# Types of Inductors

### Practical Equivalent

Inductors, like capacitors, are not ideal. Associated with every inductor are a resistance equal to the resistance of the turns and a stray capacitance due to the capacitance between the turns of the coil.
To include these effects, the equivalent circuit for the inductor is as shown in Fig. 1. However, for most applications considered in this text, the stray capacitance appearing in Fig. 1 can be ignored, resulting in the equivalent model of Fig. 2.
Fig. 1: Complete equivalent model for an inductor
Fig. 2: Practical equivalent model for an inductor
The resistance $R_l$ can play an important role in the analysis of networks with inductive elements. For most applications, we have been able to treat the capacitor as an ideal element and maintain a high degree of accuracy. For the inductor, however, $R_l$ must often be included in the analysis and can have a pronounced effect on the response of a system (Chapter "Resonance"). The level of $R_l$ can extend from a few ohms to a few hundred ohms. Keep in mind that the longer or thinner the wire used in the construction of the inductor, the greater will be the dc resistance as determined by $R = \rho l /A$. Our initial analysis will treat the inductor as an ideal element. Once a general feeling for the response of the element is established, the effects of $R_l$ will be included.
Like capacitors, commercially available inductors come in different values and types. Typical practical inductors have inductance values ranging from a few microhenrys ($\mu H$), as in communication systems, to tens of henrys (H) as in power systems. Inductors may be fixed or variable. The core may be made of iron, steel, plastic, or air. The terms coil and choke are also used for inductors.

### Appearance

All inductors, like capacitors, can be listed under two general headings: fixed and variable. The fixed air-core and iron-core inductors were described in the last section. The permeability-tuned variable coil has a ferromagnetic shaft that can be moved within the coil to vary the flux linkages of the coil and thereby its inductance. Several fixed and variable inductors appear in Fig. 3.
Fig. 3: Various types of inductors: (a) toroidal power inductor ($1.4 \mu H$ to $5.6 mH$); (b) surface-mount inductors on reels ($0.1 \mu H$ through $1000 \mu H$ on 500-piece reels in 46 values); (c) molded inductors ($0.1 \mu H$ to $10 \mu H$); (d) high-current filter inductors ($24 \mu H$ at 60 A to $500 \mu H$ at 15 A); (e) air-core inductors (1 to 32 turns) for high-frequency applications.