Like the capacitor, an inductor is a two terminal storage device except instead of storing an electrical charge, an inductor stores a magnetic field. An inductor has many uses. One use is to block instantaneous changes in current. Another use is to convert an electrical current into mechanical force in devices such as solenoids. Fuel injectors, solenoid pumps, relays, and solenoid valves are examples of the application of inductors. Since they block instantaneous changes in current, they can also be useful in the construction of filters for blocking unwanted frequency content.
The unit of measurement for inductance is the Henry. One Henry is the inductance that will establish 1V across the inductor due to a change in current of of 1 amp per second through the inductor. This will make more sense when we get into how inductors work and their transient behaviors in the sections below. One Henry is typically larger than was we will work with in electronics. More commonly we will be working with millihenry (mH) or microhenry (μH).
1 μH = 0.000001 H
1 mH = 0.001 H
Unlike capacitor, inductors have one basic architecture. All inductors are essentially coils. In fact, the inductance of an inductor depends upon the number of turns, area and length of the coil. The permeability of the material within the coil also plays a role in the inductance as shown in the equation below.
As can be seen by the inductance equation, the inductance of an inductor is very much dependent upon the physical dimensions of the coil. As such, high values of inductance are avoided in circuit design when size constrains are in effect. The permeability component of the inductance is the permeability of the material that is encircled by the coil. Material data sheets may include this value.
The time it takes for the current passing through an inductor to reach a steady state following a change in voltage across the inductor is known as the "transient period". Like the capacitor, this transient period is equivalent to five time constants. The time constant can be calculated from the resistance and inductance in the circuit.
The Multisim simulation below demonstrates the transient behavior of an inductor. This circuit is the same as that simulated on our capacitor page except the capacitor is replaced with an inductor of a value that achieves the same time constant of 60 μs. That makes the transient period 60 μs x 5 = 300 μs. Notice that the voltage across the resistor in the resistor-inductor (RL) circuit is exactly the same as the voltage across the capacitor in the RC circuit and the voltage across the inductor in the RL circuit is exactly the same as the voltage across the resistor in the RC circuit.
In the beginning of this section we said inductors block instantaneous changes in current. Notice how when the input into the circuit goes high, the voltage across the inductor does as well because the current through it is low. At the instant the voltage across the inductor is applied, the inductor behaves like an open circuit. After five time constants the voltage across the inductor becomes zero and it behaves like a short circuit.