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Components resistors


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Components resistors

  1. 1. Components - The Resistor Resistors are probably the most common component used in basic electronic circuits. Resistors are use to restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED. Circuit SymbolExample VCC + Gnd (0V) R LED
  2. 2. The Resistor Colour Code - 4 Band The Resistor Colour Code Colour Number 0 1 2 3 4 5 6 7 8 9 ±5% ±10% Commonly used resistors have four colour bands to represent the value of resistance. Three bands represent the value and the forth band for the tolerance. The first band gives the first digit of the value. The second band gives the second digit of the value. The third band indicates the number of zeros to be added to the value. This resistor has brown (1), red (2), orange (3) value bands. Indicating that its nominal value is 12,000Ω or 12kΩ The Ω sign is usually omitted on circuit diagrams and the value is written as 12k. The gold tolerance band indicates ± 5% about the nominal value. The actual value can lie between 11,400Ω and 12,600Ω.
  3. 3. The Resistor Colour Code Activity 1. A resistor has colour bands yellow, violet, orange what is its value (nominal). 2. A resistor has brown, black, red value bands and a gold tolerance band. What is the nominal value and value range for this component. 3. You are asked to obtain a 68k resistor, explain how would you recognise this. 4. A 36k and a 27k resistor are connected in series what would the be the nominal value for this combination. If they both had gold tolerance bands what would the be the upper and lower limits.
  4. 4. The Resistor Colour Code - 5 Band The Resistor Colour Code Colour Number 0 1 2 3 4 5 6 7 8 9 As component manufacturing techniques became more refined (use of laser trimming) and the use of high stability materials (metal film) instead of carbon composition. It became possible to manufacture much closer tolerance components (0.5%, 1% and 2%) allowing the production of more values in a range. Three bands represent the digits of value and the forth band the number of zeroes to be added. The fifth band is for the tolerance. What value is shown in the diagram?
  5. 5. Preferred Values To produce a sensible range of resistor values you need to increase the size of the 'step' as the value increases Standard values form a series which follows the same pattern for every multiple of ten these are known as preferred values. E6 (20%) 6 values for each multiple of ten, 10, 15, 22, 33, 47, 68 and continues 100, 150, 220, 330, 470, 680, 1000 etc. Notice how the step size increases as the value increases. For this series the step to the next value is roughly half the value. E12 (10%) 12 values for each multiple of ten, 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and continues 100, 120, 150 etc. (E6 series with an extra value in the gaps) E24 (5%) 24 values for each multiple of ten, 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91 and continues 100, 110, 120, 130 etc. (E12 series with an extra value in the gaps)
  6. 6. Film Resistors Electrical energy is converted to heat when current is passed through a resistor, however in most cases this energy is so small that the heat produced can be ignored. The amount of energy a resistor can safely convert to heat is dependant on its power rating. Where the energy is small the most common types of resistor is are the carbon and metal film. Film resistors have values ranging from tens of ohms to million of ohms. Power rating for film resistors is from 0.125Watt to 1Watt.
  7. 7. Power Resistors In some cases it is necessary to pass a high current through a resistance which can result in heat being generated in the component. We must ensure that the component continues to operate to specification and no damage is caused as a result of this current. The heat generated is also known as power dissipation. Most power resistors are wirewound (not film) and very high power types are encased in an aluminium cladding that acts as a heat sink or designed to be mounted on a heat-sink to remove the heat from the component.
  8. 8. Variable Resistors (potentiometer) Standard single turn Precision multi-turn Rotary potentiometers Single turn Preset potentiometers 10 turn precision A potentiometer is a resistor that can have its value adjusted from zero to the value stated on its case. Standard potentiometers are used for amplifier controls by the equipment user and can have either a rotary or slider wiper. Potentiometers used for volume controls have a logarithmic scale. Multi-turn rotary potentiometers used for precision instrumentation control applications operated by the equipment user. Preset potentiometers are mounted directly on the circuit board and used for setting up and calibration by the manufacturer. Presets are not accessible to the equipment operator.
  9. 9. Construction of a Potentiometer Construction of a Carbon Track Potentiometer and Circuit Symbol Terminals Control shaft Sliding contact (wiper) Carbon track { Variable resistors consist of a resistance track with a connection at either end and a wiper that moves along the track as you rotate the spindle, see diagram. The track can be made from carbon, cermet (matalised ceramic) or turns of resistance wire. θ rotation 0 330 Resistance Lin Log Construction of a Carbon Track Potentiometer and Circuit Symbol
  10. 10. Light Dependant Resistor (LDR) Light dependant resistors use a semiconductor material whose resistance varies according to the amount of light falling on it. Light provides energy to set free electrons in the semiconductor material (cadmium sulphide) thus increasing conductivity, (reducing its resistance). ORP12 Symbol Response light Resistance A common device is the ORP12 its resistance can range from 10MΩ in darkness to 500Ω in full light. LDR’s are used
  11. 11. Application of a Potentiometer The potentiometer is commonly used in amplifiers as a volume control. This control ‘sits’ between the pre-amplifier and the power amplifier and is used to limit the amount of signal fed to the power amplifier. Pre-amp Power amp Output to Speaker system Input signal gnd (0v)