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' .

: ..

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DC M TER 3.1The P?HMC galvanometer constitutes the basic movement of a de ammeter.Since the coil winding of a basic movement is small and light, it can carry onlyvery smaLL curtt ll tS. When large currents are to be measured, it is necessary tobypass a major part of the current through a resistance called a shunt, as shownio Fig. 3. I. The resistance of shunt can be calculated using conventional circuitanalysis.

Referring. to Fig. 3. IR. = h temal resistance of the movement.11, ' shunt current

I l , "'' full scale cleflcc titm current of the movementl = ful i scale current of the ammeter+ shunt (i.e. total current) -

+

____________ ___________Jfig. 3.1 Basic de Ammeter

OArsonvalMovement

Since the sh tmt resistance is in parallel with the meter movement, the voltagedrop across the shunt and movement must be the same.Therefore Vsh.= V

lsi Rsh 1 R. ..But / s11 = 1- ,,hence R = ' R.h 1-- fmFor each required value of fu ll scale meter cu rrent, we can determine the va lueof ;hunt resistance.

I I

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Ammeters 65

Example 3J a) A I mA meter movement wilft m imem l resistance of100 Q is ro be converted into a 0 _ I00 mA. Calculate the value o fshuntresistance required.

Solution GivenR,= IOOQ,l, = 1 rnA ]= lOOmAR = l,Rm =1mAxi00 2 _ IOOn1Afl_I00 2=l.OIQ

sh I I 99 rnA 99 rnA 99The shunt resistance used with a basic movement may consist of a length

of constant temperature resistance wire within the case of the instrument.Alternatively, there may be an external (manganin or constantan) shunt havingvery low resistance.

The general requirements of a shunt are as follows.I. The temperature coefficients of the shunt and instrument should be lowand nearly identical.2. The resistance of he shunt should not vary with time.3. It should carry the current without excessive temperature rise.4. It should have a low thermal emf.

Manganin is usually used as a shunt for de instruments, since it gives a lowvalue of hermal emf with copper.

Constantan is a useful material for ac circuits, since it s comparatively highthermal emf, being unidirectional , is ineffective on the these circuits.

Shunt for low current are enclosed in the meter casing, while for currentsabove 200 A, they are mounted separately.Examp le 3J b A 100 pA meter movement with an internal resisUJnce o f500 Q is to be used in a 0 - 100 mA Ammeter. Find the value oftlte requiredshunt.

Solution The shunt can also be determined by considering current I to be ntimes larger than Jm. This is called a multiplying factor and relates the totalcurrent and meter current.Therefore I = ImTherefore the equation for

R _ l , R' I.,R., l Rh - l m nl,. -1 ,. l,..(n-1) (n )Given:Step I:

Step 2:

1 = J O O ~ a n d R s o o nI 100 rnAn= =

1 1 0 0 ~R _ R, = 500Qsir ( - I 1000 - I

1000

500 =0.50Q999Copyr rtoo r1a or a

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66 Elearonic Instrumentation

MULTI RANGE AMMETERSThe current range of the de ammeter maybe further extended by a number of shunts, +selected by a range switch. Such a meteris called a multirange ammeter, shown inFig. 3.2.

The circuit has four shunts R1 R2 R3and R4 which can be placed in parallel withthe movement to give four different currentranges. Switch S is a multiposition switch,having low contact resistance and high

s

3.2

+t RmD'ArsonvalMovement

Fig 3.2 Multi range ammetercurrent carrying capacity, since its contacts arc in series with low resistanceshunts). Make before break type switch is used for range changing. This switchprotects the meter movement from being damaged without a shunt during rangechanging.

If we use an ordinary switch for range changing, the meter does not have anyshunt in parallel wh ile the range is being changed, and hence full current passesthrough the meter movement, damaging the movement. Hence a make beforebreak type switch is used The switch is so designed that when the switch positionis changed, it makes contact with the next tenninal range) before breakingcontact with the previous terminal. Therefore the meter movement is never leftunprotected. Multirange ammeters are used for ranges up to 50A . When using amultirange ammeter, first use the highest current range, then decrease the rangeuntil good upscale reading is obtained. The resistance used for the various rangesarc of very high precision values, hence the cost of he meter increases.

xample 3 2 A 1 mA meter movement having an intemal resistance of100 n is ~ ~ S e d to convert into a multirange ammeter having the range 0- 10mA 0-20 mA and 0-50 mA. Determine the value of the shunt resistancerequired.

