A resistor is a small component that's designed to provide a specific amount of resistance in an electronic circuit. Because resistance is an essential element of nearly every electronic circuit, you'll use resistors in just about every circuit that you build.
Although resistors come in a variety of sizes and shapes, the most common type of resistor for hobby electronics is the carbon film resistor. These resistors consist of a layer of carbon laid down on an insulating material and contained in a small cylinder, with wire leads attached to both ends. The resistor itself is about 1/4″ long, and the leads are about an inch long, making the entire thing about 2-1/4″ long.
Resistors are blind to the polarity in a circuit. Thus, you don't have to worry about installing them backwards. Current can pass equally through a resistor in either direction.
In schematic diagrams, a resistor is represented by a jagged line, like the one shown in the margin. The resistance value is typically written next to the resistor symbol. In addition, an identifier such as R1 or R2 is also sometimes written next to the symbol.
In some schematics, particularly those drawn in Europe, the symbol shown in the margin is used instead of the jagged line.
Resistors are used for many reasons in electronic circuits. The three most popular are
Limiting current: By introducing resistance into a circuit, resistors can limit the amount of current that flows through the circuit. In accordance with Ohm's law, if the voltage in a circuit remains the same, the current will decrease if you increase the resistance.
Many electronic components have an appetite for current that must be regulated by resistors. One of the best known are light-emitting diodes (LEDs), which are a special type of diode that emits visible light when current runs through it.
Unfortunately, LEDs don't know when to step away from the table when it comes to consuming current. That's because they have very little internal resistance. Unfortunately, LEDs don't have much tolerance for current, so too much current will burn them out.
As a result, it's always prudent — essential, in fact — to place a resistor in series with an LED to keep the LED from burning itself up.
You can use Ohm's law to your advantage when using current-limiting resistors. For example, if you know what the supply voltage is and you know how much current you need, you can use Ohm's law to determine the right resistor to use for the circuit.
Dividing voltage: You can also use resistors to reduce voltage to a level that's appropriate for specific parts of your circuit. For example, suppose your circuit is powered by a 3 V battery but a part of your circuit needs 1.5 V. You could use two resistors of equal value to split this voltage in half, yielding 1.5 V.
Resistor/capacitor networks: Resistors can be used in combination with capacitors for a variety of interesting purposes.