DEE 2313 SEM II 0809 PART 2. CHAPTER 2 AC Meters

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1 CHAPTER 2 AC Meters 2.6 Introduction to AC meters (Alternating-Current) The d Arsonval meter movement is a dc responding device. It can also be used to measure alternating current and voltage. AC voltmeter are usually of the average-responding type, with the meter scale calibrated in terms of the rms value of a sine wave. Non-sinusoidal waveforms, however, will cause this type of meter to read high or low depending on the form of the waveform. There are two types of ac meters that will be discussed. Half-wave rectifier voltmeter Full-wave rectifier voltmeter Half-wave Rectifier Voltmeter The d Arsonval meter movement only responds to the average or dc value of the current through the moving coil. In order to measure alternating voltage with d Arsonval meter, the alternating signal need to be rectified first by using diode rectifier to produce unidirectional current flow. If a diode D 1 is added to the dc voltmeter circuit as discussed earlier (Figure 1), then a circuit that is capable of measuring ac voltage is achieved. + R S _ R m I m Figure 2.11 : AC Voltmeter Using Half-wave Rectification Connecting a 10V sine wave input as shown in Figure 2.11, the voltage across the meter movement is just the positive half cycle of the sine wave of the rectifying action of the diode. The peak value of the 10V rms sine wave is E p = 10 V rms x = [V rms = V m / 2] 1

2 The dc meter will only respond to the average value of the ac sine wave. The average or dc value is given as: E ave = E dc = x E p Where E p = 2 x E rms Therefore, E dc = x 2 x E rms = 0.45 E rms The half-wave rectifier will give a reading of 4.5V for a 10V rms sinusoidal ac waveform. Therefore, using half-wave rectification the dc value is only 45% to that of rms value. S ac = 0.45 S dc... (2.21) The multiplier resistor R s (connected in series with diode) would be only 45% of the multiplier value of a dc voltmeter. The value of R s using the half-wave rectification can be calculated as: R s = (E dc / I dc ) - R m = (0.45 E rms / I dc ) - R m Where S = 1/ I dc (Ω/V) Therefore, R s = S 0.45 E rms - R m In general, multiplier resistor R s = (S x Range) - R m..(2.22) Where equation (2.22) can be R s = (S dc x Range dc ) - R m.....(2.23) OR R s = (S ac x Range ac ) - R m.....(2.24) 2

3 Example 2.7 Compute the value of the multiplier for a 10 V rms ac range on the voltmeter as shown figure 2.11 using equation (2.23) and (2.24) given that I fs = 1 ma and R m = 100Ω Full-wave Rectification Voltmeter To improve the sensitivity, the full wave rectification is usually used. It is more desirable to use a full-wave rather than a half-wave rectifier in ac voltmeter because of the higher sensitivity rating. The most frequently used circuit for full-wave rectification is the bridge type as shown in figure 2.12 Figure 2.12: AC Voltmeter Circuit Using Full-wave Rectification The average value or the dc value is then: E ave = 2 x E p E ave = E p... (2.25) OR E ave = x 2 E rms E ave = 0.9 E rms. (2.26) 3

4 Notice that the 10V rms voltage is equivalent to 9V dc.when full-wave rectification is used, the pointer will deflect to 90% of the full scale. This means an ac voltmeter using full-wave rectification has a sensitivity equal 90% of the dc sensitivity or twice the sensitivity of a circuit using half-wave rectification. The sensitivity, S ac for full-wave rectification is S ac = 0.9 S dc.(2.27) This would mean that the value of the multiplier resistor would only be 90% that of a 10 V dc voltmeter. Formula Half-wave rectifier Full-wave rectifier E p-p E p-p = 2( V rms x ) E p-p = 2( V rms x ) E max or E peak E p = V rms x E p = V rms x E ave E ave = E dc = x E p E ave = x E p Sensitivity (S ac ) S ac = 0.45 S dc S ac = 0.9 S dc Rs Rs = S x 0.45 E rms - R m Rs = S x 0.9 E rms - R m Example 2.8 Compute the value of the multiplier resistor for a 10V rms ac range on the voltmeter in figure 2.12 if the I fs = 1 ma and R m = 500Ω. 4

5 2.7 Considerations in Choosing an Analog Meter (a) For measurements involving dc applications, select the meter with broadest capability meeting the circuit s requirements. (b) For ac measurements involving sine waves with only modest amounts of distortion (< 10 percent), the average-responding voltmeter provides the best accuracy. (c) For high-frequency measurements (> 10MHz), the peak-responding voltmeter with a diode probe input is the most economical choice. Peak-responding circuits are acceptable if the inaccuracy caused by distortion in the input waveform can be tolerated. (d) For measurements where it is important to determine the effective power of waveform which depart from the true sinusoidal forms, the rms-responding voltmeter is the appropriate choice. 2.8 Comparison Between Analog Meters and Digital Meters Meters exist in both digital and analog Analog meters are electrochanical devices driving a pointer against a scale. They are prone to meaaasurement errors from a number of sources, with accuracy figures of between ± 0.1% and ± 3%. Inaccurate scale marking during manufacture, bearing friction,bent pointers and ambient temperature variations all limit measurement accuracy. Futher human errors are introduced through parallax error and mistakes in interpolating between scale readings. Digital meters give a reading in the form of a digital display. There are no problem of parallax. Digital meters are technically more superior than analog meter with accuracy of between ± 0.005% and ± 2%. However, this increases manufacturing cost. 5