Solution Given 1. = I rnA and R. = 100 QCase I: For the range 0 - I 0 rnAa R 1m RmIVeO sh l = 1 - 1 I rnA X 100 = 100 = l l l l Qm IOmA ImA 9

Case 2: For the range 0 - 20 mA1 R lmAxlOOGiven R . = ' m =h - 1 - 1 20 mA - I rnA

'Case 3: For the range 0 - 50 rnA100 =5 2Q19

GivenR =lmRm= ImAxiOO =100=2.041QshJ 1 - 1 50 rnA - I rnA 49m

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Ammeters 67

xample 3 Design a multirange ammeter with range of0- 7A, 5 A and10 A employing individual shunt in eachA D Arsonval movement with aninternal resistance of500 Q and afull scale deflection of10 mA is ava ilable.

SolutionGiven /m = 10 rnA and Rm =500 0Case 1 : For the range 0 - 1A, i.e, 1000 rnA

Given R = 1 ' R,, _ 10 mAx 500 =5000 =5.0S Os l 1 - 1 1000 rnA - I 0 rnA 990'Case 2 : For the range 0 - SA, i.e, 5000 rnA

G' R _lmRm_ IOmAxSOO _5000_ 10020IVen sh2 1 - /m -SOOOmA-IOmA - 4 9 9 0 - .

Case 3 : For the range 0 - IOA, i.e, I0000 rnAGivenR = l. R. -= IOmAxSOO = 5000

hJ I - . , 10000 rnA -10 rnA 99990 0.0500Hence the values of shunt resistances are 5.05 0 , 1.002 0 and 0.050 0 .

THE RYTON HUNT OR UNIER L HUNT 3.3The Aryton shunt eliminates the possibility of having the meter in the circuitwithout a shunt. This advantage is gained at the price of slightly higher overallresistance. Figure 3.3 shows a circuit of anAryton shunt ammeter. ln this circuit, whenthe switch is in position 1 , resistance Rais in parallel with the series combination 3of Rb, Rc and the meter movement. Hence s 12 Rb ~ _ Rmthe current through the shunt is more than ,, : :,;the current through the meter movement, L

1 -thereby protecting the mete r movementand reducing its sensitivity. If the switchis connected to position 2 , resistance

D ArsonvalMovement

R0 and Rb are together in parallel with theseries combination of Rc and the metermovement. Now the current through the

Fig. 3.3 Aryton shunt

meter is more than the current through the shunt resistance.l he switch is connected to position 3 R0 , Rband Rc are together in parallel

with the meter. Hence maximum current flows through the meter movement andvery little through the shunt. This increases the sensitivity.

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68 Electronic Instrumentation

xample 3.4 (a) Design an Aryton shunt Fig. 3.4) to provide an ammeterwith a current range of0 - 1 mA, 10 mA , 50 mA and 100 mA. A D Arsonvalmovement with an internal resistance of 00 Q and ull scale currentof50 p.Ais used.+ 1mA

SOmA1OOmA 1 mA

-

R.

R tR2

R1

+- DArsonvalMovement

Fig. 3,4(a) For Example 3.4(a)Solution Given m = 100n,1., =50 JlA.For0 - I rnA range

ForO-lOrnA

ForO - SOmA

ForO-IOOrnA99950 JlA R1) =50 JlA ( 100 + R2 + R3 + R4)

But R1 + R2 + R3 = 5.26 - R4 Substituting in Eq. 3.2, we have9950 lA (5.26- R4) = 50 JlA (I 00 + R4)

9950 lAx 5.26 - 99SO JlA x R4 = SOOO jl.A + SO JlA R4(9950 JlA x S.26 - 5000 J. .A) = 9950 JlA R4 + SO JlA R4

(3.1)

(3.2)

(3.4)

Therefore 9950 JlA X S.26 - SOOO JlAR4 = lOrnA 47377 JlA10 rnA = 4.737 n

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R4 =4.74 QIn Eq. 3.1, substituting for R4 we getR1+ R2 + R3 = 5.26 - 4.74 = 0.52

Substituting in Eq. 3.3, we have49950 J 1 A 0.52 - R3) = 50 JlA R3 + 4.74 + 100)

