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EKT112 Principles of Measurement and Instrumentation Week 4

EKT112 Principles of Measurement and Instrumentation Week 4. Current, Voltage & Resistance Measurement. Topics Outline. 1.0 Device for Current Measurement 1.1 Analog ammeter 1.2 Galvanometer 2.0 Device for Voltage Measurement 2.1 Analog voltmeter 2.2 Oscilloscope

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EKT112 Principles of Measurement and Instrumentation Week 4

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  1. EKT112Principles of Measurement and InstrumentationWeek 4

  2. Current, Voltage & Resistance Measurement

  3. Topics Outline 1.0 Device for Current Measurement 1.1 Analog ammeter 1.2 Galvanometer 2.0 Device for Voltage Measurement 2.1 Analog voltmeter 2.2 Oscilloscope 2.3 Potentiometer 3.0 Device for Resistance Measurement 3.1 Ohmmeter 3.2 Megger 4.0 Multimeter

  4. 2.0 Voltage Measurement

  5. 2.1 Voltmeter • A voltmeter is an instrument used for measuring the potential difference between two points in an electric circuit.

  6. A voltmeter is placed in parallel with a circuit element to measure the voltage drop across it and must be designed to draw very little current from the circuit so that it does not appreciably change the circuit it is measuring. • To accomplish this, a large resistor is placed in series with the galvanometer. • Its value is chosen so that the design voltage placed across the meter will cause the meter to deflect to its full-scale reading. • A galvanometer full-scale current is very small: on the order of milliamperes.

  7. Voltmeter – Principle of Operation • The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. • When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. • The angular rotation is proportional to the current that is flowing through the coil. • For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

  8. D’Ársonval Meter Movement Used In A DC Voltmeter • The basic d’Ársonval meter movement can be converted to a dc voltmeter by connecting a multiplier Rs in series with the meter movement • The purpose of the multiplier: • is to extend the voltage range of the meter • to limit current through the d’Arsonval meter movement to a maximum full-scale deflection current. Fig 2-1 The basic d’Arsonval meter Movement Used In A DC Voltmeter

  9. Cont. • To find the value of the multiplier resistor, first determine the sensitivity, S, of the meter movement.

  10. Example 1-4 Calculate the value of the multiplier resistance on the 50V range of a dc voltmeter that used a 500A meter movement with an internal resistance of 1k.

  11. Solution: Sensitivity, Multiplier, Rs = S X Range – internal Resistance = (2k X 50) – 1k = 99k

  12. Multirangevolmeter : Example 1 • A D’Arsonval movement with a full scale deflection of 50µA and having an internal resistance of 500 Ω is to be converted into a multirange voltmeter. Determine the value of multiplier required for 0-20 V, 0 – 50 V and 0-100V using individual multipliers for each range. Calculate the values of the individual resistor.

  13. Multirange voltmeter: Example 2 • Convert a basic D’Arsonval movement with a full scale deflection of 10mA and having an internal resistance of 100 Ω into a multirange voltmeter with ranges from 0-5 V, 0 – 50 V and 0-100V .

  14. Voltmeter Loading Effects When a voltmeter is used to measure the voltage across a circuit component, the voltmeter circuit itself is in parallel with the circuit component. Since the parallel combination of two resistors is less than either resistor alone, the resistance seen by the source is less with the voltmeter connected than without. Therefore, the voltage across the component is less whenever the voltmeter is connected. The decrease in voltage may be negligible or it may be appreciable, depending on the sensitivity of the voltmeter being used. This effect is called voltmeter loading. The resulting error is called a loading error.

  15. Example 1-5 Two different voltmeters are used to measure the voltage across resistor RB in the circuit of Figure 2-2. The meters are as follows. Meter A : S = 1k/V, Rm = 0.2k, range = 10V Meter B : S = 20k/V, Rm = 1.5k, range=10V Calculate: • Voltage across RB without any meter connected across it. (b) Voltage across RB when meter A is used. (c) Voltage across RB when meter B is used (d) Error in voltmeter readings. Fig. 2.2

  16. Solution: (a) The voltage across resistor RB without either meter connected is found Using the voltage divider equation:

  17. Cont. (b) starting with meter A, the total resistance it presents to the circuit is The parallel combination of RB and meter A is Therefore, the voltage reading obtained with meter A, determined by the voltage divider equation, is

