The previous tutorial presented the voltage source as a circuit element that supplies energy to an electrical network. In a functional circuit, this potential energy is manifested as the flow of electric charge, which we call current.
Clearly, then, there is a relationship between voltage and current: if a voltage source is connected to a component or a network of components, current will flow.
The amount of current, however, is not determined solely by the voltage source. The wires and components in the circuit will present some amount of opposition to the movement of electric charge, and this opposition influences the magnitude of the current produced by a given voltage source. We use the term resistance (R) to capture the idea of this inherent tendency to oppose electric current, and resistance comes with a unit—called the ohm—that allows us to quantify opposition to electric current.
All materials (except superconductors) present some amount of resistance. The terms conductor and insulator do not identify materials that have, respectively, zero resistance and infinite resistance. Rather, they are convenient ways of referring to materials that exhibit very low resistance and very high resistance.
In the context of electrical engineering, we deal with conductors and insulators in different ways according to the characteristics of a given application. For example, the resistance offered by short copper traces on a printed circuit board is often low enough to be ignored, and we can analyze the circuit as though the connections between components have zero resistance. In contrast, the resistive imbalances in a cable consisting of plated copper wires might cause significant problems in a high-precision sensor application.
When the goal is simply to transfer electrical energy, resistance is a nuisance: it reduces the amount of current delivered to the user and dissipates power as unwanted heat. However, resistance plays an essential role in low-voltage circuit design. Electrical engineers constantly use resistance for such tasks as reducing the amplitude of a voltage signal, establishing the gain of an amplifier circuit, limiting the current through a light-emitting diode, and creating frequency-dependent networks called filters.
The intentional resistance that we incorporate into electronic circuits comes in the form of individual components called resistors. Two symbols are used to represent resistors in a circuit diagram:
Engineers determine the amount of resistance needed in different portions of a circuit, and then the physical circuit is created by installing resistors that have the appropriate resistance specification.
Electrical circuits are filled with resistance that does not originate from resistors. Wires, printed-circuit-board traces, pins on integrated circuits, leads on capacitors and inductors—when we’re working with typical electronic systems, there is no such thing as a perfect conductor.
This type of resistance is called parasitic resistance, and the degree to which it affects the functionality of a circuit depends on various factors. In many cases, parasitic resistance can be ignored because it will not alter the circuit’s functionality in any noticeable way.
The following diagram depicts how a simplified equivalent circuit for a capacitor uses an additional resistor to represent the component’s parasitic resistance.
The unit that is most closely associated with resistance is the ohm. The symbol for the ohm is Ω, so a fifty ohm resistance is written as follows: 50 Ω. The significance of the ohm will be discussed in a future tutorial when we study Ohm’s law.
The ohm focuses on a material’s tendency to oppose current. If we want to quantify a material’s tendency to allow the flow of electric current, we can use the siemens (the symbol for this unit is S). The ohm quantifies resistance, and the siemens quantifies conductance. If we take the reciprocal of a resistance in ohms, we obtain the conductance in siemens. Thus, a 50 Ω resistor has a conductance of 1/50 = 0.02 S.
We’ve covered some essential information related to resistance, which is an extremely important aspect of electronic systems. In the next tutorial we’ll explore two other fundamental circuit concepts: capacitance and inductance.
by Luke James
In Partnership with Mentor Graphics
by Gary Elinoff