Tech Course

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  1. What are the type of fixed resistors
  2. . Carbon composition resistors
    • • Carbon film resistors
    • • Metallic film resistors
    • • Wire-wound resistors
    • • Thermistors
    • • Voltage dependant resistors
  3. Describe carbon resistor
    Construction Carbon composition resistors are very common types of resistors which employ a mix of materials consisting of a conducting carbon material and a filler or binder.  The filler is mixed with the carbon material to establish the ohmic value of the resistor. Stability Carbon composition resistors are the most unstable when exposed to temperature. Tolerance These resistors commonly have tolerances of 10% to 20%.  For example, as follows:  100 Ω at 20% = 100Ω + 120Ω/ - 80Ω.

    Performance Limitations These resistors are no longer commonly used due to their tendency to generate noise (unwanted electrical disturbances), as they age. A
  4. Describe
    Metallic Film Resistor Properties 
    • Construction
    • Metallic Film resistors are similar to carbon film resistors in make-up, but the conducting material in the metal film resistor is a metallised oxide film coated onto the insulator. Stability The metallic film resistors have a very good temperature stability. Tolerance These resistors are sometimes called precision resistors because they commonly have a tolerance of 1% or less. Performance Limitations They have no performance limitations, except when used in applications associated with high frequencies. An example of a metallic film resistor is shown in Figure 2.  Figure 2: Metallic Film Resistor Wire-wound Resistors Properties Construction Wire-wound resistors are made up of resistance wire that is wound on a porcelain or ceramic tube and attached to terminal lugs at each end.  In order to avoid a short circuit, the turns of resistance wire are spaced so that they do not touch. To insulate the entire assembly and prevent damage to the resistance wire, the unit is covered with an insulating material which is baked-on at a high temperature. Stability These resistors have a good temperature stability and are usually used in circuits requiring high power.                                                                                                   Tolerances The majority of wire-wound resistors have a tolerance of either 5% or 10%. Performance Limitations These resistors can have a significant inductance even at low frequencies and this must be considered during the circuit design. They are also limited to values between 0.1 ohm and 200 k ohms.
  5. Describe

    Voltage Dependant Resistor Properties
    Voltage dependant resistors (VDR) are also known as varistors.  They operate utilising a characteristic where, as voltage increases, resistance decreases and vice versa. Varistors are often used to remove disturbances on mains power caused by lightning strikes, particularly on circuits where computers are connected. Construction They are constructed from zinc-oxide based powders or carbide granules and ceramic disks.  The powder is infused into the ceramic disk at high temperatures creating a semiconductorlike material. Stability, Tolerance and Performance Limitations The performance limitations, stability and tolerances are very similar to that of the thermistor. Operation During normal operation the varistor’s high resistance will have no affect on the circuit.  However, as a voltage surge (a large increase in voltage) appears, the varistor’s resistance will drop low.  The low resistance shunts current away from the equipment and attempts to drop the voltage.  Varistors react very quickly, thus limiting the amount of the voltage surge which will be felt by the circuit being protected.  Dependant on the application, varistors can be manufactured to operate continuously from 50 V to 2800 V, whilst current ratings may vary from 100 A to 70,000 A.
  6. What do the colours mean for a resister
    Colour Digit Multiplier Tolerance Mnemonic BLACK 0 1  Bye BROWN 1 10 ± 1% Bye RED 2 100 ± 2% Rosey ORANGE 3 1000 ± 3% Off YELLOW 4 10000 ± 4 % You GREEN 5 100000  Go BLUE 6 1000000  Blacktown VIOLET 7 10000000  Via GREY 8 100000000  Great WHITE 9 1000000000  Western GOLD  0.1 (10-1) ± 5%  SILVER  0.01 (10-2) ± 10%     ± 20%
  7. What type of resistor would be required in a circuit where high power can be expected?
  8. A circuit requires very accurate voltages; what type of resistor would fill this role?
  9. State the two most commonly used methods of identifying resistor values.
  10. Give the resistor value and tolerance for a resistor with colour bands: yellow, grey, brown and brown. 5
  11. Using the new method of expressing resistor values, state the expression for a 1.9 Ω resistor. 6
  12. The three basic resistive materials used in variable resistors are……………?
  13. A resistor that has three electrical terminals with one terminal at each end and the centre terminal as a pickoff is called a ………………………..?
  14. The difference in construction between a rheostat and potentiometers is?
  15. What are two
    Types of Variable Resistors
    1 Potentiometers 

    2 Rheostats.
  16. Describe a
    Potentiometers A variable resistor which has three active electrical terminals: one terminal at each end of the resistive element and a centre terminal which is usually connected to the wiper arm and is called a potentiometer, often referred to as a POT.  The electrical symbol for a potentiometer is shown in Figure 15 (a).  Pots are normally used as a variable voltage divider to control voltage to a circuit, as shown in Figure 15 (b
  17. Describe a Rheostat
    A variable resistor which uses two active electrical terminals, one terminal being connected to the resistive element (one end only) and a second being connected to the wiper arm, is called a rheostat.  The symbol for a rheostat is shown in Figure 16a. Figure 16b shows the common alternative symbol used for depicting a rheostat.  Notice that a rheostat is simply a potentiometer with one leg left open.  The circuit diagrams shown in this topic will use the symbol shown in Figure 16c to aid in explanations.  Figure 16: Rheostat Rheostats are placed in series with the circuit load to control the circuit current. Referring to Figure 16c as the wiper of the rheostat moves away from terminal 1, the resistance between terminals 1 and 3 increases reducing the current.  Since terminal 2 is not connected, the resistance between terminals 2 and 3 has no effect on the circuit.  Terminals 2 and 3 could be connected and the variable resistor would still behave as a rheostat.  When connected in this way, the resistance between terminals 2 and 3 would always be zero ohms (or shortcircuited) and thus have no effect.
  18. What is
    Kirchhoff’s Voltage Law
    When calculating the voltage drops across the resistors, the addition of the individual voltage drops across the resistors was equal to the applied voltage.  This is the basis of Kirchhoff’s Voltage Law which states: The algebraic sum of the voltage source and voltage drops in a series circuit is equal to zero.   VS + VR1 + VR2 + ...  = 0
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Tech Course
2015-12-21 10:59:30

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