test failure and the system's overall

design constraints.

 

 

 

---

 

 

 

6. Final Thoughts

Understanding inrush current is not just for textbook knowledge — it’s a practical necessity in real-

world engineering.

 

 

Whether you're designing consumer electronics, industrial controls, or automotive systems, managing

inrush current is key to ensuring product longevity and safety.

 

 

 

From basic circuit theory to advanced testing protocols, inrush current affects more than we realize.

Hopefully, this post has given you both the fundamental understanding and practical tools needed to

address it effectively.

Thanks for reading!

If you have questions or want to learn more about a specific topic, feel free to leave a comment.