8 example 21 calculate the value of the shunt

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8 Example 2.1 Calculate the value of the shunt resistance required to convert a 1mA meter movement, with a 100Ω internal resistance, into a 0-10mA ammeter Solution Cont’d… DC Ammeter 11 . 11 9 1 . 0 9 1 10 1 . 0 1 . 0 100 1 mA V R mA mA mA I I I V V V V mA R I V sh m sh m sh m m m
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12/07/18 Chapter 2 : DC Meter 9 The purpose of designing the shunt circuit is to allow us to measure a current I that is some number n times larger than I m . The number n is called a multiplying factor and relates total current and meter current as I = nI m Substituting this for I in equation 2.1: Cont’d… DC Ammeter ) ( 1 n R I nI I R R m m m m m sh
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12/07/18 Chapter 2 : DC Meter 10 Aryton shunt On a multirange ammeter, the Aryton shunt or the universal shunt is frequently a more suitable design. Advantages: It can eliminates the possibility of current overloading of the moving coil i.e they protect the deflection instrument of an ammeter from an excessive current flow. It may be used with a wide range of meter movements. The Aryton shunt is shown in Figure 2.3 Cont’d… DC Ammeter
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12/07/18 Chapter 2 : DC Meter 11 Cont’d… DC Ammeter Figure 2.3 Computing resistance values for the Aryton shunt
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12/07/18 Chapter 2 : DC Meter 12 Part A The individual resistance values of the shunts are calculated by starting with the most sensitive range, i.e. the most sensitive range is the I 1 =10mA range (Part A with R sh = R c +R b +R a ) and can be computed by: Cont’d… DC Ammeter or I R I R sh m m sh 1 where: I sh1 = I 1 -I m and m I I n 1 1 n R R m sh
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12/07/18 Chapter 2 : DC Meter 13 Part B Since the resistance R c +R b is in parallel with R m +R a , the voltage across each parallel branch should be equal and can be written as: V Rc+Rb = V Rm+Ra In current and resistance term: (R b +R c )I sh2 =I m (R a +R m ) (R b +R c )(I 2 -I m )=I m (R a +R m ) I 2 (R b +R c )-I m (R b +R c )=I m [R a +R m ] Cont’d… DC Ammeter ) ( ) ( 2 I R R I R R m sh m c b can determine R a as R a =R sh -(R b +R c ) (Ω)
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12/07/18 Chapter 2 : DC Meter 14 Part C This part is to determine the value of R c and having I 3 =1A as the maximum current range. R c can be written as: I sh3 R c =I m (R a + R b +R m ) (I 3 -I m ) R c = I m (R a + R b +R m ) The resistor R b can be computed as R b = (R b +R c )- R c (Ω) Cont’d… DC Ammeter ) ( ) ( 3 I R R I R m sh m c
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12/07/18 Chapter 2 : DC Meter 15 Example 2.2 Compute the value of the shunt resistor for the circuit as shown in Figure 2.3 Soln: Part A R sh = R c +R b +R a 1 . 10 1 100 1 1 100 100 10 1 k n R R A mA I I n m sh m Cont’d… DC Ammeter
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12/07/18 Chapter 2 : DC Meter 16 Part B R a =R sh -(R b +R c ) (Ω)=10.1 -1.01 =9.09 Part C The resistor R b can be computed as R b = (R b +R c )- R c (Ω)=1.01 - 0.101 =0.909 Cont’d… DC Ammeter 01 . 1 100 ) 1 1 . 10 ( 100 ) ( ) ( 2 mA k I R R I R R m sh m c b 101 . 0
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  • Fall '17
  • puan nor azilah binti ahmad
  • Resistor, Ohmmeter, Voltmeter, Ammeter

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