B - A - 3.7 - Resistance and Resistor PDF

Module 3: Electrical Fundaments Topic 3.7: Resistance and Resistor  INTRODUCTION On completion of this topic you should be able to: 3.7.1 Describe resistance and factors that affect it. 3.7.2 Describe the following regarding resistance:...

Module 3: Electrical Fundaments Topic 3.7: Resistance and Resistor  INTRODUCTION On completion of this topic you should be able to: 3.7.1 Describe resistance and factors that affect it. 3.7.2 Describe the following regarding resistance: Specific resistance Resistor colour code Values and tolerances Preferred values Wattage ratings 3.7.3 Describe the effects of connecting resistors in series and parallel. 3.7.4 Perform calculations of total resistance using the following configurations: Series Parallel Series parallel combinations 3.7.5 Identify potentiometers and rheostats and describe their operation and use. 30-03-2024 Slide No. 2  INTRODUCTION On completion of this topic you should be able to: 3.7.6 Describe the operation of a Wheatstone bridge and calculate the value of an unknown resistor in a balanced Wheatstone bridge circuit. 3.7.7 Define conductance and positive and negative temperature coefficients of resistors. 3.7.8 Identify fixed resistors and define the following regarding them: Stability Tolerance and limitations Methods of construction 3.7.9 Identify the following variable resistors and define their operation and use: Thermistors Voltage dependent resistors 3.7.10 Define the construction of potentiometers and rheostats. 3.7.11 Define the construction of a Wheatstone bridge. 30-03-2024 Slide No. 3  RESISTANCE Resistance is defined as: ‘The opposition offered by a material to the free flow of electrons’ Opposition offered by a conductor or component to electric current flow. Measured in Ohms – represented by Greek symbol omega –  Larger units of measurement: Kilohms (k) = (1000 OHMS) – eg. 5 kΩ Megohms (M) = (1,000,000 OHMS) – eg. 10 MΩ Represented by letter R – eg. R = 2.2 kΩ 30-03-2024 Slide No. 4  FACTORS AFFECTING RESISTANCE Factors that affect the resistance of a conductor are: Length Cross-sectional area (Thickness) Material Temperature 30-03-2024 Slide No. 5  RESISTOR LENGTH AND THICKNESS Length 2 lengths of wire, one being twice as long as the other. Longer length has twice the resistance of the other. The resistance of a conductor is directly proportional to the length. Cross-Sectional Area / Thickness 2 identical lengths of wire, 1 has twice the cross-sectional area as the other. Thicker conductor has half the resistance of the other. The resistance of a conductor is inversely proportional to the cross-sectional area. 30-03-2024 Slide No. 6  SPECIFIC RESISTANCE / RESISTIVITY Resistance of a piece of material having unit length and unit sectional area. Stated in circular-mil-foot ,  / cm ,  / m. Measurement is known as the resistivity, or specific resistance, of the material. Resistance of a conductor is dependent on type of conductor material. Represented by Greek letter rho () – eg. Copper:  = 1.724 x 10-6 /cm. 30-03-2024 Slide No. 7  RESISTOR COLOUR CODE Black beetles running on your garden bring very good weather. First letter of ‘saying’ corresponds to sequential colour. These numeric colour values used on majority of resistors. Also used on some capacitors. Remember – Start counting at zero – Black is zero. 30-03-2024 Slide No. 8  4 BAND RESISTOR COLOUR CODE Ignore colour of resistor body. Most resistors have 3 coloured bands together at one end and 1 single band at other. 3 adjacent bands give resistor value. Band nearest the wire lead gives the value of first digit. Next band gives the value of the next digit. 3rd band gives the number of zeros which follows the two digits. Therefore a resistor with red, violet, and green bands would have a value of? 2700000 ohms or 2.7 MΩ A green, blue, black resistor would be 56 ohms (black indicates there are NO zeros). 