Sanna Taking & Mohamad Nazri Abdul Halif School of Microelectronic Engineering Prepared by ALTERNATING CURRENT METERS Part 2 Syarifah Norfaezah Edited by

2 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.

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

4 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.

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

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

7 Solution 1-2 The dc sensitivity is The ac sensitivity is S ac = 0.9S dc = 0.9 (1k) = 900  /V

8 Cont. Therefore the multiplier resistor is R s = S ac x Range – R m = 900 x 10V rms – 500 = 8.5k 

9 Each diode in full-wave rectifier circuit above has an average forward resistance of 50Ω and is assumed to have an infinite resistance in the reverse direction. Calculate: Multiplier resistance, R S AC sensitivity The equivalent dc sensitivity. E in = 10V rms RSRS R sh = 0.5kΩ I fs = 1mA R m = 0.5kΩ Assignment

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

11 Electrodynamometer Movement Fixed Coil Moving Coil Fixed Coil Source Most fundamental and versatile meter use today. Is a current-sensitive device – the pointer deflects up scale because of current flow through moving coil. Most important applications: voltmeter and ammeter standard.

12 Electrodynamometer (Cont..) The single-coil electrodynamometer movement consists of a fixed coil divided into two equal halves. Both halves of the split fixed coil and the moving coil are connected in series – current from the circuit being measured passed through all the coils causing magnetic field around the fixed coils. The moving coil rotates in this magnetic field. The electrodynamometer – handle much more current than d’ Arsonval movement. It can handle ~ 100mA. The electrodynamometer – have a very low sensitivity rating of ~ 20 to 100 Ω/V. Most extensive application: Wattmeter. The magnetic torque that cause pointer deflect up scale: Θ m – angular deflection of the pointer E – rms value of source voltage K m – instrument constant (degrees/watt) l – rms value of source current cos θ – power factor

13 Iron-vane Meter Movement I The iron-vane meter movement consists of a fixed coil of many turns and two iron vanes placed inside the fixed coil. it is widely used in industry. the current can be measured passes through the winding of the fixed coil setting up a magnetic field that magnetized the two iron vanes with the same polarity.

14 Iron-vane Meter (Cont…) If one iron vanes is attached to the frame of a fixed coil – the other iron vane will then be repelled by amount related to the square of current. Although it is responsive to direct current (the hysteresis) – the iron vanes causes appreciable error. (used only for a very inexpensive indicators, i.e charge-discharge indicators on automobiles). It is used extensively in industry for measuring ac when errors on the order of 5% to 10% are acceptable. Iron-vane movement very sensitive to frequency change (25 – 125 Hz) - it is because the magnetization of the iron vane is nonlinear.

15 Thermocouple Meter d’ Arsonval meter movement Heater Insulating bead Thermocouple Source I  Basic thermocouple meter Usually consists a heater element – fine wire, a thermocouple, and d’Arsonval meter movement. to measure a very high-frequency ac (very accurate well > 50Hz).

16 Loading effects of AC Voltmeter The sensitivity of ac voltmeters, using either half wave or full wave rectification, is less than the sensitivity of dc voltmeters. Therefore, loading effect of an ac voltmeter is greater than that of a dc voltmeter. S ac = 0.45S dc S ac = 0.9S dc

17 Calibrating AC voltmeters and ammeters for different full-scale ranges of operation is much the same as with DC instruments: series "multiplier" resistors are used to give voltmeter movement a higher range, and parallel "shunt" resistors are used to allow ammeter movements to measure currents beyond their natural range. However, we are not limited to these techniques as we were with DC: because we can use transformers with AC too. Voltage and Current Transformer Applications

18 Summary For general purposes, the d’Arsonval movement – either half- wave or full-wave rectifier, is widely used. Meter MovementApp. Voltage & Freq.Reading Obtained D’Arsonval10V rms, 60 Hz0V Iron vane10V rms, 60 Hz10V Electrodynamometer10V rms, 60 Hz10V Thermocouple10V rms, 60 Hz10V D’Arsonval with half- wave rectifier 10V rms, 60 Hz4.5V D’Arsonval with full- wave rectifier 10V rms, 60 Hz9.0V Iron vane10V dc 10V Electrodynamometer10V dc 10V Thermocouple10V dc 10V

19 NEXT LECTURE DC & AC BRIDGE: Part 1 (DC bridge) The Wheatstone bridge The Kelvin bridge