  18. Cont. (c) The total resistance that meter B presents to the circuit is RTB = S x Range = 20k/V x 10 V = 200 k The parallel combination of RB and meter B is Re2 = (RB x RTB)/(RB + RTB) = (5kx200k)/(5k+200k) = 4.88 k Therefore, the voltage reading obtained with meter B, determined by use of the voltage divider equation, is VRB = E(Re2)/(Re2+RA) = 30 V x (4.88k)/(4.88k+25k) = 4.9 V

  19. Cont. (d) Voltmeter A error = (5 V – 3.53 V)/5 V x (100% = 29.4% Voltmeter B error = (5 V – 4.9 V)/5 V x (100%) = 2 %

  20. Five principal meter movements used in ac instrument 1. Electrodynamometer 2. Iron Vane 3. Electrostatic 4. Thermocouple 5. D’Arsonval with rectifier

  21. Application of meter movements:

  22. PMMC Instrument on AC • The PMMC instrument is polarized (terminals +ve & -ve) - it must be connected correctly for positive (on scale) deflection to occur. • When an AC with a very low frequency is passed through a PMMC, the pointer tends to follow the instantaneous level of the AC • As the current grows positively, the pointer deflection increases to a maximum at the peak of the AC • As the instantaneous current level falls, the pointer deflection decreases toward zero. When the AC goes negative, the pointer deflected (off scale) to the left of zero • This kind of pointer movement can occur only with AC having a frequency of perhaps 0.1Hz or lower

  23. PMMC Instrument on AC • At 50Hz or higher supply frequencies - the damping mechanism of the instrument and the inertia of the meter movement prevent the pointer from following the changing instantaneous levels. • The average value of purely sinusoidal AC is zero. • Therefore, a PMMC instrument connected directly to measure 50Hz AC indicates zero average value. • It is important to note that although a PMMC instrument connected to an ac supply may indicating zero, there can actually be very large rms current flowing in its coils

  24. Two types of PMMC meter used in AC measurement : 1. Half wave rectification 2. Full wave rectification

  25. D’Arsonval meter movement used with half wave rectification To convert alternating current (AC) to unidirectional current flow, which produces positive deflection when passed through a PMMC, the diode rectifier is used. Several types of rectifiers are selected such as a copper oxide rectifier, a vacuum diode, or semiconductor or “crystal diode”.

  26. Cont… • For example, if the output voltage from a half wave rectifier is 10Vrms so the dc voltmeter will provide an indication of approximately 4.5V dc  Therefore, the pointer deflected full scale when 10V dc signal is applied. • When we apply a 10Vrms sinusoidal AC waveform, the pointer will deflect to 4.5V  This means that the AC voltmeter is not as sensitive as DC voltmeter. • In fact, an AC voltmeter using half wave rectification is only approximately 45% as sensitive as a dc voltmeter.

  27. Cont… • Actually, the circuit would probably be designed for full-scale deflection with a 10V rms AC applied, which means the multiplier resistor would be only 45% of the value of the multiplier resistor for 10V dc voltmeter. Since we have seen that the equivalent dc voltage is equal to 45% of the rms value of the ac voltage. Sac = 0.45Sdc

  28. Cont.. Commercially produced ac voltmeters that use half wave rectification also has an additional diode and a shunt as shown in Figure below:

  29. Cont… • The additional diode D2 is reverse biased on the positive half cycle and has virtually no effect on the behavior of the circuit. • In the negative half cycle, D2 is forward biased and provides an alternate path for reverse biased leakage current that would normally through the meter movement and diode D1. • The purpose of the shunt resistor Rsh is to increase the current flow through D1 during positive half cycle so that the diode is operating in a more linear portion of its characteristic curve. • Although this shunt resistor improves the linearity of the meter on its low voltage ac ranges, it also further reduces the AC sensitivity.

  30. Example 1-6 Compute the value of the multiplier resistor for a 15Vrms ac range on the voltmeter shown in Fig. 1. RS Ifs = 1mA Ein = 15Vrms Rm = 300Ω Fig. 1: AC voltmeter using half wave rectification

  31. Rs = Sdc × Rangedc – Rm - Rm = 1k × = 1k × 0.45(10) – 300 = 4.2k Solution: Method 1 The sensitivity of the meter movement,

  32. Rs = Sac × Rangeac – Rm = 450 × 10 –300 = 4.2k Cont. Method 2 The AC sensitivity for half wave rectifier, Sac = 0.45Sdc = 0.45(1k) = 450/V

  33. Rs = = = 4.2k Cont. Method 3

  34. Example 1-7 Calculate the ac and dc sensitivity and the value of the multiplier resistor required to limit the full scale deflection current in the circuit shown in Fig above.

