Resistors make up an important part of electronic circuits. A resistor opposes the flow of electrons, and the electron flow can be controlled by varying the resistance in a circuit. Resistors enable us to control the current in a circuit by controlling the opposition to electron flow. As the resistors oppose the flow of electrons they dissipate electrical energy in the form of heat. The more energy a resistor dissipates, the hotter it will become.
Most common resistors have standard fixed values and are called fixed resistors. Variable resistors, also known as potentiometers, allow us to change the value of resistance without removing and replacing the component. These can be used to adjust the voltage in a circuit. A common example of potentiometers in use are the volume and tone controls in most stereo amplifiers.
Because of the size of most resistors it is not practical to print the resistance values on the side of a resistor. Instead, a color code shows the value of each resistor. There are four color bands on each resistor. The color of the first three bands shows the resistor value and the color of the fourth band indicates the resistor tolerance.
Standard resistors are usually found in values ranging from 2.7 ohms to 22 megaohms. Standard resistance tolerances can be + or - 20%, 10%, 5%, and 1%. For example a tolerance of 10% on a 200 ohm resistor means that the actual resistance of the resistor may be any where from 180 ohms to 220 ohms. There are also high precision resistors that can be found with tolerances of 0.5%, 0.25%, 0.1%, and 0.05% tolerance ratings.
The first three bands are represented by twelve different colors with the values as follows: black - 0, brown - 1, red - 2, orange - 3, yellow -4, green - 5, blue -6, violet - 7, grey - 8, white - 9, silver - 0.01, and gold - 0.1. The fourth band has only three different colors as follows: gold - +/- 5%, silver - +/-10%, and no color - +/- 20%.
Resistors are also rated according to the amount of power they can safely handle. These ratings typically start at one watt and range to 10 watts or larger.
A switch simply connects or disconnects a single electrical contact. Two wires are connected to the switch and when you turn the switch on, the two wires have a complete path. When you turn the switch off, the wires have an open path, and are disconnected. This is called a single-pole, single-throw switch. It connects a single pair of wires (single pole) and has only two positions, on or off (single throw). Single-pole, single-throw switch is often abbreviated as a SPST switch.
If you want to control more devices with a single switch, more contacts are needed. A single-pole, double-throw switch, or SPDT switch, connects a single wire (single pole) to one of two other contacts (double throw).
If you want to switch two circuits on and off at the same time, then you would use a double-pole, single-throw switch, or DPST switch. A DPST switch connects two input wires to their respective output wires at the same time.
If you want to add even more contacts to a switch, you need a double-pole, double-throw switch, or DPDT switch. You can think of DPDT switch as two SPDT switches in the same box with their handles connected. A DPDT switch has two center contacts and connects these two contacts to one of two other contacts.
A fuse is simply a device made of metal that will heat up and melt after a certain amount of current flows through it. These are used to protect against unexpected short circuits and other electrical problems. The amount of current needed to melt (or blow) a fuse is determined by the manufacturer. When a fuse blows, it creates an open circuit and stops the current.
Fuses can range in ratings at which they are designed to blow from as little as 250 mA or less to 20 A or more. Remember that fuses are designed to protect against too much current - not too much voltage. Circuit breakers are commonly used instead of fuses now to protect house circuits from overload. Circuit breakers are simply a spring loaded switch that unlatches and opens if too much current flows through it.
Everyone is familiar with batteries. They come in many different shapes and sizes. But what exactly do they do? A battery changes chemical energy into electrical energy. When a wire is connected between the terminals of a battery, a chemical reaction takes place inside the battery. This produces free electrons and causes them to flow from the negative terminal to the positive terminal. Batteries are made up of cells, which each cell having a positive electrode and a negative electrode. The number of cells in a battery depends on the voltage you want to get out of the battery.
Transistors and Diodes
Diodes and transistors are two types of semiconductors, the solid state devices that have replaced vacuum tubes in most uses and created many new applications. Almost all semiconductor devices are much smaller than their comparable tubes and produce much less heat. They are also much less expensive than tubes.
Transistors come in many shapes and sizes. One of the most common types of transistors is the bipolar transistor. Bipolar transistors are made up of two different kinds of material. There are two kinds of semiconductor materials. They are N-type material and P-type material. The N refers to the way electrons move through the material. The P refers to the way positive charges move through this material.
There are commonly two kinds of bipolar transistors - NPN and PNP. A NPN transistor has a layer of P-type material between two layers of N-type material. A PNP transistor has a layer of N-type material between two layers of P-type material.
Each transistor has three leads or electrodes, and each of these electrodes connects to a different part of the transistor. The three different electrodes are labeled base, emitter, and collector. Transistors can amplify small signals, which is what makes them so useful. Usually we use a low-voltage signal applied to the base of the transistor to control the current through the collector and emitter.
A diode is simply an electronic component which allows current to flow through it in one direction but not the other. A diode's main function is to change AC voltage into DC voltage. A diode has two leads - the one closest to the printed grey line on the device is connected to a cathode and the other is connected to an anode. There are several different types of diodes. We have pictured some of them below. There are even diodes that emit light - these are called LEDs.
An integrated circuit (or IC) performs several circuit functions in one simple package, and sometimes an entire application will only require the addition of a few external components. Like transistors, ICs are made completely of semiconductor materials and use very low operating voltages. This makes them low power devices. Below are some examples of typical ICs. One of the most complex use of ICs is the microprocessor.
The inductance of a coil increases as an iron core is inserted. Many practical inductors of this type use screw threads to allow moving of the core in precise amounts when making small changes in inductance. The adjustable inductor uses a movable powdered-core or slug. They are sometimes called slug-tuned or permeability-tuned inductors. Some inductors and some circuits will interact with magnetic fields of other inductors. To prevent this a metal shield is used to enclose them. Pictured above are several types of common inductors, although there are many more.
A capacitor is made up of two or more conductive plates with an insulating material between them. There are four factors that determine the capacitance of a capacitor: 1) the type of insulating material, or dielectric used between the plates, 2) the area of the surface of one plate, 3) the number of plates, and 4) the distance between the plates.
Practical capacitors are described by the material used for their dielectric. Mica, ceramic, plastic-film, polystyrene, paper, and electrolytic capacitors are in common use today. Each of these types have properties that make them more or less suitable for a particular application.
Another type of capacitor is the variable capacitor. This type of capacitor allows you to vary the capacitance at some point. The best example is the tuning circuit of a receiver or transmitter VFO. The capacitance is varied by changing the amount of space between the plates.