An coil (or inductor) is a component that stores energy in a magnetic field and opposes changes in current.
At first glance, a coil is nothing more than copper wire wound around a core (air, iron, or ferrite). It looks like a short circuit, right? If you measure its resistance with a multimeter, it will read almost 0 Ohms.
When the current changes, the phenomenon of self-inductance appears.
The Concept: Current Inertia
The best way to understand a coil is to compare it to a heavy Flywheel in mechanics.
- At start-up: It is very hard to get it moving. Initially, the coil “resists” the flow of current.
- In motion: Once it is spinning, it is unstoppable. Current flows freely.
- When braking: If you try to stop it abruptly, the flywheel will rip your hand off. If you suddenly cut the current in a coil, it will generate a brutal voltage to try and keep the current flowing.
A capacitor opposes sudden changes in voltage. A coil opposes sudden changes in current.
The Unit: The Henry (H)
Inductance is measured in Henries (H). Like the Farad, a Henry is a gigantic unit.
(Microhenries): Standard for switch-mode power supplies (Buck/Boost). (Millihenries): For audio filters or motors.
How It Works: The Physics
When current passes through a wire, it creates a small magnetic field around it. If we coil the wire, these fields add up and concentrate in the center.
The fundamental formula defining the inductor is:
Translation for humans: The voltage (
- If the current is constant (DC), the change is zero, the induced voltage is zero. The coil is just a regular wire.
- If the current changes very fast (AC or pulses), the coil generates a huge opposing voltage that resists that change.
Common Types
Air Core
Simply wound wire (springs). They are used in radio frequency (FM, antennas) because iron cores do not work at those speeds.
Ferrite/Toroidal Core
The wire is wound around a grey or black “donut”. The ferrite concentrates the magnetism, multiplying the inductance. They are the kings of power supplies.
Axial
They look like fat, greenish-blue resistors. They are used for small signals.
Real-world Applications
What do we use this “magnetic spring” for?
Filters
The coil has a property called Inductive Reactance (
- DC Signal (0Hz): Passes freely.
- High-Frequency Noise: The coil becomes an impassable wall.
- Use: At the input of power supplies to prevent mains noise from entering your circuit (Common-mode choke).
Buck and Boost Converters
This is the star application nowadays. To step down from 12V to 5V without generating heat (as a linear regulator would), we use a switch that turns on and off very fast. The coil is responsible for smoothing out those pulses.
- ON: The coil “charges” magnetically, slowing the current rise.
- OFF: The coil “discharges” its magnetic energy, converting it back to electrical energy, maintaining current flow to the output. Result: A stable and efficient voltage.
Electromagnets and Relays
If you place a movable iron core inside the coil, the magnetic field will attract it. This is how relays, speakers, and electronic locks work.
The Danger: The Inductive Spike (Flyback)
We mentioned this in the Diode and LED articles, but now you understand the why.
If you have a coil carrying 10 Amperes and you instantly open the switch): According to the formula, if the time is almost zero, the voltage tends towards infinity.
The coil, desperate to maintain those 10 Amperes, will raise the voltage to 1000V, 2000V, or whatever it takes to create an electric arc (spark) through the air or punch through your transistor.
Solution: A flyback diode in anti-parallel. It gives the current a path to recirculate and die peacefully.