30-03-2024 Slide No. 9  4 BAND RESISTOR COLOUR CODE If 3rd band is silver – divide the value of the first 2 digits by 100. If 3rd band is gold – divide by 10. e.g. red, violet, gold is 2.7 ohms. 4th band indicates the tolerance. Brown 1% Red 2% Gold 5% Silver 10% None 20% 30-03-2024 Slide No. 10  5 BAND RESISTOR COLOUR CODE Some resistors have 5 bands for identification. Typically used on higher precision resistors (eg. 1940 Ω / 1k94). Same colour codes apply. Black beetles running on your garden bring very good weather. Differences are: 3rd band is the 3rd digit 4th band is the multiplier 5th band is for tolerance 30-03-2024 Slide No. 11  RESISTOR COLOUR CODE – 4 & 5 BAND 30-03-2024 Slide No. 12  RESISTOR MARKINGS Circuit Resistance value Marking R05 0.05 ohms 1R5 1.5 ohms 15R 15 ohms 1k5 1500 ohms or 1.5 kilohms 15k 15 000 ohms or 15 kilohms 1M5 1.5 megohms Sometimes value of resistor is printed on component itself. The letters "R“, "k“ and “M” have a purpose here: R – denotes the decimal point k – denotes the 1000 point M – denotes the 1000000 point Typically used on wiring diagrams/schematics and printed circuit boards. 30-03-2024 Slide No. 13  RESISTOR COLOUR CODE EXERCISES Colours used for Gold, Orange, Grey, and Silver Tolerance band colour 1st band, denominator: Brown (1) 2nd band, denominator: Black (0) 3rd band, how many zeros (1) 4th band, tolerance in %: gold (5) Answer: 1 0 1 = 100 Ω or 100R, 5% tolerance 1st band: _______ Red (2) Red (2) 2nd band: _______ Orange (3) 3rd band: _______ 4th band, tolerance in %: Gold (5%) ______ 22000 Ω or 22k, 5% tolerance Answer: ___________________ 30-03-2024 Slide No. 14  RESISTOR COLOUR CODE EXERCISES 1st band: _______ Red (2) Violet (7) 2nd band: _______ Red (2) 3rd band: _______ 4th band, tolerance in %: Gold (5%) ______ 2700 Ω or 2k7, 5% tolerance Answer: ___________________ 1st band: _______ Yellow (4) Violet (7) 2nd band: _______ Orange (3) 3rd band: _______ 4th band, tolerance in %: Red (2%) ______ 47000 Ω or 47k , 2% tolerance Answer: ___________________ 30-03-2024 Slide No. 15  SYSTEM OF PREFERRED VALUES Electronic Industries Association (EIA), and other authorities, specify standard values for resistors – referred to as "preferred value" system. Preferred value system has its origins in early years of last century at a time when most resistors were carbon-graphite with relatively poor tolerances. Rationale is simple - select values for components based on tolerances. Using 10% tolerance as an example (silver band). Suppose the first preferred value is 100 Ω – (90 to 110 Ω). Makes little sense to make a 105 Ω resistor since 105 Ω falls within 10% tolerance. Next reasonable size is 120 Ω (10% tolerance) – 108 to 132 Ω – and so on. 30-03-2024 Slide No. 16  SYSTEM OF PREFERRED VALUES Following this logic, the preferred values for 10% tolerance resistors between 100 and 1,000 ohms would be 100, 120, 150, 180, 220, 270, 330 and so on. This enables nearly all resistive values to be covered. The EIA "E" series specify preferred values for various tolerances. The number following "E" specifies the number of logarithmic steps per decade. E6 20% tolerance (now seldom used) E12 10% tolerance E24 5% tolerance E48 2% tolerance E96 1% tolerance 30-03-2024 Slide No. 17  WATTAGE RATINGS When current flows through a resistance, electrical energy is converted into heat. Resistor's ability to lose heat depends largely upon its surface area. A small resistor with a limited surface area cannot dissipate (lose) heat quickly. Larger resistors dissipate heat more effectively. Some resistors are designed to pass very large currents. Cased in aluminium with fins - increases surface area & promotes heat dissipation 30-03-2024 Slide No. 18  WATTAGE RATINGS 1/2 Watt Carbon Composition Resistors 1/2 Watt Carbon Film Resistors 1 Watt Carbon Film Resistors 2 Watt Metal Oxide Resistors 5 Watt Cement Wire wound Resistors Usual ratings are 0.125, 0.25, 0.5, 1, and 2 watt in carbon resistors. Above these levels, wire-wound or other types are used. 30-03-2024 Slide No. 19  RESISTORS CONNECTED IN SERIES A series circuit is defined as a circuit that contains only ONE PATH for current flow. Current in a series circuit must flow through each resistor to complete circuit. Total circuit resistance (RT) is equal to the sum of the individual resistances. As an equation: RT = R1 + R2 + R3 RT = Total Resistance Any number of additional resistances may be added in the equation. 