Component Classifications (Part 3)

Generally, passive devices have no functional gain or control over voltage or current, their controlling function is basically linear as in the case of a resistor. Active devices on the other hand has the ability to electrically control electron flow to manipulate and transform a signal's properties. For a circuit to be considered electronic, it must contain at least one active device.

Diodes and Zeners

A diode, by definition, is a two-terminal non-linear device that works on the p-n junction principle that exhibits forward and reverse biasing characteristics. There are many types of diodes, from the common rectifier to the varactor, the light-emitting to the photo-laser, and the step recovery to the zener, etc.

So what is the difference between a diode and a zener?

While a diode operates on the forward voltage region and may suffer catastrophic failure if subjected to voltages that exceed its reverse breakdown limit, the zener can safely operate on the reverse voltage region. In other words, while a diode conducts current in only one direction, a zener, by virtue of its heavy narrow junction doping, can conduct current in both directions with the ability to maintain a stable voltage in the reverse bias condition. This makes the zener ideal for applications such as voltage references in comparator circuits and voltage stabilizers in low-current circuits.

As you'd probably realized by now, unlike the axial-lead diodes, SMD packages can contain more than one diode per package type, such as the BAV99 example just mentioned. Things can get really tricky, however, when you work on PCBs with many of these SMD type components present, so it's good to download a copy of the SMD Codebook and keep it handy for quick reference.

Ps: While there are many editions of free SMD Codebook online, readers who buy my book will get a copy of the SMD Code Reference Book, which contains over 500 pages listing of more than 23,000 SMD type component markings.

Component Classifications (Part 2)

Before going into active components (semiconductors and ICs), let's take a look at one type of device

that is widely used in many digital and processor-based PCB designs—the crystal oscillator.

Crystal oscillators, whether excited by the host ICs which they are connected to, or powered by external sources, exhibit high quality factors (Q) above 10,000 and are suitable for use as resonators and in high stability tuned circuits.

In its simplest form, a crystal oscillator has a fixed frequency vibration with accuracy up to ±30ppm, but greater accuracy and stability can be further achieved in mission critical system using special variants like the voltage-controlled crystal oscillator (VCXO), temperature compensated crystal oscillator (TCXO), and the oven-controlled crystal oscillator (OCXO).

Component Classifications (Part 1)

Components are the basic building blocks in electronic circuits. They come in all shapes and sizes and perform different functions based on their electrical characteristics, some of which are simple and straightforward, while others are complex and cascaded in nature,

Generally, components are classified into discrete and integrated, the former being a single entity or unit, while the latter is made up of multiple discrete entities ranging from a few to over a million, depending on the manufacturing process and technology. These are further differentiated into passive and active components, the former requiring no external sources to power or operate, whereas the latter uses and manipulates energy to effect gain or change in a signal's condition.

Passive Discrete Components

Some examples of passive discrete components are shown in the photos below:

The resistor is one of the most basic element in an electrical circuit. Types of resistor include carbon composites, chip SMDs, PTC thermistors, aluminum clads, and networks. Second only to resistors, capacitors are one of the most common components you'll find on PCBs. Types of capacitor include electrolytics, micas, chip SMDs, radial-lead ceramics, and axial-lead tantalums.

Together with resistors and capacitors, inductors belong to the same passive linear circuit elements found in electrical circuits. Inductors and capacitors are two types of energy storage elements—the former deals with current while the later with voltage. Types of inductor include axial-lead, radial-lead, torroids, ferrite-cores, unshielded and shielded SMDs.

A fuse is a kind of resistor with low resistance and built to tolerate a certain amount of current, which if exceeded, will cause it to blow up and become an open. In other words, it's a protection device. Types of fuse include axial-lead, glass cartridges, radial, SMDs, resettable and blade.

Most relays are electro-mechanical in structure in which contacts make-and-break by means of a relay coil being energized or de-energized by a control voltage. For this reason, they are also called electrically operated switches. Types of relay include electro-mechanical, reeds, TO-5, SMDs, solid-state and hermetically sealed.

It is important to be able to identify these discrete components when doing PCB reverse engineering, especially with modern surface-mounted boards where such components are shrinking in size and their markings on the PCB are kept to a minimum.