Electronics Cookbook

Chapter 4. Diodes

4.0 Introduction

The first diodes to be used in electronics were cat’s whisker detectors used in crystal radios. These comprised a crystal of a semiconductor material (often lead sulphide or silicon). The cat’s whisker is simply a bare wire that is held in an adjustable bracket that touches the semiconductor crystal. By carefully moving the whisker around, at certain points of contact the arrangement would act as a diode, only allowing current to flow in one direction. This property is needed in a simple radio receiver to detect the radio signal so that it can be heard (see Chapter 19).

Today diodes are much easier to use and come in all sorts of shapes and sizes.

4.1 Block the Flow of Current in One Direction

Problem

You want a component that allows current to flow one way but not the other.

Solution

A diode is a component that only allows current to flow through it in one direction. It’s a kind of one-way valve if you want to think of it in terms of water running through pipes, which of course is a simplification. In reality the diode offers very low resistance in one direction and very high resistance in the other. In other words, the one-way valve restricts the flow a tiny bit when open and also leaks slightly when closed. But most of the time, thinking of a diode as a one-way valve for electrical current works just fine.

There are lots of specialized types of diodes, but let’s start with the most common and basic diode, the rectifier diode. Figure 4-1 ...