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Rectifier type instruments

• These are used for ac measurements with the help of a rectifier that convert ac in to a dc.

• These are suited for measurements in communication circuits .

Types : 1. voltmeters

2. ammeters

3. multi meters

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Fig. 1

Rectifier type voltmeter

Bridge rectifier operation

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Construction

• Figure1 shows Rectifier type ac voltmeter

• According the construction R1,R2,&R3 connected in series through the switch.

• D1 is connected in series with the meter

• D2 is connected in shunt with the meter in series with the diode.

• Rsh is connected in shunt with the meter.

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Rectifier type voltmeter

Principle of operation • A PMMC movement is used in combination with

rectifier arrangement.• According to the Fig.1 Diode D1 conducts

during Positive half cycle • It causes meter to deflect average value of this

half cycle.• During the negative half cycle D2 conducts,

current through the measuring circuit,• It conducts in the opposite direction.

Rectifier Type Ammeter.

AC Input

Current Transformer

Full wave Bridge rectifier

PMMC Instrument

Basic block diagram of rectifier type ammeter for AC current measurement .

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Rectifier type Ammeter…

• Rectifier type instruments generally used a PMMC

movement in combination with some Rectifier

arrangement.

• Moving coil meter connected across the secondary of a

current transformer.

• The rectifier converts ac to dc and the dc current is

passed through ammeter for measurement.

operation

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Rectifier type Ammeter

• An ac ammeter must have a very low resistance because it is always connected in series with the circuit in which current is to be measured.

• The low resistance requirement means that the voltage drop across the ammeter must be very small, not greater than 100mv.

• The use of current Transformer gives ammeter a low terminal Resistance and low voltage drop.

• The Transformer steps up the input voltage (more secondary turns than primary turns) to provide sufficient voltage to operate the Rectifiers.

Rectifier type Ammeter

• The Transformer also used to steps down the primary current to a level suitable for measurement by a PMMC meter.

• As the Transformer is used in an Ammeter circuit the current transformation ratio Ip/Is=Ns/Np is very important.

Rectifier type Ammeter…

• Rectifier type Ammeter circuit with current Transformer, full wave rectifier and PMMC instrument.

Fig :1

Rectifier type Ammeter…

• A load resistor RL is connected across the secondary winding of the Transformer.

• This is selected to take the portion of secondary current not required by the meter.

• The range of the instrument can be changed by switching-in different valves of load resistance.

• Another method of range changing involves the use of additional terminals (or taps) on the primary winding to alter the number of primary turns, as shown in Fig 1

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Fig :2

In some instruments bridge rectifier is used in place of full wave rectifier.

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• Alternating current generally expressed in RMS values hence the Rectifier type Ammeter is calibrated in terms of RMS values.

Construction of Rectifier type Ammeter