Chapter 2

• The two main characteristics of a resistor are its resistance R in ohms and its power rating in watts (W).
• Resistors main function in any circuit is to limit the amount of current or to produce a desired drop in voltage.
• The R is the resistance value required to provide the desired current or voltage
• -          Also important is the wattage rating because it specifies the maximum power the resistor can dissipate without excessive heat.
• The resistor is a passive electrical component that creates resistance in the flow of electric current
• Resistors are used for many purposes. A few examples include limiting electric current, voltage division, heat generation, matching and loading circuits, gain control, and setting time constants
• The resistor is removing the energy from the circuit
• It is turning the electrical energy into heat in order to remove it
• Resistors make it harder for electrons to flow, so they add resistance to a circuit
• Resistance is a measurement of how easily electrons can flow through a material. It restricting how many electrons can flow. The speed of the electrons remains the same.
• We have a voltage drop across the resistor. These collisions converts the kinetic energy into heat, that’s why resistors become hot
• Power Rating this indicates the maximum amount of power the resistor can handle without getting damaged.
• When current passes through a resistor, energy is dissipated as heat due to the resistance. The power rating tells you the maximum amount of this heat that the resistor can safely dissipate.
• Dissipation means that the power is wasted, since the resultant heat is not used. Too much heat can make the resistor burn
• Power Dissipation refers to the actual heat generated within the resistor during operation
• power rating is a specification that sets the upper limit for how much heat the resistor can handle without being damaged
• resistors are not polarity-sensitive devices. This means that it does not matter which way the leads of a resistor are connected in a circuit.

   
• Resistors with higher R values usually have lower wattage ratings because they have less current.
• several carbon resistors with the same physical size but different resistance values. The different color bands on each resistor indicate a different ohmic value.
• The lower the power rating, the smaller the actual size of the resistor. However, the resistance value is not related to physical size.
• Wire-wound Resistors a special type of wire called resistance wire is wrapped around an insulating core. The length of wire and its specific resistivity determine the R of the unit.
• in wire-wound resistor material is used to indicate the resistance
• in wire wound resistor the insulated core is commonly:

  • porcelain
  • cement
  • plain pressed paper
• in wire-wound resistor wire materials are

  • alloy
  • Pure metals that have a high-temperature coefficient of resistance
• The temperature coefficient is a measure of how much the resistance will change as the temperature changes.
• The temperature dependence of electrical resistivity is determined by the material:

  -          Number of phonons

  -          Coefficient of expansion of the material
• two types of wire-wound resistors

  Fixed and variable
• wire-wound resistors are generally used for high-current applications with low resistance and appreciable power
• wire-wound resistors are available in wattage ratings from 1W up to 100W or more.
• wire wound resistors are probably the oldest type and can be used for both high precision as well as high power applications.
• wire-wound resistors used where accurate, stable resistance values are necessary
• Wire-wound resistors can roughly be classified in two types: precision and power
• Potentiometers are often wirewound resistors
• A potentiometer is a resistor that has three terminals.
• axial leads – because they come straight out from the ends
• • Carbon Composite Resistor are very common and very cheap to produce
• in carbon composition resistor the larger the resistor the more heat it can dissipate because of the larger surface area
• in carbon composition the bigger the resistor the larger the power rating
• The big advantage of carbon composition resistors is their ability to withstand high energy pulses - When current flows through the resistor, the entire carbon composition body conducts the energy
• Carbon resistors can be made with a higher resistance than wirewound resistors and are considerably cheaper