KickStart by Mike Tooley
Part 3: Making sense of inductors Our occasional KickStart series aims to show readers how to use readily available low-cost components and devices to solve a wide range of common problems in the shortest possible time. Each of the examples and projects can be completed in no more than a couple of hours using
off-the-shelf parts. As well as briefly explaining the underlying principles and technology used, the series will provide you with a variety of representative solutions and examples, along with just enough information to enable you to adapt and extend them for your own use.
This third instalment shows you how to design and realise inductive components in a wide range of electronic applications and, in keeping with the KickStart philosophy, we’ve provided ideas for you to start making use of inductors in your own projects.
frequency-selective circuits. Inductors are also used in noise and interference suppression applications across a wide range of frequencies, from mains (50/60Hz) to RF. The most basic form of air-cored inductor is nothing more than a coil of insulated wire wound on a non-conductive former. Larger values of inductor can benefit from the enhanced magnetic properties offered by a core made from a ferromagnetic material such as steel (often laminated) or ferrite (a ceramiclike material with magnetic properties). Inductors are widely available in a range of off-the-shelf values and ratings, but they can also be made at home relatively easily, though this can sometimes be a hit and miss process. Before we explain this, take a look a typical selection of the small inductors that you might encounter in electronics, as shown in Fig.3.1 and Table 3.1.
When is an inductor not an inductor?
Fig.3.1. A typical selection of small inductors.
W
hat are inductors? They are components that
store energy in the form of an electromagnetic field, and like resistors and capacitors, they are classified as passive devices. When a changing or alternating current is applied to an inductor it will respond by temporarily storing energy in an electromagnetic field, releasing it back at some later time. This temporary storage of energy has the effect of slowing the change in current that causes it.
Unfortunately, compared with resistors or even capacitors, inductors rarely behave perfectly, and this can be an important factor in their design and application. Fig.3.2 shows an ‘ideal’ inductor together with its real-world equivalent which incorporates the losses and stray reactance found in ‘real’ components. The essential features shown in Fig.3.2(b) include: L The inductance value, determined by the number of turns and magnetic properties of the core material. Cp The distributed capacitance (ie, the lumped-together capacitance between adjacent turns, connecting wires and/or terminals). This capacitance can be problematic at high frequencies where the capacitive reactance may fall to a relatively low value.
When an alternating current (eg, a sinewave) is applied to an inductor the effect of the inductor (in terms of opposition to current flow) is referred to as ‘inductive reactance’. The faster the current changes, the greater will be the Table 3.1 Brief details of the inductors shown in Fig. 3.1 reactance, and vice versa. Hence inductive reactance is directly proportional to the Ref. Inductance Notes frequency of the applied current. Like resistance (which strictly applies to steady A 3.5mH Adjustable ferrite inductor for PCB mounting direct current) reactance is measured in B 2.8mH Choke for use in low-frequency radio applications ohms (ȍ) and is defined simply as the ratio of applied alternating voltage to the current C 1mH Small ferrite pot-cored inductor for general-purpose applications flowing. A 100mH inductor will exhibit a D 100µH Large ferrite pot-cored inductor for high-current applications reactance of approximately 63ȍ at 100Hz, E 100µH Fixed toroidal inductor for use in filters 630ȍ at 10kHz, and 6.3kȍ at 100kHz. Inductors are used in power supplies F 33µH Wire-ended axial-lead choke for RF applications to filter alternating current from direct G 3.9µH Small axial-lead inductor for general-purpose applications current, in which case they are sometimes referred to as ‘chokes’ or ‘reactors’. In audio and communication systems inductors are often employed in filters and other 36
H
2µH
Adjustable ferrite-cored inductor for use in RF tuned circuits
I
300nH
Air-cored inductor for use in VHF and UHF radio applications Practical Electronics | June | 2021
