Learning Inductance Using an Oscilloscope

Among the basic components such as resistors, capacitors and inductors, for most of us electronics enthusiasts and even hardware engineers, inductors are often the last ones to master.

Today, we use an oscilloscope to learn about inductance!

What is Inductance

Generally speaking, an inductor refers to a device that temporarily stores energy in the form of a magnetic field.

Strictly speaking, inductance should be called inductor. Inductor has the property of resisting changes in current. This property is called inductance, but this article does not distinguish between inductance and inductor.

Inductors store energy in the form of magnetic fields

Below are some pictures of the inductors:

Common Inductor Pictures

An inductor is usually just a coil of wire. One of the basic properties of electromagnetism is that when current flows through a wire, it creates a small magnetic field around the wire.

Electric current generates magnetic field

The more coils you wrap around, the stronger the magnetic field you create.

When current starts flowing through the coil, the magnetic field starts to increase... and then stabilizes... so the coil has stored some electrical energy through the magnetic field.

Inductive charging animation

When the current stops flowing, the magnetic field begins to collapse and the magnetic energy begins to be converted back into electrical energy.

Inductor discharge animation

As we all know, capacitors store electrical energy in the form of static charge and resist sudden changes in voltage. Inductors are very similar, storing energy in the form of magnetic fields and resisting sudden changes in current.

If you can only remember one thing about inductors, please remember that the current in an inductor cannot change suddenly. It always lags behind for a certain period of time.

Minimum inductance circuit

Now let's learn about inductance through a small circuit. The square wave signal source is set as follows: Duty cycle: 50%, peak-to-peak value: 10V, bias: 5V.

Small circuit

1 kHz square wave

At a signal frequency of 1 kHz, the square wave looks perfect:

Perfect square wave

Let's see how the waveform changes after adding a 10 mH inductor in series with the circuit:

Minimum inductance circuit

Square waves aren’t so perfect anymore

With the inductor, the square wave isn't quite so perfect anymore. There's some lag in the voltage change. This is because it takes some time for the inductor to store and release the energy.

10 kHz square wave

Now let's try it with a higher frequency 10 kHz square wave:

10 kHz square wave

With the 10kHz waveform, it is more obvious that the inductor is resisting the sudden change in current.

100 kHz square wave

The square wave almost disappeared

As the frequency of the input square wave increases further, the waveform flattens further. At 100 kHz, there is almost no square wave anymore. At this point, it takes longer to store and release energy in the inductor than it takes for the input square wave to switch from high to low. So, in this case, the inductor starts to gradually flatten the voltage.

DC power supply?

If you add a 1000 uF capacitor after the inductor, you will get a very smooth DC voltage.

Magic capacitor

DC power supply?

Does not work with DC

Capacitors have the function of "passing AC and blocking DC", while inductors have the opposite function of "passing DC and blocking AC". Inductors have almost no effect on DC. They are just a few turns of wire with a resistance of only a few milliohms.