Sanna Taking & Mohamad Nazri Abdul Halif

School of Microelectronic Engineering

Prepared by

ALTERNATING CURRENT METERS

Part 2Syarifah Norfaezah

Edited by

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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.

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Consider the circuit shown in Fig. 1-2

Fig. 1-2: AC voltmeter using full wave rectification

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Cont.When the 10Vrms of AC signal is applied to the circuit above, where the peak value of the AC input signal is

V14.14)10(x414.1xE2E rmsp

And the average full wave output signal is

V914.14x636.0xE636.0EE pdcave

Therefore, we can see that a 10Vrms voltage is equivalent to 9Vdc for full-scale deflection.

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Cont.

Sac = 0.9 Sdc

rmsrmspavg E9.0)xE2(636.0E636.0E

Or

This means an ac voltmeter using full wave rectification has a sensitivity equal to 90% of the dc sensitivity

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Example 1-2Compute 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

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Solution 1-2

The dc sensitivity is

V/k1mA1

1

I

1S

fsdc

The ac sensitivity is

Sac = 0.9Sdc = 0.9 (1k) = 900 /V

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Cont.

Therefore the multiplier resistor is

Rs = Sac x Range – Rm

= 900 x 10Vrms – 500

= 8.5k

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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, RS

• AC sensitivity• The equivalent dc sensitivity.

Ein = 10Vrms

RS

Rsh = 0.5kΩ

Ifs = 1mA

Rm = 0.5kΩ

Assignment

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Cont.

Note: Voltmeters using half wave and

full wave rectification are suitable for measuring only sinusoidal ac voltages.

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Electrodynamometer Movement

Fixed CoilMoving 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.

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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:

cosElKmm Θm – angular deflection of the pointer E – rms value of source voltageKm – instrument constant (degrees/watt) l – rms value of source currentcos θ – power factor

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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.

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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.

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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).

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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.

Sac = 0.45Sdc

Sac = 0.9Sdc

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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

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SummaryFor general purposes, the d’Arsonval movement – either half-wave

or full-wave rectifier, is widely used.

Meter Movement App. Voltage & Freq. Reading Obtained

D’Arsonval 10Vrms, 60 Hz 0V

Iron vane 10Vrms, 60 Hz 10V

Electrodynamometer 10Vrms, 60 Hz 10V

Thermocouple 10Vrms, 60 Hz 10V

D’Arsonval with half-wave rectifier

10Vrms, 60 Hz 4.5V

D’Arsonval with full-wave rectifier

10Vrms, 60 Hz 9.0V

Iron vane 10Vdc 10V

Electrodynamometer 10Vdc 10V

Thermocouple 10Vdc 10V

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NEXT LECTURE

DC & AC BRIDGE: Part 1 (DC bridge)

• The Wheatstone bridge• The Kelvin bridge