Passive Elements

Introduction to Electronic Circuits: 1 of 3

The electronic devices we encounter all around us are driven and controlled by the flow of electrical current through electronic circuits. Each circuit is an arrangement of electrical elements designed to perform specific functions. Circuits can be engineered to carry out a wide variety of operations, from simple actions to complex tasks, according to the job(s) the system must perform.

Let's begin by looking at how the key passive elements found in most electronic circuits work.

A passive element is an electrical component that does not generate power, but instead dissipates, stores, and/or releases it. Passive elements include resistances, capacitors, and coils (also called inductors). These components are labeled in circuit diagrams as Rs, Cs and Ls, respectively. In most circuits, they are connected to active elements, typically semiconductor devices such as amplifiers and digital logic chips.

Resistors

A resistor is a primary type of physical component that is used in electronic circuits. It has two (interchangeable) leads. The material placed internally between the two leads of a resistor opposes (restricts) the flow of current. The amount of that opposition is called its resistance, which is measured in ohms (Ω). Resistors are used to control the various currents in areas of a circuit and to manage voltage levels at different points therein by producing voltage drops. When a voltage is applied across a resistor, current flows through it. Ohm's law for resistors is E = IR, where E is the voltage across the resistor, R is the resistance of the resistor, and I is the current flowing through the resistor. That current is proportional to the applied voltage, and inversely proportional to the resistance. Thus, as resistance goes up, the current through the element comes down, so that at high resistances the current is very small.

Ohm's law makes it possible to calculate any one of three circuit values (current, voltage, or resistance) from the other two.

Capacitors

A capacitor is another primary type of physical component used in electronic circuits. It has two leads and is used to store and release electric charge. A capacitor's ability to store charge is referred to as its capacitance, measured in farads (F).

A typical capacitor takes the form of two conductive plates separated by an insulator (dielectric). This type of circuit element cannot pass direct current (DC) because electrons cannot flow through the dielectric. However, a capacitor does pass alternating current (AC) because an alternating voltage causes the capacitor to repeatedly charge and discharge, storing and releasing energy. Indeed, one of the major uses of capacitors is to pass alternating current while blocking direct current, a function called 'AC coupling'.

When a direct current flows into a capacitor, a positive charge rapidly builds up on the positive plate and a corresponding negative charge fills the negative plate (see Figure 1). The buildup continues until the capacitor is fully charged—i.e., when the plates have accumulated as much charge (Q) as they can hold. This amount is determined by the capacitance value (C) and the voltage applied across the component: (Q = CV). At that point, current stops flowing (see Figure 2).

Figure 1: The capacitor is charging / Figure 2: The capacitor is charged (and stable)