30-03-2024 Slide No. 20  RESISTORS CONNECTED IN PARALLEL Resistors in parallel means their ends are connected by a conductor. This puts their ends at a common potential. That means exactly same potential difference is across each of resistors. Equation for calculating resistance in parallel is: Any number of additional resistances may be added in the equation. 30-03-2024 Slide No. 21  RESISTORS CONNECTED IN PARALLEL If 2 equal resistances are in parallel – total resistance is ½ of common value. Eg. If two 4Ω resistors in parallel – total resistance is 2Ω (½ x 4Ω). If 3 equal resistances are in parallel – total resistance is ⅓ of common value. Eg. If three 60Ω resistors in parallel – total resistance is 20Ω (⅓ x 60Ω). To determine total resistance of a parallel circuit when resistors are of equal value. Divide the value of one resistor by the number of resistors. 30-03-2024 Slide No. 22  RESISTORS CONNECTED IN PARALLEL For calculating resistance with different value resistances – use normal formula Alternative formula for calculating resistance in parallel when ONLY 2 resistances Otherwise, for more than 2 resistances, use normal formula 30-03-2024 Slide No. 23  SERIES-PARALLEL COMBINATIONS Combination circuit is a circuit that is a blend of series paths and parallel paths. Must first figure out resistance of each individual parallel path in circuit. Example – two 4Ω resistors in parallel an 8Ω resistor in series (R1). To figure out total resistance of parallel section of circuit we use the formula: 1/Rt = 1/R2 + 1/R3 Rt (R2||R3) = 2 Ohms (ll denotes in parallel). 30-03-2024 Slide No. 24  SERIES-PARALLEL COMBINATIONS Knowing total resistance of parallel 'sub circuit‘ – add all series resistances. Parallel leg resistance (R2||R3) can now be used in series calculation. Find total resistance in circuit by adding R1 and R2||R3 Rt = R1 + (R2||R3) Rt = 8 Ohms + 2 Ohms R total = 10 Ohms 30-03-2024 Slide No. 25  SERIES / PARALLEL CALCULATIONS To calculate total resistance of above circuit, first step is to? Calculate resistance of each individual parallel path in circuit. Parallel calculations must be performed first. Series calculations can then follow. What is the total resistance of above circuit? 30-03-2024 Slide No. 26  SERIES / PARALLEL CALCULATIONS Order for calculations: Each parallel leg resistance is calculated R4-5-6-7 = R5-6-7+ R4 = 10 + 10 Series calculation is performed – (R4 + R5,6,7) = 20 ohms Another parallel calculation is performed Another series calculation is performed 30-03-2024 Slide No. 27  SERIES / PARALLEL CALCULATIONS First calculate total resistance of each leg of a parallel circuit. then total parallel resistance. then series resistance, and so on, until total circuit resistance is calculated. 30-03-2024 Slide No. 28  EFFECT ON CIRCUIT RESISTORS IN SERIES Rt = R1 + R2 Rt = 11,000 Ω + 2,000 Ω Rt = 13,000 Ω Total circuit resistance – calculated by adding resistances in series. If more resistors are connected in series – total circuit resistance increases. Using Ohm’s Law – can calculate that current flow will decrease. 30-03-2024 Slide No. 29  EFFECT ON CIRCUIT RESISTORS IN SERIES Voltage drop across each resistance – product of resistance x current (Ohm’s Law). Using Kirchhoff's Voltage Law – all voltage drops in circuit will equal voltage source. 30-03-2024 Slide No. 30  EFFECT ON CIRCUIT RESISTORS IN PARALLEL Rt = 15 Ω = 5 ΩΩ 7.5 2 3 Total circuit resistance – calculated by using formula. This example has equal resistances – may use alternate formula. If more resistors are connected in parallel – total circuit resistance decreases. Using Ohm’s Law – can calculate that current flow will increase. 30-03-2024 Slide No. 31  EFFECT ON CIRCUIT RESISTORS IN PARALLEL Resistors in parallel across a voltage source – same voltage across each resistor. Using Ohm’s Law – current flow through each resistor can be calculated. 