Significance

• Resistors are essential cogs in the majority of circuits. Their ability to limit current allows them to protect electronics from circuit overload or destruction. They can be combined with other electrical components to form important fundamental circuits. For example, they can be coupled with capacitors to perform as filters or voltage dividers. Another role is that of the formation of oscillatory AC circuits when they are coupled with capacitors and inductors.
  
  

Construction

• Resistors are typically constructed from conductors or semiconductors, with the most common ones being formed from carbon encased in lacquer. Wire-wound ones are made from coils of metal wire and are extremely accurate and heat resistant. Carbon film resistors are made from carbon on a ceramic cylinder. Photoresistors are also called photocells, and these are made from materials such as cadmium-sulfide.
  
  

Types

• Regular resistors are basically linear and thus follow Ohm's law. Ohm's law states that the current I is proportional to a voltage V, where the constant of proportionality is the resistance R. Resistors that do not obey Ohm's Law are known as non-ohmic, and they may be variable, nonlinear or a combination of both. Non-ohmic ones include potentiometers, varistors, thermistors and photoresistors.
  
  

Function

• Resistors convert electrical energy into heat, and so function as heating elements in devices such as irons, toasters, heaters, electric stoves and hair dryers. Their resistive properties also allow them to produce light, and so they form filaments in light bulbs. Variable resistors function as sensors, switches or voltage dividers.
  
  

Tips

• High-voltage circuits require resistors that are able to handle a heavy load of current. Where the power rating is not specified, resistors that are only one-quarter or one-half watts can be used.