RESISTORS, Types

There are many different types of resistors used in electronics. Each type is made from different materials. Resistors are also made to handle different amounts of electrical power. Some resistors may change their value when voltages are placed across them. These are called voltage dependent resistors or nonlinear resistors. Most resistors are designed to change their value when the temperature of the resistor changes. Some resistors are also made with a control attached that allows the user to mechanically change the resistance. These are called variable resistors or potentiometers.

The value of wirewound resistors remain fairly flat with increasing temperature, but change greatly with frequency. It is also difficult to precisely control the value of the resistor during construction so they must be measured and sorted after they are built.
1.THE CARBON COMPOSITION RESISTOR
      By grinding carbon into a fine powder and mixing it with resin, a material can be made with different resistive values. Conductive leads are placed on each end of a cylinder of this material and the unit is then heated or cured in an oven. The body of the resistor is then painted with an insulating paint to prevent it from shorting if touched by another component. The finished resistors are then measured and sorted by value . If these resistors are overloaded by a circuit, their resistance will permanently decrease. It is important that the power rating of the carbon composition resistor is not exceeded.

Resistor

2.THE WIREWOUND RESISTOR

     The first commercial resistors made were formed by wrapping a resistive wire around a non-conducting rod . The rod was usually made of some form of ceramic that had the desired heat properties since the wires could become quite hot during use. End caps with leads attached were then placed over the ends of the rod making contact to the resistive wire, usually a nickel chromium alloy.

Resistor

3.CARBON FILM RESISTORS

      Carbon film resistors are made by depositing a very thin layer of carbon on a ceramic rod. The resistor is then protected by a flameproof jacket since this type of resistor will burn if overloaded sufficiently. Carbon film resistors produce less electrical noise than carbon composition and their values are constant at high frequencies. You can substitute a carbon film resistor for most carbon composition resistors if the power ratings are carefully observed. The construction of carbon film resistors require temperatures in excess of 

1,000OC.

Resistor

4.THE VARIABLE RESISTOR

      When a resistor is constructed so its value can be adjusted, it is called a variable resistor. Figure 6 shows the basic elements present in all variable resistors. First a resistive material is deposited on a non-conducting base. Next, stationary contacts are connected to each end of the resistive material. Finally, a moving contact or wiper is constructed to move along the resistive material and tap off the desired resistance. There are many methods for constructing variable resistors, but they all contain these three basic principles.

Resistor

5.METAL OXIDE RESISTORS

           Metal oxide resistors are also constructed in a similar manner as the carbon film resistor with the exception that the film is made of tin chloride at temperatures as high as 5,000OC. Metal oxide resistors are covered with epoxy or some similar plastic coating. These resistors are more costly than other types and therefore are only used when circuit constraints make them necessary.

6.METAL FILM RESISTORS

          Metal film resistors are also made by depositing a film of metal (usually nickel alloy) onto a ceramic rod. These resistors are very stable with temperature and frequency, but cost more than the carbon film or carbon composition types. In some instances, these resistors are cased in a ceramic tube instead of the usual plastic or epoxy coating