30-03-2024 Slide No. 32  PARALLEL CIRCUIT CALCULATIONS Calculate the following: Total circuit current flow Current flow through each resistor 30-03-2024 Slide No. 33  POTENTIOMETER Symbols Manually adjustable, variable, electrical resistor. Typically, wire wound resistance element, 3 terminals, 1 sliding contact. Ends of resistance element attached to input– sliding contact is output. Effectively divides resistor element into 2. Position of slider determines what percentage of input becomes output. Can be used to vary magnitude of the voltage – sometimes called a voltage divider. Typical uses – radio volume controls and television brightness controls. 30-03-2024 Slide No. 34  RHEOSTAT Symbols Typically, wire wound resistance element with 1 sliding contact and 2 terminals. Attached to circuit via 2 terminals. Sliding contact moved along resistance element to vary resistance. Therefore, current through circuit can be increased or decreased. Widely used for controlling speed of electric motors and dimming electric lights. Have been replaced in many applications by semiconductor based systems. Semiconductor devices waste far less power. 30-03-2024 Slide No. 35  GALVANOMETER Galvanometer – historical name given to a moving coil electric current detector. Current is passed through a coil in a magnetic field. Coil experiences a torque proportional to current. Accuracy is dependent upon having a uniform and constant magnetic field. Such "meter movements" were heart of moving coil meters. Now largely replaced with solid state meters or sensors. 30-03-2024 Slide No. 36  WHEATSTONE BRIDGE Electrical bridge circuit for precise comparison of resistances – consists of: Common source of electrical current (such as a battery) Galvanometer that connects two parallel branches 4 resistors (3 of which are known) Well suited for measurement of small changes of a resistance. Suitable to measure resistance change in a strain gauge or temperature probe. Strain gauge transforms strain applied to it into a proportional change of resistance. 30-03-2024 Slide No. 37  WHEATSTONE BRIDGE OPERATION Normal operation – current in each of the arms ABC and ADC of circuit. Potential drop (IR) in the direction of current. Point A is at a higher potential than point B and likewise B higher potential than C. Note both points B and D are at potentials lower than A and higher than C. Potentials of points B and D are not necessarily equal. Arm BD contains galvanometer (G). 30-03-2024 Slide No. 38  WHEATSTONE BRIDGE OPERATION Other representation Current direction through galvanometer is dependent on potentials at B and D. In event potential at both B and D are equal – no current through galvanometer. Galvanometer would indicate this lack of current. Wheatstone bridge is now balanced. Balanced if ratio of resistances R1 and R2 is equal to ratio of R3 and R4. If values of (3) resistances are known – then unknown value can be calculated. 30-03-2024 Slide No. 39  WHEATSTONE BRIDGE FORMULA Ratio is written from supply input to supply output: R1 R2 = R3 R4 Simply transpose to calculate unknown. 30-03-2024 Slide No. 40  WHEATSTONE BRIDGE Balance occurs when: Transposed: R1 x R4 = R2 x R3 When all 4 resistances are equal, current flow through galvanometer equals…??? …………..zero To calculate unknown resistance R4 R4 = (R2 x R3) ÷ R1 30-03-2024 Slide No. 41  WHEATSTONE BRIDGE CALCULATIONS Example No. 1 If R1 = 25 Ω R2 = 20 Ω R3 = 50 Ω then R4 = ……??? 40 Ω R4 = (R2 x R3) ÷ R1 30-03-2024 Slide No. 42  WHEATSTONE BRIDGE CALCULATIONS Example No. 2 If R1 = 300 Ω R2 = 150 Ω R3 = 400 Ω then R4 = ……??? 200 Ω R4 = (R2 x R3) ÷ R1 30-03-2024 Slide No. 43  WHEATSTONE BRIDGE APPLICATION Typical application: Oil temperature system Temperature probe is variable resistor R4 Resistance varies with temperature of oil flowing past probe 30-03-2024 Slide No. 44  CONDUCTANCE The opposite of resistance is conductance (G). Conductance is the ability of a material to pass electrons. Factors that affect resistance – exactly same for conductance (but in opposite). Therefore, conductance is: Directly proportional to cross-sectional area Inversely proportional to length Unit of conductance is SIEMEN (S). Previously known as MHO (G) – ohm spelled backwards. Relationship that exists between resistance and conductance is a reciprocal one. Siemens (mhos) = 1 R 30-03-2024 Slide No. 45  TEMPERATURE EFFECT ON RESISTANCE Resistance of most materials will vary as temperature varies. Amount of change in resistance due to temperature is specific for each material. This variation is known as the temperature co-efficient. Defined as: Temperature co-efficient is expressed by the Greek symbol alpha (). May be positive, negative or zero – depending on type of material. 