In this guide, we are going to dive deep into everything you want to know about inductors.
This is part of our basics series on resistors, capacitors, and inductors.
What Is An Inductor?
How do we define inductor? Here is the best inductor definition:
inductor =electrical component that stores electrical energy in the form of a magnetic field
#1 Lesson: The main thing to remember about inductors is that they "love" to keep current steady, and will use voltage to make it happen.
Let's discover why these statements are true by looking a little more closely at inductance.
Inductance
The basic property of inductors is something called inductance. We will look at a great inductance definition next.
What Is Inductance?
Great question. The answer starts with magnetic fields, as covered next.
Magnetic Fields
Imagine we have a nice piece of copper wire in front of us. If the wire is just sitting there, it simply acts like a tube of copper.
That means that the electrons in the copper atoms all have their own individual magnetic fields. However, the electrons are randomly orientated and therefore all of their collective magnetic fields cancel each other out.
Nothing is apparently happening.
Imagine a big crowd of people all saying something different at the same time. It's just a bunch of noise that doesn't do much.
Now imagine that we hook up a voltage source to the copper wire and this causes current to flow through the wire. Now all of the sudden, the current flow causes the electrons' individual magnetic fields to align with each other.
Stacking all of those fields together, it creates a uniform magnetic field that extends beyond the copper wire.
This is like our crowd of people starting to sing the same song together. It's loud, it's clear, and it's meaningful because their voices are all adding together and aligned.
Current flowing through a conductor creates a magnetic field by aligning electrons.
Electromotive Force
Now that we see that current through a conductor causes a magnetic field to be generated by the conductor, what does that do for us?
Well, that was using current to create a magnetic field with a conductor. Did you know that you can reverse it and use a magnetic field to create current in a conductor? It works both ways.
One thing to remember is that in order to get current to flow in a conductor with a magnetic field, we need the magnetic field to be changing in time. A constant magnetic field will not cause current to flow.
When a changing magnetic field causes current to flow in a conductor, this creates a voltage, which we can refer to as a electromotive force, or EMF.
The trick is a changing magnetic field. There are all sorts of tricks one can play to make a magnetic field change in time. For example, you can move a permanent magnet back and forth with motion.
Or, you could change the current flowing through a conductor, making its magnetic field that it generates change in time. This changing magnetic field will not only affect the conductor itself, called self induction, but also any other nearby conductors.
There are many examples of devices to generate magnetic fields like this including solenoids.
And that is exactly how we get back to inductance. A great inductance definition from physics is as follows:
inductance = degree to which a changing current causes a electromotive force in itself and other nearby conductors
The change in nearby conductors due to EMF is known as mutual inductance.
Inductance Units
The units of inductance is the Henry, H. It is named after Joseph Henry, who discovered electromagnetic induction.
The best way to wrap your head around the inductor units of henries is:
1 Henry = current changing at 1 Amp per second creating an EMF of 1 Volt
For inductors, we use henries to have a way of comparing its capabilities to that of 1 henry.
What Does An Inductor Do?
With inductance in mind, you can begin to see how convenient it can be to be able to manipulate electrical energy in the form of magnetic fields.
An inductor is a nice device that allows us to do just that.
We can exploit the laws of physics by creating devices like inductors so that we can take advantage of their properties and capabilities.
From power electronics to filtering and countless other circuit applications, inductors are one of the fundamental building blocks of electronics.
Inductor Symbol
There happens to be a great way to represent inductors in circuits.
Many inductors are just loops of wire with an air core. They get the simple symbol which looks like some squiggly lines with two terminals.
Note that inductors can also be drawn with interlocking circles instead of squiggles.
You can also make inductors with a loop of wire around a material core. The symbol for those usually has two bars near the squiggles.
And did you know that there are inductors that can be adjusted? They are called variable inductors, and have an arrow crossing over the squiggles.
Check out the symbol for inductors in the following diagram:
How Does An Inductor Work?
Now it's time to take a closer look to see just how inductors work.
Air Core Inductor
An air core inductor is simply a coil of wire. The easiest example of this is if you look at a spool of magnet wire as seen below.
Remember, we mentioned that current flow through a conductor creates a magnetic field. It turns out that if use just a straight wire, then the magnetic fields will form rings around the wire.
You can use something called the right hand rule to point your thumb in the direction of current flow, and then curl your fingers which will tell you the orientation of the magnetic field lines.
There is a notorious experiment you can do if you use some iron filings, a piece of paper, a wire, and a voltage source. The experiment allows you to see this effect in action.
I'll save you the time of doing the experiment by telling you that you can form concentric circle patterns with the iron filings by running current through the conductor.
What is interesting is that when you wrap a conductor in loops, like you do in a coil, then the magnetic fields created by each loop start stacking together to form an even stronger magnetic field.
This is the idea behind an inductor coil. You can manipulate the effects at play here by creating more loops, commonly referred to as N loops.
Iron Core Inductor
What's even cooler than a coil with an air center? An iron core inductor, that's what.
So what does sticking some metal inside the loops of a coil do for us? Well, the metal helps steer the magnetic field even more, which enhances the ability of the inductor.
The cause is known as magnetic permeability, which is the ability of a material to support the formation of a magnetic field within itself.
Inductance Formula
We now have the main ingredients of the things we can control in an inductor. Earlier, we noted that the number of loops, N, is a factor.
Looking at a coil of wire, we must also point out that the physical size plays a role. For example, the cross area of the loops is important as well as the length of the coil. Bigger is stronger right?
