Real and Ideal Circuit Components: You are required to produce a report on real components. Focussing on how they are constructed and the ways in which they are less than ideal. Illustrate your essay with specific examples.

It is vital when it comes to understanding the intricacies behind all electrical and electronic circuits to investigate the characteristics of the sometimes complex components which depend on many and various factors. An ideal model is frequently used to simplify the analysis and distinguish from real world circumstances. This report will focus on how these components have features which are less than ideal in a real world environment.

Resistors

The resistor introduces resistance into a circuit and determines, for a given current, the rate at which electric energy is converted into heat or radiant energy (Howe, 1969).

Power dissipation

The first feature associated with resistors is its resistance which is measured in ohms (Ω). Resistors have either fixed or variable resistance depending on their structure and the application they are used. Because resistors dissipate heat as energy, they are also rated in regards to the power of this heat dissipation, with breakdown threshold the point where overheating leads to damage (Kuphaldt, 2001).

Construction

A basic modern construction of a low-power resistor consists of a cylindrical ceramic core coated with a uniform layer of resistance material, bounded with conducting material over each end as depicted in Figure 1. By changing the angle of the spiral groove, cut around the resistor body, the length and width of the spiral stripe can very accurately be adjusted and hence the resistance of the unit. The resistor is further coated with an insulating material such as phenolic or ceramic, and is colour coded to indicate the value of the resistor (see Appendix I) (Bigelow 2007).

Applications, Range of values available, Sizes

Physically these resistors are of the range of approximately 3mm long and 1.5mm in diameter depending on application. Resistor applications are electrical/electronic devices from portable radio to modems, having purposes such as setting biases, controlling gain, fixing time constants, matching and loading circuits, voltage division and heat generation. High-power resistors that can handle large currents and high temperatures have larger physical attributes because of the dependence of size to power dissipation. There are many types of resistors which can be grouped in three categories: fixed resistors (wire-wound, metal-film, etc.), variable resistors (potentiometers, rheostats) and special purpose resistors (integrated circuit, varistors, etc.) (Gore 1982). Resistance range, power dissipation and operating temperatures are illustrated below.

Temperature effects

From the structure of the resistor, factors which affect its performance can be deduced.

1. Temperature measured on the resistor body due to internal heating and the ambient operating temperature, define the maximum temperature at which the resistor is functional and can dissipate power.
2. An unwanted voltage fluctuation generated within the fixed resistor which includes Johnson noise (due to thermal agitation) and the noise caused by the current flow, cracked bodies, loose end caps or leads also degrade the resistors function. In variable resistors noise can be generated by changing values and the imperfect electrical path between the contact and resistance element (Gore, 1982).

Nonlinearity

Resistors are manufactured to provide a definite, stable value of resistance, behaving like the linear model depicted below in Figure 2. In reality, current conductions in circuits where the medium is either wire or gas high temperatures develop or ionization commences respectively and the model takes the form of a non-linear curve as shown (Kuphaldt, 2001).

Capacitors

The capacitor is a passive electric-circuit component. Purpose of the capacitor is to store a large electric charge in a small volume, which defines capacitance measured in Farads (F) (Grillo & Pelton, 2001).

Saturation

The capacitor consists of two metal electrodes or plates, which are not necessarily flat, separated by a dielectric (insulator). The amount of charge a capacitor can hold depends on the size of the plate surfaces and the length between them. When the capacitor is fully charged (saturation point) it can only gradually react to any further changes in voltage applied (Peck & Martin, 1969).

Construction

A modern construction of a capacitor consists of a disc of ceramic material. Both sides of the disc are coated with solder, which consists of tin and lead and form the plates of the capacitor. Wire leads are attached to the solder plates to form the structure shown (Bigelow 2007).
The completed construction has another ceramic coat applied (Figure 3, right), insulating the entire structure providing some additional protection. Different types of ceramic can be used in order to control how the capacitor behaves as the temperature and applied voltage change (Bigelow 2007).

Applications, Range of values available, Sizes

There are various types of capacitors which are used to filter, couple, tune, block dc, pass ac, bypass, shift phase, compensate, feed through, isolate, store energy, suppress noise, and start motors. They must also be small, lightweight, reliable, and withstand adverse conditions. Capacitors are grouped according to their dielectric material and mechanical configuration (Table Appendix II) (Shu-Park, 2000).

Non Linearity, Power Dissipation, Temperature effects

Capacitors in real world applications are sustainable to performance losses. A real capacitor also includes an inductance and resistance and can be portrayed as the equivalent circuit below.
Where Rs is the series resistance due to wire leads, the electrodes, and contact terminations. Rp is the shunt resistance due to resistivity of dielectric and case material and dielectric losses. L is the inductance due to leads and electrodes.
The power factor defines the electrical losses in a capacitor under an ac voltage. Ideally the current leads the applied voltage by 90o. Due to the dielectric, the electrode, and contact termination losses, the capacitor exhibits a phase angle less than 90o (Gore, 1982).
(Peck & Martin, 1969)
3. Dielectric material other than a perfect vacuum creates dielectric absorption. During the period when the voltage is applied to the capacitor a part of the leakage current is trapped in the dielectric.
4. The dielectric strength of the capacitor identifies the maximum voltage that can be applied preventing the capacitor to conduct (breakdown voltage).
5. Capacitors are associated with a temperature coefficient which determines their stability under varying temperatures (Peck & Martin, 1969).

Inductors

The inductor is another passive component which has the function of storing magnetic energy. The magnetic energy is continually stored if current constantly flows through the inductor. This conductor defines the inductance in a circuit, measured in Henry (H). In opposition with the capacitor, the inductor strives to resist change in the current flowing through it. The inductance depends on the number of coils, the cross sectional area covered from them, the core and the length of the coil (Shu-Park, 2000).

Construction

The structure of an inductor comprises a wire in the form of a coil which is where the magnetic field is concentrated on. A core of ferrite (powdered iron) is usually placed which is laminated to increase the concentration of the magnetic field as seen in figure 7 (Bigelow 2007).
Applications, Range of values available, Sizes
Depending on the application, different types of inductors are manufactured. Solenoids which are basic coil wounded formers with or without a core, adjustable inductors and power transformers are a mere example of the inductors made to suit the needs of a circuit. Low inductance values are wound on nonmagnetic form, powdered iron cores are used for intermediate values and a ferrite core in addition to a magnetic sleeve of the same material is used for high values. Inductors are usually used in tuned circuits as filters and along with capacitors creating oscillators (Gikow, 1969).

Saturation, Non Linearity, Power Dissipation, Temperature effects

The diagram of flux density against the magnetic field intensity shows how the increasing current leads to the saturation point of the inductor.
In a non ideal environment there are losses leading to power dissipation (Plant, 1990).
– Resistance in coiled wire which leads to loss in inductance.
– Eddy current loss (The parasitic current induced by varying magnetic field)
– Hysteresis loss (Heat due to alternating magnetic field)
Temperature change influences alloys which succumb under the heat making the inductor prone to malfunctioning (Dakin, 1969).