- A resistor basically reduces the amount of current flowing through a circuit. Or as its name implies, it resists current flow. Using a water analogy for electricity (that is, think of current in a circuit as water moving through pipes), resistors are the narrow pipes that reduce the amount of water that is allowed to pass. In a sense, resistors are doing the same thing because they are limiting the number of electrons that can flow past a given point at one time.
- In general electrical terms, materials are either conductors or insulators. Conductive materials have a molecular structure that allows electrons to flow freely through them. On the other hand, insulators have molecular structures that block or limit the flow of electrons. Resistors can be made of polymers, ceramics or carbon or metal film. Some types of resistors are even made up of wire filaments or wound coils of wire.
Resistors often release heat as current passes through them. This heat energy can be extremely useful. Coiled wire resistors are common in coffee pots, toasters and space heaters. The most common wire filament resistor is an incandescent light bulb. The wire filament inside a light bulb offers a little resistance to the flow of current. This resistance generates heat and the heat gives off light. - Why you need resistors has a lot to do with your power supply. Sometimes a component in a circuit needs less current than what the power supply is providing. So the incoming current needs to be reduced. This is important in complex pieces of equipment where one power source drives several different functions in a device. Think of your computer. Your hard drive is plugged into a single outlet. But there are many different circuits at work inside there. Each circuit has different power demands.
Another time you may need a resistor is when you want to vary the current that you are using. This can be done through a device called a potentiometer, or a "pot." A potentiometer is a variable resistor. You see these on lights with dimmer switches. When you lower the light level in a room, you are actually turning up the resistance on the pot. Other common uses for pots include volume control for stereos and television sets. When the resistance to the current coming into the speakers is at its lowest, the volume is loudest. Conversely, when you turn the resistance all the way up, you turn the volume all the way down. - It is important to know the right size of the resistor needed for an application. Resistance is measured in ohms, and this is a variable in the computation of Ohm's law. Ohm's law says that V = IR, or Voltage = Current (I) x Resistance. Voltage is the potential of a power supply to release electricity. Current is the actual flow of electrons through a circuit, and resistance is the resistance to the flow of electricity. So if we know how much voltage we have in a power supply and how much current we need for a particular operation, we can then determine how much resistance we need in our circuit.
- Instead of having numbers that tell you how many ohms a particular resistor has, a color code is used. There are four colored bands on a resistor. The color system is universal, so on each resistor, if the third band is red, it means the same thing. The color code is listed below.
Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet =7
Gray =8
White = 9
The first two bands on a resistor represent the first two digits in the number of ohms. But the third band is a multiplier. For example, if the bands on a resistor are yellow, violet and brown, the yellow represents 4, the violet 7. But the brown band tells you how many 0s to put behind the first two digits. In this case, you have one 0, which means the resistor is 470 ohms.
There is one last band called a "tolerance band." This band indicates the percentage of accuracy of the resistor value. The tolerance values are listed below.
Brown = +/- 1 percent
Red = +/- 2 percent
Gold = +/- 5 percent
Silver = +/- 1 percent
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