The resistor is the most fundamental component of an electrical circuit. The resistor does exactly what it's name implies. It resists the flow of current. All materials have resistance, but the resistor is a component that is designed to have a very specific resistance value. As was mentioned in our Ohm's law tutorial, the value of resistance is measured in Ohms which uses the Ω symbol. Resistors used in a circuit are named with a capital "R" and a number such as R1 or R503. Usually a schematic of a circuit will include both the resistance value and the name next to the the symbol for the resistor. The symbol used to show a resistor component in a schematic is shown in the figure in the next section.
The simple circuit shown below includes a 12V DC power supply named V1, three resistors named R1, R2, R3, and a ground. When resistors are placed one after another like this, they are said to be in series. This circuit is being simulated in Multisim by National Instrument, and virtual multi-meters are being used to measure the voltage across each resistor. A fourth multi-meter is being used to measure the current flowing through the circuit. To learn more about how multi-meters work, visit our Tools of the Trade tutorial. Notice how R1 has a value of 1kΩ, R2 has a value of 2kΩ, and R3 has a value of 3kΩ. That is the same as saying R1 has a value of 1000 Ohms, R2 has a value of 2000 Ohms, and R3 have a value of, you guessed it, 3000 Ohms. When resistors are placed in series like this, they add to each other so that the total resistance across the circuit shown is 1000 Ohms + 2000 Ohms + 3000 Ohms = 6000 or 6kΩ. From Ohm's law, current through the circuit should be 12V / 6kΩ = 0.002A or 2 mA. From the fourth multi-meter (XMM4) display in the simulation, we can see the simulation agrees with Ohms law. We dig into this circuit further in our Kirchhoff's Laws tutorial.
There are many different types of resistors, but we are going to be mostly using just a couple of them in our tutorials. The main difference we care about for now is in how they attach to a circuit board. One attaches with through holes in the board and the other is a surface mount. For our prototype work we will be using the through hole type because they fit nicely into a prototype board. However, a surface mount resistor can be made much smaller to save space on a production board. To the right you can see R27 is a through hole type while R37 and R36 are surface mount.
There are a few parameters of the resistor that we need to be aware of. These are the nominal resistance, the tolerance for that resistance, and the power rating. For the through hole resistors that we will usually use for our prototype projects, the nominal resistance and tolerance can be determined from color codes on the resistor itself. We will need to keep track of the wattage or power rating. We will mostly be using 0.25W resistors in our projects, but may occasionally use 1.0W or even 10W resistors. Heating elements used in devices such as electric ovens or water heaters, by the way, are high wattage resistors.
BLACK BROWN RED ORANGE YELLOW GREEN BLUE VIOLET GRAY WHITE
In the resistor to the right you can see four stripes. From left to right they are green, brown, black, and gold. For this resistor the first three stripes indicate the resistance value. The first two tripes represent the first two first digits of the value according to the table to the left. The third stripe represents the number of zeros after those first two numbers according to the same table. This resistor has a nominal value of 51Ω. Green represents the 5. Brown represents 1. Black means there are 0 zeros after 51. The last stripe tells us the tolerance. Brown = +-1%, Red = +-2%, Gold = +-5%, and Silver = +-10%. This resistor has a tolerance of +-5% as indicated by the gold stripe.
One note of caution here. Its a good idea to always measure the value of a resistor with a multi-meter to make sure the resistor is both within tolerance and you didn't mistake orange or red or blue for violet. The colors can sometimes be difficult to see clearly.
There are many factors that may go into what the value of a resistor needs to be. We will learn about these as we progress through other topics.