30-03-2024 Slide No. 46  TEMPERATURE CO-EFFICIENT Positive Temperature Co-efficient (PTC) Material’s resistance increases with an increase in temperature Most metals exhibit a PTC (e.g.: copper, gold, aluminium) Negative Temperature Co-efficient (NTC) Material’s resistance decreases with an increase in temperature Most non-metals exhibit a NTC (e.g.: glass, ceramics, semi-conductors, carbon electrolyte and ionised gases) Zero Temperature Co-efficient (ZTC) Material’s resistance does NOT change with changes in temperature Certain alloys, such as manganin and eureka, have ZTC. Resistance 30-03-2024 0 Temperature Slide No. 47  FIXED RESISTORS Fixed resistors may be: Carbon composition Carbon film Wire wound Metal film Metal oxide Resistance values from a fraction of an ohm up to many megohms. 30-03-2024 Slide No. 48  CARBON COMPOSITION FIXED RESISTOR Most common resistor type in electronic devices and systems. Mix of materials consisting of a conducting carbon material and a filler or binder. Filler is mixed with carbon material to establish ohmic value of resistor. Sensitive to temperature change. Typical tolerance from 10% to 20%. Tendency to generate “noise” (unwanted electrical disturbances) as they age. 30-03-2024 Slide No. 49  CARBON FILM FIXED RESISTOR Thin film of carbon deposited onto a small ceramic rod. Carbon film is spiralled onto ceramic rod until correct resistance reached. Metal leads and end caps added and resistor covered with insulating coating. Painted with coloured bands to indicate resistor value. Cheap & easily available – values within ±10% or ±5%. 30-03-2024 Slide No. 50  WIRE WOUND RESISTORS Made from winding Nickel Copper or Nickel Chromium wire on an insulating form. Able to handle higher currents than carbon-composition resistor of same physical size. Good temperature stability. Because wire is wound into a coil, component may act as an inductor with AC. 30-03-2024 Slide No. 51  METAL FILM RESISTORS High precision resistors – typically 1% to 0.1% tolerance. Metal alloy deposited onto specially treated ceramic in ultra high vacuum. Ideal where close tolerance and low temperature coefficients are required. 30-03-2024 Slide No. 52  METAL OXIDE RESISTOR High precision resistors. Excellent high temperature stability. Made in a similar way to carbon film resistors. Metal oxide film wound on a high purity ceramic former sealed and coated. Ideal where close tolerance and low temperature coefficients are required. 30-03-2024 Slide No. 53  THERMISTORS Thermally sensitive resistors – temperature sensitive passive semiconductors. Essentially a resistor whose resistance varies depending on the temperature. Capitalises on fact that electrical resistance changes as temperature changes. Have either a negative (NTC) or positive (PTC) temperature coefficient. Exhibit large change in resistance with a relatively minute change in temperature. Offer mechanical, thermal and electrical stability with a high degree of sensitivity. Low cost and small size. Permit fast response to temperature changes. 