1. CHAPTER 4
AC Meters.
Prepared By:
Rosemizi B. Abd. Rahim

2. 4.1 Introduction to AC Meters.
Five principal meter movement that are commonly used in ac instruments;
(i) Electrodynamometer.
(ii) Iron-Vane.
(iii) Electrostatic.
(iv) Thermocouple.
(v) D’Arsonval (PMMC) with rectifier.
The d’Arsonval meter is the most frequently used meter movement, event though it cannot directly measure alternating current or voltage.
In this chapter it will discuss the instruments for measuring alternating signal that use the d’Arsonval meter movement.

3. Figure 4.1: D’Arsonal Electromechanical Meter Movement.
Cont’d…
(a) AC voltmeters and ammeters
AC electromechanical meter movements come in two basic arrangements:
(1) Based on DC movement designs.
(2) Engineered specifically for AC use.
Permanent-magnet moving coil (PMMC) meter movements will not work correctly if directly connected to alternating current, because the direction of needle movement will change with each half-cycle of the AC.
Permanent-magnet meter movements, like permanent-magnet
motors, are devices whose motion
depends on the polarity of the
applied voltage, Figure 4.1.
Figure 4.1: D’Arsonal Electromechanical Meter Movement.

4. Figure 4.2: Rectified D’Arsonal Electromechanical Meter Movement.
Cont’d…
(b) DC-style Meter Movement for AC application.
If we want to use a DC-style meter movement such as the D'Arsonval design, the alternating current must be rectified into DC, Figure 4.2.
This can be accomplished through the use of devices called diodes. The diodes are arranged in a bridge, four diodes will serve to steer AC through the meter movement in a constant direction throughout all portions of the AC cycle:
Figure 4.2: Rectified D’Arsonal Electromechanical Meter Movement.

5. Figure 4.3: Iron-Vane Electromachanical Meter Movement.
Cont’d…
(c) Iron-Vane Electromechanical.
The AC meter movement without the inherent polarity sensitivity of the DC types.
This design avoid using the permanent magnets. The simplest design is to use a non-magnetized iron vane to move the needle against spring tension, the vane being attracted toward a stationary coil of wire energized by the
AC quantity to be measured, Figure 4.3.
The electrostatic meter movements
are capable of measuring very high
voltages without need for range
resistors or other, external apparatus.
Figure 4.3: Iron-Vane Electromachanical Meter Movement.

6. Figure 4.4: AC Voltmeter with Resistive Divider.
Cont’d…
(d) AC Voltmeter with Resistive Divider.
When a sensitive meter movement needs to be re-ranged to function as an AC voltmeter, series-connected "multiplier" resistors and/or resistive voltage dividers may be employed just as in DC meter design, Figure 4.4.
Figure 4.4: AC Voltmeter with Resistive Divider.

7. Figure 4.5: AC Voltmeter with Capacitive Divider.
Cont’d…
(e) AC Voltmeter with Capacitive Divider.
Capacitors may be used instead of resistors, though, to make voltmeter divider circuits. This strategy has the advantage of being non-dissipative; no true power consumed and no heat produced. Refer to Figure 4.5.
Figure 4.5: AC Voltmeter with Capacitive Divider.

8. Figure 4.6: DC Voltmeter Circuit Modified to Measure AC Voltage.
4.2 D’Arsonval Meter Movement with Half-Wave Rectification.
In order to measure the alternating current with the d’Arsonval meter movement, we must rectify the alternating current by use of diode rectifier .
Figure 4.6 is the DC voltmeter circuit modified to measure AC voltage.
The forward diode, assume to be ideal diode, has no effect on the operation of the circuit .
For example if the 10 V sine-wave input is fed as the source of the circuit, the voltage across the meter movement is just the positive half-cycle of the sine wave due to the rectifying effect of the diode.
Figure 4.6: DC Voltmeter Circuit Modified to Measure AC Voltage.

9. The peak value of 10 V rms sine wave is,
Cont’d…
The peak value of 10 V rms sine wave is,
Shown that a dc voltmeter will provide 45% of the ac input
From the above equation, the sensitivity of dc voltmeter is

10. Figure 4.9: Full Wave Bridge Rectifier Used in AC Voltmeter Circuit.
4.3 D’Arsonval Meter Movement with Full-Wave Rectification.
The full-wave rectifier provide higher sensitivity rating compare to the half-wave rectifier.
Bridge type rectifier is the most commonly used, Figure 4.9.
Figure 4.9: Full Wave Bridge Rectifier Used in AC Voltmeter Circuit.

11. (b) Vise versa for the negative half cycle (blue arrow).
Cont’d…
Operation;
(a) During the positive half cycle (red arrow), 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.
- If the deflection remains the same, the instrument using full wave rectification will have a greater sensitivity.
(b) Vise versa for the negative half cycle (blue arrow).

12. The average dc value of the pulsating sine wave is,
Cont’d…
From the circuit in Figure 4.9, the peak value of the 10 Vrms signal with the half-wave rectifier is,
The average dc value of the pulsating sine wave is,
The AC voltmeter using full-wave rectification has a sensitivity equal to 90% of the dc sensitivity or twice the sensitivity using half-wave rectification.

13. Figure 4.10: AC Voltmeter Using Full-Wave Rectification and Shunt.
Example 4.2: D’Arsonval Meter Full-Wave Rectifier.
Each diode in the full-wave rectifier circuit in Figure 4.10 has an average forward bias resistance of 50 Ohm and is assumed to have an infinite resistance in the reverse direction. Calculate,
a) The multiplier Rs.
b) The AC sensitivity.
c) The equivalent DC sensitivity.
Solution:
The multiplier Rs
Calculate the current shunt and total current,
Figure 4.10: AC Voltmeter Using Full-Wave Rectification and Shunt.

14. Cont’d…Example The equivalent DC voltage is, a) The multiplier Rs
(b) The ac sensitivity,
(c.) The dc sensitivity,

15. Quiz on Chapter 4
1) Calculate the ac and dc sensitivity and the value of the multiplier resistor required to limit current to the full-scale deflection current in the circuit shown in Figure below. All diodes have forward resistance of 300 and an infinite reverse resistance.

16. Quiz on Chapter 4
2) A DC multimeter can be designed by using a movement full-scale meter connecting with Rshunt , Rmultiplier1 , Rmultiplier2 , R and a battery as shown in Figure below.
Determine the value of each resistor to make the circuit become a dc multimeter.
that it can be used to measure 10V voltage, 5A current and 0Ω resistance.
(assume the full scale meter is 1 mA, 500 Ω and 9V battery)