Now we are ready to put all of these factors together for a good inductor equation. The inductance equation is as follows:
In the equation, we have the magnetic permeability (u), the number of loops (N), the cross section area of the loop (A), and the length of the coil (l). The equation results are in henries (H).
Energy Storage
The energy of running current through an inductor is stored as a magnetic field.
In other words, if we turn on a voltage and current flows through an inductor, then a magnetic field will form. That magnetic field represents the energy in an inductor that we spent in creating it with voltage and current.
Given that inductors always fight changing currents, there's a certain amount of extra work our current will have to do initially to create a stable magnetic field.
Once we stabilize the current and magnetic fields, we can calculate all of the energy that we used to store that magnetic field.
It turns out by some hefty math derivation, with integrals, you come down to a simple inductor formula for energy stored in an inductor as follows:
For this equation, we have the inductance of the inductor (L), and the final current established (I).
Inductor Impedance
The AC impedance of an inductor, also known as inductor reactance is pretty straight forward.
The inductive reactance formula is as follows:
From the equation, we can see that we have the frequency in radians (w) and the inductance (L) of the inductor.
As you can see, the impedance increases with increasing frequency.
Inductor Packaging
Through Hole
You can get inductors in many different package options. In fact, you can even make your own if you have some iron cores and some wire. Magnet wire works better because it has much thinner coating, allowing you to get more and tighter loops.
Typically, in higher current circuits or for prototype breadboarding, its more convenient to get inductors that are through hole, meaning they have leads that can be easily manipulated.
For example, if you buy a pack of inductors from Radio Shack, you can get some windings like the one shown below.
You can also get inductors that are much smaller depending on what you need to use them for. The smaller ones almost look like through hole resistors and even have color codes to identify them.
Surface Mount
Given our age of mass manufacturing, it makes more sense to have inductors in surface mount form so that they are better handled by pick and place machines during circuit board assembly.
Depending on the inductor size, there is an array of different package options.
An example of a bigger inductor used in a power electronics circuit can be seen in the picture below.
You can see the magnet wire windings in the inductor in the picture. To the left of this inductor, you can see a grey rectangle component that is a ferrite bead, which we will cover later.
Standard Inductor Values
Inductor values are typically found in the nano, micro, and milli Henry ranges. Different applications require different values.
Typical popular values include 1 nH, 10 nH, 100 nH, 1 uH, 10 uH, 100 uH, and 1 mH and many other values in between. As you go higher, the size of the inductors starts increasing as well as the price.
Coilcraft is a well known inductor company and has many great tools on their website to help select the right inductor for different circuit designs.
Using Inductors
Inductors In Parallel
What happens when we put inductors in parallel? It turns out that they are similar to resistors.
The equation to calculate the equivalent inductance is as follows:
Here is an example circuit. Let's work through an example inductor circuit to demonstrate the equation. The circuit has three inductors: L1, L2, and L3. Note that uH stands for micro-Henries.
To calculate the total inductance, we do the following:
total L = 1 / (1/L1 + 1/L2 + 1/L3) = 1 / (1/1 uH + 1/2 uH + 1/3 uH) = 0.545 uH
Inductors In Series
When we have inductors in series, it's much easier to calculate the equivalent inductance.
The equation in this case is as follows:
And here is an example circuit where we have three inductors:
To calculate the total inductance, we do the following:
total L = L1 + L2 + L3 = 1 uH + 2 uH + 3 uH = 6 uH
Power Electronics
For digital electronics these days, you may never come across an inductor. However, since switching power supplies are much more common, you will often find inductors in these circuits.
Inductors are a great choice here for energy storage because as discussed earlier, inductors love stable current. The inductor voltage changes to maintain current.
This ability allows the switching controller to store the energy it needs externally in order to maintain a desired output voltage of the regulator circuit.
Induction Coil
An induction coil is a neat device that allows you to create high voltage pulses from a low voltage DC power supply.
How is this done?
It uses the fundamentals that we have discussed. It can be thought of as a transformer with a set of loops around a common core for both primary and secondary windings.
The primary might have 100 loops and be hooked to a DC voltage. This establishes a magnetic field that is constant.
Now when the DC voltage is quickly removed, the inductor (primary windings around the core) tries to keep the current that was flowing stable and creates a high EMF voltage spike.
There is also a secondary winding around the same core that might have thousands of loops. So the EMF voltage spike that was just created now gets amplified by the higher number of loops because of the common core.
Usually, the primary and secondary windings are arranged in order to generate an output voltage that can be thousands of Volts, which is enough to create an electrical spark across an air gap.
This simple principle is behind the ignition coil in your car's engine. There are many other applications for it as well.
Chokes
An inductor choke is simply an inductor used to block higher frequencies of AC signals.
One popular type of choke is called ferrite beads. They are simply a conductor or conductors that are wrapped around a common core. You will often find this in surface mount chip form near the front end of the DC portion of the power supply circuit.
You can also get chokes as clamp-on components that are great little devices to help you remove cable line noise. You have probably seen these on your older VGA video cable in the past.
A picture of a typical common mode choke can be seen below. Notice the clamshell design where the user can open the assembly, place it around a cable, and then lock it closed.
Companies will often put them on a DC power cable near the input of the device they are powering if the power supply is separated from the unit by a cable.
It turns out that if you look into the physics of it, you will find that by putting conductors around the same iron core, the common mode noise flowing in the conductors will create fields that will help cancel themselves out.
These choke inductors are great to have in the lab to troubleshoot possible noise sources.
What was the most interesting thing you discovered in this article? Let me know in the comments.
If you are ready to move on to more advanced topics, check out diodes or transistors next.