30-03-2024 Slide No. 54  NTC THERMISTORS NTCs – decrease in resistance when subjected to an increase in temperature. Extreme sensitivity to minute temperature changes. Can be brought about either by: A change in the ambient temperature, or Internally by self-heating resulting from current flowing through device Typical uses: Temperature measurement and control Temperature compensation Surge suppression 30-03-2024 Slide No. 55  NTC THERMISTORS CIRCUIT PROTECTION DEVICES Used to limit inrush current when power is applied to a system – switch on protection. Example – Projector lamp: Used to limit current until lamp has warmed up correctly Current will increase as it warms Require time to return to their ambient resistance state – min. 60 secs recommended. Ensures adequate ‘inrush current limiting’ at each power turn-on. 30-03-2024 Slide No. 56  NTC THERMISTORS – TEMPERATURE MEASUREMENT AND CONTROL DEVICES Time and temperature are 2 of the most frequently measured variables. Thermocouples and NTCs – most common way of measuring temperature electronically. NTCs used for general purpose temperature measurement Operate over a wide temperature range (-55 to +300°C) Stable throughout a long lifetime Small and comparatively inexpensive 30-03-2024 Slide No. 57  PTC THERMISTORS Thermally sensitive semiconductor resistors. Exhibit an increase in resistance at a specified temperature. Most applications can be classified in 2 main groups where: Temperature is determined by surrounding environment Temperature is determined by current flowing through device 30-03-2024 Slide No. 58  PTC THERMISTORS OVER-TEMPERATURE PROTECTORS Solid state over-temperature protectors. Designed to sense the temperature of power transistors, heat sinks, chassis, etc. Remain in their low resistance state at all temperatures below switch temperature. When switch temperature is reached or exceeded – rapid increase in resistance. Thereby limiting current to the drive circuitry to protect critical components. Once temperature decreases to normal level – device resets to low resistance state. 30-03-2024 Slide No. 59  PTC THERMISTORS OVER-CURRENT PROTECTORS PTC over-current protectors – connected in series with load to be protected. During normal operating conditions – remains in its low resistance state. Low resistance state results in negligible attenuation to current flow. When short circuit or over current condition occurs – switches to high resistance state. Thereby limiting current flow in circuit to a point well below the normal operating level. When fault condition is removed – device returns to its low resistance state. 30-03-2024 Slide No. 60  VOLTAGE DEPENDENT RESISTORS Used as surge protection device – protects against transient spikes (lightning etc.). Connected directly across AC input to electronic components or circuits. VDR does not change characteristics of circuit. Rest state – high impedance (several MΩ) in relation to component being protected. When a power surge or voltage spike is sensed – resistance rapidly decreases. Creates instant shunt path for over-voltage – thereby saving sensitive components. Because shunt path creates a short circuit – line fuse subject to damage in process. 30-03-2024 Slide No. 61  CONCLUSION Now that you have completed this topic, you should be able to: 3.7.1 Describe resistance and factors that affect it. 3.7.2 Describe the following regarding resistance: Specific resistance Resistor colour code Values and tolerances Preferred values Wattage ratings 3.7.3 Describe the effects of connecting resistors in series and parallel. 3.7.4 Perform calculations of total resistance using the following configurations: Series Parallel Series parallel combinations 3.7.5 Identify potentiometers and rheostats and describe their operation and use. 30-03-2024 Slide No. 62  CONCLUSION Now that you have completed this topic, you should be able to: 3.7.6 Describe the operation of a Wheatstone bridge and calculate the value of an unknown resistor in a balanced Wheatstone bridge circuit. 3.7.7 Define conductance and positive and negative temperature coefficients of resistors. 3.7.8 Identify fixed resistors and define the following regarding them: Stability Tolerance and limitations Methods of construction 3.7.9 Identify the following variable resistors and define their operation and use: Thermistors Voltage dependent resistors 3.7.10 Define the construction of potentiometers and rheostats. 3.7.11 Define the construction of a Wheatstone bridge. 30-03-2024 Slide No. 63  This concludes: Module 3: Electrical Fundaments Topic 3.7: Resistance and Resistor