The Resistor

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. Resistors act to reduce current flow, and, at the same time, act to lower voltage levels within circuits. Every resistor has a specific electrical resistance which limits the flow of electrons though a circuit. A resistor can be compared to a valve or tap in a water line. The more you close the valve, the higher the resistance and the less pressure there is and the less water flows.

Symbol

So a resistor is an electrical component used to:

  • reduce current
  • lower a voltage level
  • adjust signal levels
  • ...

They also come in different packages with colours or numbers on them. There are different types of packages

  • Two leads and 4, 5 or 6 coloured bands
  • Two leads and letters printed on it. These are generally bigger and bulkier
  • Small rectangular black block with tiny numbers printer in it

The first type is the more common one. These types of resistors are commonly used in hobby projects and can generally dissipate a quarter watt (0.25W) of energy. When doing a project on a breadboard this is the resistor type you'll use. The coloured bands represent the value of the resistor. There are charts available that allow you to convert the colours to numbers and let you calculate the value of the resistor.

The second type is called Power Resistor because, due to its size, can dissipate a lot more energy. Most of the time they look like long white blocks with a lead sticking out at each end. The higher power ones can get very, very hot and are even used as heating elements. These resistors are build using a resistor wire in a white ceramic container filled with sand. that's why they are also called sand bar resistors. The value of the resistor is printed on the housing. Ex. 10Ω 5W.

The third kind is the smallest kind and does not have any wires. It is soldered directly on the surface of a printed circuit board and is therefor called a Surface Mount Device. Generally these tiny blocks have three letters on them. The first two are the first two letter of the value and the last one is the number of zeroes that are following. So a resistor with the number 103 on it has a value of 10 kilo ohm as it is a 10 followed by three zeroes.

Converting the colours to a value

The most common resistors have 4 or 5 colour bands. The difference between the two is the possible detail in value. With 4 bands the first two are defining the first two digits, the third one defines the number of zeroes and the fourth band is the tolerance. With 5 bands the first three bands define the first three digits, the fourth one defines the number of zeroes and the last band defines the tolerance. So a band is added in the middle giving an extra factor of resolution.

Here is a graphical overview:

Here is a link to a page containing a resistor value calcutaor where you can select the colours of the bands and the application gives you the value of the resistor : http://www.resistor-calculator.com/