AC Meters

continued

Warm-ups

Outlines

• d’Arsonval MM with Full Wave Rectification.

• Electro-Dynamometer Movement• Iron-vane meter movement.• Loading effects of AC Voltmeters.

d’Arsonval MM withFull Wave Rectification.

• Frequently, it is more desirable to use a full-wave rectifier in AC voltmeters because it shows higher sensitivity rating compared to ½ wave rectifier.

• The most frequently used circuit for full-wave rectification is the bridge-type rectifier, as shown in next figure.

d’Arsonval MM withFull Wave Rectification.

d’Arsonval MM withFull Wave Rectification.

D1 D2

D3

D4

Rs

Vin

Rm

IfsVout

Figure 2: Full-Wave Bridge Rectifier used in an AC voltmeter

d’Arsonval MM withFull Wave Rectification.

• Positive ½ cycle– Current flows from Vin trough D2, trough the MM

from positive to negative, then trough D3.

• Negative ½ cycle– Current flows from Vin trough D4, trough the MM

from positive to negative, then trough D1.

d’Arsonval MM withFull Wave Rectification.

• Even though the polarity of Vin changes in every ½ cycle, the direction of current that flows through MM does not change.

• Since current flows through MM on both ½ cycles, then the deflection of MM will be greater compared to ½ wave rectification.

d’Arsonval MM withFull Wave Rectification.

D1 D2

D3

D4

Rs

Vin

Rm

IfsVout

Example 01.

For the circuit below, if given that Vin = 10Vrms, Rs= 10kΩ and Ifs = 1mA. Calculate: a) Vp

b) Vave

c) Sac

d) Sdc

d’Arsonval MM withFull Wave Rectification.

D1 D2

D3

D4

Rs

Vin

Rm

IfsVout

Example 02:

By using the same circuit, with Vin = 10Vrms, Rm=500Ω and Ifs = 1mA, calculate: a) Sdc

b) Sac

c) Rs

d’Arsonval MM withFull Wave Rectification.

D1 D2

D3

D4

Rs

Vin

Rm

IfsRsh

Example 03

For the circuit below, given that Vin = 10Vrms, Rm=500Ω, Ifs = 1mA, Rsh=500Ω, average forward resistance of 50Ω and infinite reverse resistance of each diode, calculate the following:a) the multiplier, Rs

b) the ac sensitivity, Sac

c) the dc sensitivity Sdc.

The Electro-Dynamometer

Movement• It is one of the most fundamental meters

that is widely used today.• It is a current –sensitive device: similar to

the D’Arsonval MM, which uses essentially the same principle of movement.

• Remember that the D’Arsonval movement is a DC device and can only measure DC current or AC current rectified to DC.

The Electro-Dynamometer

Movement

The Electro-Dynamometer

Movement

The Electro-Dynamometer

Movement• The Electrodynamometer movement has

the same basic operating principle as the D’Arsonval meter movement, except that the permanent magnet is replaced by fixed coils.

• The moving coil and pointer, which are attached to the coil, are suspended between and connected in series with the two field coils.

The Electro-Dynamometer

Movement• The two field coils and moving coil are

connected in series such that the same current flows through each coil.

• Current flow through the three coils in either direction causes a magnetic field to be produced between the field coils.

• The same current flow through the moving coil causes it to act as a magnet exerting a force against the spring.

The Electro-Dynamometer

Movement• If the current is reversed, the field polarity and the

polarity of the moving coil reverse, and the force continues in the same direction.

• Due to this characteristic of the Electro-dynamometer movement, it can be used in both AC and DC systems to measure current.

• Some voltmeters and ammeters use the electrodynamometer.

• However, its most important use is in the wattmeter.

The Electro-Dynamometer

Movement• The basic EDMM is capable of handling

much more current than a d’Arsonval could handle.

• However, it has much less Sensitivity rating compared to d’Arsonval MM, which ranges about 20Ω/V up to only 100Ω/V.

The Electro-Dynamometer

Movement

Example 04

An EDMM that has a full scale deflection current rating of 10mA is to be used in a voltmeter circuit. Calculate the value of the multiplier for a 10-V range if Rm = 50Ω.

The Electro-Dynamometer

Movement

Example 05

An EDMM with a full scale deflection rating of 10mA is to be used as a 1-mA ammeter. If the resistance of the moving coil is 40Ω, what is the value of the shunt resistor, Rsh?

Loading Effects of AC Voltmeters

• As already being discussed, the sensitivity of AC Voltmeters, using either ½ wave or Full wave rectification, is always less than the sensitivity of the DC Voltmeters.

• Therefore, the loading effect of an AC Voltmeter is always greater than that of a DC Voltmeter.

Loading Effects of AC Voltmeters

Example 06a) Determine the reading obtained with a

DC voltmeter at RB when the switch is set at point A.

b) Determine the reading at the same RB using ½ wave and Full wave rectifier AC meter respectively when the switch is set at point B.

Given that Ifs = 100-A and set at 10-V dc or rms range.

RB

E = 20V

RA 10kΩ

5.5kΩ

AC20Vrms

A B

Conclusions

Summary

Q&A