Inductors and Transformers

 An inductor is simply a coil or loop of wire which opposes a change in the current within it. Inductance of a coil is measured in Henry. Inductance of a coil will depend on the number of turns in the coil, permeability of the medium inside the coil, cross section of the core, and the spacing between the turns of the coil. The more number of turns there are in the coil, higher the inductance. Higher permeability of the core also increases inductance. Coils with larger cross section have higher inductance. In the case of spacing between the turns of the coil, it is the other way round. Smaller the spacing between the turns of the coil, higher the inductance. The inductance in this discussion is actually self inductance of the coil. A change in the current in the coil induces a voltage within it. One Henry is the amount of inductance required for generating one Volt of induced voltage in the coil when the current is changing at the rate of one ampere per second. As in case of capacitance, smaller units are millihenry and microhenry.

Just as the change of current in a coil can induce a voltage in it, the change in current with corresponding change in magnetic flux can induce a voltage in a coupled coil which is very near to it or wound over it. That is known as mutual inductance and forms the basis of transformers. This applies only to alternating current which changes continuously. In case of direct current, change occurs only during the moment the circuit is switched on. So practically, there is very little opposition to a direct current by an inductor except the minimal resistance of the wire. But when an alternating current is applied to the same coil, there is significant impedance to current flow depending on the frequency of the alternating current, which is higher at higher frequencies than at the usual 50 Hz of the supply voltage.

An inductor is also called a choke because it tends to choke the flow of a fluctuating current through it. This property is used in the process of filtering a rectified direct current along with high value capacitors. As the inductance opposes changes in the current, it smoothes out the ripples in a rectified DC. But they are not as commonly used as filter capacitors in rectifier circuits as the inductance needed will make them quite bulky at the low frequency of 50 Hz. At higher frequencies, as the effect is much more, inductors can be used. An important use of an inductor in amateur radio is as a common mode choke in feedlines to prevent something known as common mode current, which has been covered in another video on this channel.