EEE3406 Instrumentation & easurements LABOATOY Experiment 2 Name Class Date Class No. arks Voltmeter and Ammeter Design Objectives: After completing this lab, you will be able to measure the full-scale deflection current and internal resistance of a moving coil meter, use the moving coil meter as a voltmeter, use the moving coil meter as an ammeter. Equipment/Components equired Digital multimeter (D) Variable DC power supply oving coil meter (100 µa 1 ma full-scale) Decade resistance box x 2 Ver Author(s) Date emark 1.0 YH WONG 1/07 For EEE3406 (PT) 1.1 YH WONG 7/07 Block for students name/class, date of experiment added. Laboratory Experiment 2 1
Part A: Background In the days before the launching of electronic instruments, a wide range of analogue instruments was developed. They are generally robust so most of us will use them quite often. They also have the attraction of being cheaper in price. The most common analogue instrument is the permanent-magnet moving-coil instrument and moving-iron instrument. They are widely used in electrical or electronic products for the indication of voltage and current. The major differences between moving-coil and moving-iron instruments are listed in Table 1. Here, we will use moving coil meter as the heart of this mini-project. Table 1. ajor differences between moving-coil and moving iron instruments oving-coil oving-iron oving element Coil Iron Deflection (or Scale) Linear Non-linear AC/DC DC only Both AC and DC Sensitivity High Low oment of inertia of the pointer Low High Figure 1 oving-coil meter Figure 2 Principle of operation The typical construction of the moving-coil meter and its principle of operation are shown in Figure 1 and 2 respectively. The moving coil is mounted on the two pivots and bearings which allow the coil free to rotate. The coil and the pointer are fixed to point at zero by the spiral spring. When current passes through the coil, magnetic field is produced according to the magnitude of the current. The deflection of the pointer is caused by the interaction of the magnetic field produced by the coil and the permanent magnet. When the force produced by the interaction of the two magnetic fields equals to the backward force produced by the spiral spring, the pointer will stop at that particular position on the meter scale which indicates the values of current flowing through the coil. 2 Experiment 2 Laboratory
EEE3406 Instrumentation & easurements Since the moving coil meter is a current operating device, we use the symbol shown in Figure 3 to represent it. + A - Figure 3 Symbol The current sensitivity of the moving-coil meter is normally 100µA, 500µA or 1mA. For higher sensitivity meter, the full-scale deflection current can be as low as 1µA. Voltmeter and ammeter can be built from this low current moving-coil meter. Assume the resistance of the moving coil meter is 0.5Ω and full-scale deflection current is 1mA. We have to build a 1V voltmeter and 1A ammeter using this moving coil meter. Voltmeter The connection of the voltmeter is shown in Figure 4. The resistor added in series with the meter is used to limit the current flowing through the meter, which causes a larger voltage drop in the meter circuit. The full-scale deflection of the meter is 1mA and the total voltage drop across the meter and the series resistor is 1V at full-scale deflection current. Therefore, we can calculate the resistance value by using Ohm s law. V = I 1 = 1 10 3 3 = 1 10 ( + m ) m = 1 10 3 0.5 = 999.5Ω where m = the internal resistance of the meter (= 0.5Ω) + A - Figure 4 V Voltmeter That is, by adding a 999.5Ω resistor in series with the meter, we can convert the meter to a 1V voltmeter. The full-scale deflection of the voltmeter is still 1mA and the input resistance of the voltmeter is 1kΩ. The power rating of the series resistor is: Power = I 2 = (1 10-3 ) 2 999.5 = 1mW. Therefore, a 1/4W 999.5Ω resistor is used. Laboratory Experiment 2 3
Ammeter The connection of the ammeter is shown in Figure 5. A shunt resistor is added in parallel with the meter to by-pass part of the current. The full-scale deflection current of the meter is 1mA and the total current flow through the circuit 1A at full-scale deflection. Therefore, we can calculate the resistance value by using Kirchhoff s law. By Kirchhoff s current law, I = I m + I For I = 1A and I m = 1mA, I = 1 0.001 = 0. 999A By Kirchhoff s voltage law, I I m m = 3 1 10 0.5 = = 5.005 10 0.999 4 Ω + A - Figure 5 Ammeter series + A - The rating of the shunt resistor is shunt Power = I 2 Figure 6 Ammeter with = (0.999) 2 5.005 10-4 higher input resistance = 0.4995mW. Therefore, a 1/4W 0.5005mΩ resistor is used. By adding the 0.5005mΩ resistor in parallel with the meter, the full-scale deflection of the ammeter is 1A and the input resistance of the ammeter is about 0.5005mΩ. However, it is very difficult to add such a low value resistor in parallel with the meter because value of the contact resistance will be of the same order as the shunt resistor. Also, the effect of the environmental temperature on the drift of the resistance value of this low value shunt resistor will be significant. The environmental temperature changes will affect the full-scale deflection current of the ammeter. We can solve this problem by using the circuit as shown in Figure 6. We add a series resistor to the meter and then connect a shunt resistor to them. However, you should take the value of the input resistance of the ammeter into account for the evaluation of the values of the two resistors. The calculation of these resistor values will be left as exercise for the students. I I m I 4 Experiment 2 Laboratory
EEE3406 Instrumentation & easurements Part B: Procedures Voltmeter Design 1. Connect the circuit as in Figure 7. Increase the supply voltage slowly until the moving coil meter deflects full-scale. easure the full-scale current with the D. Full-scale deflection current I fsd = D A 0.5-1 kω oving-coil meter Figure 7 2. Connect the D across the terminals of the moving coil meter and measure the internal resistance of the meter. Internal resistance int = 3. Use the voltmeter circuit as in Figure 4. Calculate the multiplier resistance required to convert the meter as a 20 V range voltmeter (i.e. the meter will deflect full-scale when 20 V is applied across the voltmeter circuit). 4. Connect the testing circuit as in Figure 8. Increase the input voltage from 0 to 20 V in step of 2 V and record the corresponding reading of your designed voltmeter in Table 1. V D 0 20 V voltmeter Figure 8 Laboratory Experiment 2 5
Table 1 Input voltage (V) eter reading 0 2 4 6 8 10 12 14 16 18 20 Ammeter Design 5. Use the ammeter circuit as in Figure 6. Calculate the series and shunt resistance required to convert the meter as a 100 ma range ammeter (i.e. the meter will deflect full-scale when 100 ma is injected to the ammeter circuit) and input resistance of 50 Ω [i.e. ( int + series )// shunt = 50 Ω] 6 Experiment 2 Laboratory
EEE3406 Instrumentation & easurements 6. Connect the testing circuit as in Figure 9. Increase the input current from 0 to 100 ma in step of 10 ma and record the corresponding reading of your designed ammeter in Table 2. D A shunt series 0 100 ma ammeter Figure 9 Table 2 Input current (ma) 0 10 20 30 40 50 60 70 80 90 100 eter reading Laboratory Experiment 2 7
Part C Discussion 1. Calculate the minimum power rating of the multiplier resistor of your designed voltmeter. 2. What is the sensitivity (in Ω/V) of your designed voltmeter? 3. What is the purpose of the series resistor in your designed ammeter circuit? PAT D : SUAY In less than 100 words, summarize what you have learnt from this experiment. 8 Experiment 2 Laboratory