Temperature Compensation Arrangement

PointerScale

PointerScale

Swamping Resistors

Coil (Cu)

Swamping Resistors

Coil (Cu)

Temp. increases

R il increases

Swamping ResistorsR1 R2

(M ) (M ) Sh t R i t

Swamping ResistorsR1 R2

(M ) (M ) Sh t R i t Rcoil increases(Mn) (Mn)

(Cu)

R3

Shunt Resistor(Mn) (Mn)

(Cu)

R3

Shunt Resistor

(Cu)(Cu)

DC Ammeter

I

PMMC can be used as ammeter

Im

Rm

Range selection of R

II

Range selection of Rsh

Rsh =1−

=−

=nR

IIRI

IRI m

m

mm

sh

mmIm

Rm

Rsh

IshIm

Rm

Im

Rm

Rsh

Ish

msh

where n = I/Im.

Multirange Ammeter

ImRmR3R2R1

•I Im

R3•S 1A

•••S

10A5A

1A

RmR2

••

• 5A

10AI

Simple •

R1

Arrangement Ayrton Shunt Arrangement

Selection of resistances for different range

Ammeter Sensitivity yDetermined by the amount of current required

by the meter coil to produce full-scale deflectionby the meter coil to produce full scale deflection of the pointer

Ammeter LoadingRRRR

Im1

RE

RIm1

R

Im1

RE

R

E

R

IE

R

IRmRmRm

RIm1/I =

mRRR+

DC Voltmeter

RSRS

PMMC can be used as Voltmeter

Im

RDC Input, Vin

Im

R

Im

RDC Input, Vin

RmRmRm

Range selection - Rs depends on meter range

RS = min R

IV

−mI

Multirange VoltmeterRRRR R4R3R2R1

I10V Im

RS

•• •

•1V

10V50V100V

Rm

DC Input,

S

DC Input, Vin

Value of Resistances depends on Voltmeter range

Voltmeter Sensitivity (Ohms per volt rating)

Sensitivity S =VoltOhm

Sm RR += = I

1

inV fsdI

Voltmeter LoadingVoltmeter Loading

The insertion of voltmeter connection changeThe insertion of voltmeter connection changethe operation of circuit, is called Loading.

OhmmeterAn instrument used to measure resistance orchecking continuity

Type of ohmmeter:

1. Series Type

2 Shunt Type2. Shunt Type

Series Type Ohmmeteryp

Unknown resistance is connected in series

R1 A

with meter

R1

R

•A

I2R2

RmRx

EE•

I B

Series ohm meter DesignHalf Deflection Method (To calculate R1 and R2 )

Let Rh = Half deflection resistance.For unknown R = R I = I /2For unknown Rx= Rh , I = Ifsd/2.At half deflection:

RR

m

m

RRRR

R+

+2

21

E E

Rh =

Ih = ,2 hR

EItotal =

hRE

R1 A1

R2Rm Rx

•A

I2

E•

I BItotal

Let R = Half deflection resistance

Designing series ohm meterLet Rh = Half deflection resistance.For unknown resistance Rx = Rh ,I = Ifsd/2.Further, at half deflection

Rh =m

m

RRRR

R+

+2

21

Ih =

m2

hRE

2, Itotal =

hRE , I2 = Itotal - Ifsd and I2R2 = IfsdRm

hR2 hR

R2 =hmfsd

RIERRI

2hfsd RIE −

RRIR1 = Rh -

ERRI hmfsd

Shunt Type

Unknown resistance is connected in parallel with meter

R1•AR1•A

with meter

E

•ImIt Ish Ix

E

•ImIt Ish Ix

RmRx

E

BS

RshRmRx

E

BS

Rsh

•B•

B

Designing shunt ohm meterHalf Deflection Method (To calculate R1 and R2 )

Let Rh = Half deflection resistance.For R = R I = I /2For Rx= Rh , I = Ifsd/2.At half deflection

Ix = Ish + Im

R1 AR1 A

R RE

•Im

R h

It Ish Ix

R RE

•Im

R h

It Ish Ix

Rm Rx

•BS

RshRm Rx

•BS

Rsh

To calculate R1 and Rsh (Half Deflection Method)

Let Rh = Half deflection resistance.For this value of Rx, Im = Ifsd/2.Further, at half deflectionFurther, at half deflection

Ix = Ish + Im

Ix = m

RE mm

RRI

= xhR hR

− )( hRRI RR −Ish = =)(h

hmm R

RRI 2/)(h

hmfsd R

RRI

To calculate R and RTo calculate R1 and Rsh(Half Deflection Method)

Solving these equations, we get

sh

mm

IRI

Rsh = sh

It = Ish + Ix + Im = 2(Im + Ish) = 2Im(Rm/Rh)

R1 = (E – ImRm)/It

Ohmmeter Scale:

•Scale is logarithmic – expand at lower end andg pcompressed in higher range.

•Series type is used for relatively higher values ofresistances while shunt type is used for low values ofresistances.