An inductor is a component that stores energy in a magnetic field and opposes changes in the current flowing through it.
It is often the most intimidating or most overlooked component in basic electronics, because its operation is not as intuitive as that of a resistor. Without inductors, modern high-efficiency electronics would not exist. Your mobile phone would not fit in your pocket because the power supply would be the size of a brick.
Today we will see what they are, how they work, and why they are the heart of switched-mode power supplies.
What Does an Inductor Do?
An inductor is, basically, a wire wound around a core (air or ferrite). When we pass a current through it, it generates a magnetic field.
The key physics is this: creating that magnetic field costs energy, and maintaining it stores energy.
The inductor has current inertia. It opposes sudden changes in the current flowing through it.
- If you try to increase the current suddenly: the inductor “slows down” the rise, generating a counter voltage while charging its magnetic field.
- If you cut off the current suddenly: the magnetic field collapses and returns the stored energy in the form of current, trying to maintain the flow even though you have opened the circuit.
If a mechanical analogy helps, imagine a heavy wheel (flywheel) spinning. It takes a lot of effort to get it spinning.
But once it is spinning, it takes a lot of effort to stop it; if you try to brake it suddenly, the force of the wheel will drag you.
Inductors in Switched-Mode Power Supplies
One of their main applications appears in switched-mode power supplies. In module 5 we will look at these power supplies in depth, but we first need to understand the basic concept.
Older (linear) power supplies lowered the voltage by “burning off” the excess as heat (using resistors or transistors). That is very inefficient.
Switched-mode power supplies (buck/boost) use inductors to transform energy instead of wasting it.
How Does a Buck Converter Work?
Suppose we want to step down from 12V to 5V efficiently:
- Switch ON: we connect 12V to the inductor. The current starts to rise slowly (slowed down by the coil). The inductor is “charging” with magnetic energy.
- Switch OFF: we cut off the 12V input. But the current does not stop! The inductor, acting like inertia, releases its stored energy and continues pushing electrons toward the output (5V) for an instant.
- We repeat this thousands of times per second.
The result is that we have transferred energy from the input to the output almost without losses, using the magnetic field as a “bucket” for transport.
LC Filtering
We already saw that capacitors clean up voltage noise. But sometimes, especially at high frequencies (radio frequency or noise from switched-mode power supplies), the capacitor is not enough.
Here we use the LC filter (inductor in series and capacitor in parallel).
- The inductor blocks (presents high impedance) high-frequency signals (fast noise), preventing them from passing.
- The capacitor absorbs whatever little may have managed to pass.
It is a much more effective cleaning team than the capacitor alone. That is why you will see coils at the input of laptop power supplies: they are there to prevent the power supply’s noise from entering the electrical grid or vice versa.
Real-World Parameters
When you buy or salvage a coil, pay attention to two things in addition to its inductance (henries, H):
- Saturation current (
): if you exceed a certain current, the core saturates and the inductance decreases. The current can then rise rapidly and damage the converter if its protections do not act. - DC Resistance (DCR): in the end, it is a long wound wire. It has ohmic resistance. If the DCR is high, it will heat up.
