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For this practice you will need:

• No materials needed this time

After this lesson you should be able to know what is a resistor, what is it for, and the resistor color code.

Fixed Resistors

Resistors are very common types of components used in analog circuits. A resistors as the name indicates is a form of resistance. Although they do not show an active function, they are needed for other components to work properly; that's why they are in the group of the passive components.

Resistors are a complement to other types of resistances. They are very useful in most analog and digital circuits. These components are used reduce high voltages and currents that can damage other semiconductor components such as transistors, integrated circuits, or LEDs. Sometimes they are used in amplifiers along with transistors to get the best ouptup gain of a certain transistors; but that's another story. Resistors can also work just to experiment with them. We can make measurements of current, resistance and voltage, learn the color code, etc.

There are different types of resistors, each one made from different materials. The most common resistors are the carbon composition. These resistors are the smallest and more reliable. Carbon composition resistors vary in physical size depending on the wattage they have. The smallest resistors are the ones rated 1/4 of a watt, then there are the 1/2-watt resistors, and the largest rated 1 Watt. As the size and wattage increases, they can handle more power.

Another type or resistors are the cerment resistors. These resistors are designed to work at high temperatures and voltages. Most of these resistors look like a small clay brick. Even though they are small, they are larger than the 1-watt carbon resistors. This makes them not be as common in household devices.

Last, there are the wire wound resistors. These resistors are the strongest. They can handle the most heat, voltage, and current. These devices are made of wire winded around a metal tube, leading to a very strong resistor of more than 10 Watts.

These three types of resistors are the most common types fixed resistor. There are many other kinds that are not as common but they too operate the same way. Some of these other resistors are the surface mount resistors, deposited film resistors, and some that are common but are not fixed, are the variable resistors. We are going to talk about those comming up next.

Variable Resistors

Variable resistors have the same function as the fixed resistors. The only difference is that they have a range of resistance that can be varied with the help of a dial. I bet you have seen this type of resistors before. They are found in the volume dials of radios, in speed controllers, picture placement in monitors, etc. These resistors most of the time are made of carbon but in some divices that require high currents, you may find a wire wound variable resistor.

There are two types of variable resistors: the rheostat, that has only two terminals, and the potentiometer that has three terminals. Variable resistors work by a dial, a metallic pin inside it, and a carbon or wire wound rail. When you turn the dial, a metal pin moves through the carbon rail away or near the terminal of a fixed point, causing more or less resistance between the variable and the fixed point. Depending how far is the metal pin from the fixed terminal the resistance is determined.

How resistors look

Fixed Resistors

Variable Resistors

Potentiometer

Symbols to remember

1. Variable resistor
2. Fixed resistor
3. Diode LED

This is a simple use of a fixed resistor.

Notice that the resistor is connected in series with the LED to protect it from burning at a high voltage. Now lets go to the resistor color code.

The resistor color code

Have you ever wonder what do those color lines in a resistor mean? No, well I have and I've researched about it. And now I want to tell about it to all of you = ). Carbon composition resistors often have four bands of color that represent the value of the resistor. The first three bands represent the "average" value of the resistor, and the fourth band, which is either silver or gold, just represents how accurate can this value be. The resistor color code works as following:

Assume you have a resistor like this. You first look for the color digit opposite to the gold or silver digit. In this case is yellow. The color yellow represents the number 4, so your first number is 4. Then the next line, in this case purple, represents the second digit; a 7. Your first two digits are 4 and 7, or 47. Then you read the third line. This time you don't use the number itself but you add as many zeros as the value of the number at the end of the last two numbers. In this case it is just one, so our final number is 470 ohms. This resistor has a value of 470 ohms, but what about the fourth line? Well, the last line just tells how accurate this reading is. A gold line means it has a 5% tolerance, in other words it can be 5% off value above or below the actual reading of the color bands. A silver line means it has a 10% tolerance or a 10% off value from the reading.

The resistor color code table

Use the table above to try to find out the value of these three resistors:

R1: R2: R3:

Lesson 1, The electric circuit | Back to menu | More to come!