49950 JlA X 0.52- 49950 JlA X R3

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= 50 J I A X R3 + 50 J I A X 4. 74 + 50 J I A X I0049950 JlA x 0.52- 50 JlA x 4.74 = 49950 J I A X R3 +50 JlA X R3 + 5000 J I A

25974- 237) JlA = 50 mAX R3 + 5000 JlA

ButTherefore.From Eq. 3.4

25737 JlA = 50 rnA x R3 + 5000 J I AR = 25737 J.lA 5000 lA = 20737 .lAo SOmA SOmA

R3 = 0.4147 = 0.42 QR + R2= 0.52 - RJR1+R2 = 0.52-0.4147 = 0.10526

3.5)99950 J 1 A R 1) = 50 JlA x 100 + R2 +R3 + R4)

But R2 +R3 + R4 = 5.26-R1 from Eq. 3.1)Substituting in Eq. 3.499950 JlA x R1 = 50 JlAX 100 + 5 6 - R1)99950 JlA x R1 = 5000 JlA + 50 JlA x 5.26) - R1 x 50 JlA)

99950 JlA X R + 50 JlA X R = 5000 JlA + 50 JlA X 5.2699950 JlA +50 JlA) R1= 5000 J 1A +263 J 1A

100 rnA x R1 = 5263 JlA5263 J LAR1 = LOOmA = 0.05263

Therefore R1 = 0.05263 QFrom Eq. 3.5, we haveR2 = 0.10526 - R1= 0.10526 - 0.05263 = 0.05263 QHence the value of shunts areR1= 0.05263 Q R2 = 0.05263 QR3 = 0.4147U; R4 = 4.74Q

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70 Electronic Instrumentation

Example 34 b) Calculate the value _of the shunt resistors for the circuitshown below

Rm 1knr - - - - { f } - - - - .

1A10mA

Fg 3.4 b) For Example 3.4 b)Solution The total shunt resistance Rshis determined by

R = m where n = III,.sh (n - IGiven lm = 100 )lA and R,. = I 000 QStep I: For I0 rnA range:

n = _ = I 0 mA I00lm 100J 1A

Step 2: When the meter is set on the I 00 rnA range, the resistance R, and Rcprovides the shunt.The shunt can be found from the equationR. =(R +R )=lm Rm+R h)=100J.1A IO. I +1000) = l.OIQslrz b c l 100 nlA

Step 3: The resistor which provides the shunt resistance on the lA range can befound from the equation

Step 4: But Rb Rc = 1.01 Q

100 J lA 10.1+ 1000) = 0.101 QIOOOmA

R, = 1 01 -Rc = 1.01-0.101 Q =0.909 QStep 5: Resistor R0 is found by

Ra = Rs, - R, + Rc) = 10.1 - 0.909 + .101) Q= 10.1- 1 01 Q

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=9.09 1Hence a = 9.09 Q b 0.909 Q and Rc 0.101 Q

REQUIREMENTS OF SHUNT 3.4The type ofmaterial that should be used to join the shunts shouldl have two mainproperties.1. Minimum Thermo Dielectri Voltage DropSoldering of oint should not cause a voltage drop.2. SolderabilityResistance ofdifferent sizes and values must be soldered with minimum changein value.

The following precautions should be observed when using an ammeter formeasurement.I. Never connect an ammeter across a source of emf. Because of its low

resistance it would draw a high current and destroy the movement.Always connect an ammeter in series with a load capable of limiting thecurrent.2 Observe the correct polarity. Reverse polarity causes the meter to deflectagainst the mechanical stopper, which may damage the pointer.3 When using a multirange meter, first use the highest current range, thendecrease the current range until substantial deflection is obtained. Toincrease the accuracy use the range that will give a reading as near fullscale as possible.

EXTEND NG OF AMMETER RANGES 3.5'The range of an ammeter can be extended to measure high current values byusing external shunts connected to the basic meter movement (usually the lowestcurrent range), as given in Fig. 3.5.

Meter ettoLowesturrentRangeExternalShunt

TestLeads

Fig 3 5 Extending of ammetersNote that the range of he basic meter movement cannot be lowered. (For ex-

ample, if a I00 flA movement with I00 scale division is used to measure I ~ Athe meter will deflect by only one division. Hence ranges lower than the basicrange are not practically possible.)