  35. D’Arsonval meter movement used with full wave rectification Fig. 2: Full bridge rectifier used in an ac voltmeter circuit During the positive half cycle, currents flows through diode D2, through the meter movement from positive to negative, and through diode D3. The polarities in circles on the transformer secondary are for the positive half cycle. Since current flows through the meter movement on both half cycles, we can expect the deflection of the pointer to be greater than with the half wave cycle, which allows current to flow only on every other half cycle; if the deflection remains the same, the instrument using full wave rectification will have a greater sensitivity.

  36. Consider the circuit shown in Fig. 1-2 Fig. 1-2: AC voltmeter using full wave rectification

  37. Cont. When the 10Vrms of AC signal is applied to the circuit above, where the peak value of the AC input signal is And the average full wave output signal is Therefore, we can see that a 10Vrms voltage is equivalent to 9Vdc for full-scale deflection.

  38. Cont. Or This means an ac voltmeter using full wave rectification has a sensitivity equal to 90% of the dc sensitivity Sac = 0.9 Sdc

  39. Example 1-8 Compute the value of the multiplier resistor for a 10Vrms ac range on the voltmeter in Figure 1-2. Fig. 1-2: AC voltmeter circuit using full wave rectification

  40. Solution 1-8 The dc sensitivity is The ac sensitivity is Sac = 0.9Sdc = 0.9 (1k) = 900 /V

  41. Cont. Therefore the multiplier resistor is Rs = Sac x Range – Rm = 900 x 10Vrms – 500 = 8.5k

  42. Cont. Note: Voltmeters using half wave and full wave rectification are suitable for measuring only sinusoidal ac voltages.

  43. 2.2 Oscilloscope • An oscilloscope is a piece of electronic test equipment that allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electrical potential differences (vertical axis) plotted as a function of time or of some other voltage (horizontal axis • Perform some computations using data taken from the voltage waveform that is displayed such as: * Rms value * Average Amplitude * Peak-to-peak Amplitude * Frequency

  44. Oscilloscope • An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. • Using for signal/wave display – Winamp Music Player, Electrocardiogram,

  45. 2.3 Potentiometer • A potentiometer is a variable resistor that functions as a voltage divider • It is a simple electro-mechanical transducer • It converts rotary or linear motion from the operator into a change of resistance, and this change is (or can be) used to control any volume.

  46. Potentiometer • Schematic symbol for a potentiometer. The arrow represents the moving terminal, called the wiper. • Usually, this is a three-terminal resistor with a sliding contact in the center (the wiper) - user-adjustable resistance • If all three terminals are used, it can act as a variable voltage divider • If only two terminals are used (one side and the wiper), it acts as a variable resistor

  47. Potentiometer Circuit • Any current flow through the Galvanometer, G, wpuld be a result of an imbalance in the measured voltage, Vm and the voltage imposed across points A to B, VAB. • If Vm is not equal to VAB, a current will flow through the galvanometer, G. • Galvanometer detects current flow due to imbalance in voltage Vm and VAB. When Vm = VAB, there is a balance and no current, means no displacement in Galvanometer.

  48. Potentiometer – Application • In modern usage, a potentiometer is a potential divider, a three terminal resistor where the position of the sliding connection is user adjustable via a knob or slider. For instance, when attached to a volume control, the knob can also function as an on/off switch at the lowest volume • Potentiometers are frequently used to adjust the level of analog signals (e.g. volume controls on audio equipment) and as control inputs for electronic circuits (e.g. a typical domestic light dimmer).

  49. 3.0 Resistance Measurement

  50. Resistance Measurement • The resistances are classified as follow: • 1. Low Resistance : All resistances of the order of 1 ohm and below. example: Machine armature, series field winding shunt etc. • 2. Medium Resistance : All resistances of the order of 1 ohm to 100,000 ohms. example: Winding resistance, multiplier resistance.

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