PERTEMUAN 9

JEMBATAN DC&AC dan APLIKASINYA

PENDAHULUAN

• Sirkuit jembatan biasanya digunakan untuk mengukur nilai komponen, seperti resistansi, induktansi, kapasitansi dan parameter sirkuit lainnya • Keakurasiannya sangat tinggi

Wheatstone bridge

[ CONTENTS ]

• Photograph of the instrument • Simplified schematic of the bridge circuit----------------- 

• Operasi dasar

R x



R

3

R

2

R

1 • Measurement errors : – Found in the limiting errors of the three known resistors – Insufficient sensitivity of the null detector – Changes in resistance of the bridge arms due to the heating effect of the current through the resistors – Thermal emfs in the bridge circuit or the galvanometer circuit (when low value resistors are being measured) – Errors due to the resistance of leads and contacts exterior to the actual bridge circuit

Thevenin Equivalent Circuit • Wheatstone bridge configuration -------------------  • Thevenin resistance looking into terminals c and d -------  • Complete Thevenin circuit, with the galvanometer connected to terminals c and d

• The Thevenin, or open circuit voltage :

E cd



E

 

R

1

R

1 

R

3 

R

2

R

2 

R

4   • The Thevenin resistance :

R TH



R

1

R

1

R

3 

R

3 

R R

2 2 

R

4

R

4 • The galvanometer current :

I g



R TH E TH



R g

Kelvin Bridge

• Wheatstone bridge circuit, showing resistance

R y

of the lead from point m to point n

R x



R R

2 1

R

3 • Basic Kelvin double bridge circuit ------------------------- 

R x



R

3

R

1

R

2

Loop Tests with The Portable Test Set

• Murray-loop test, used for locating a ground fault (short circuit) ---------------------------- 

l x

 2

l A B



B

• • Varley-loop test: (a) no.1; (b) no. 2; (c) no.3, used to locate grounds, crosses, or short circuits in multiconductor cable

X

1 

R

2 2 (b)

Guarded Wheatstone Bridge

• Used for high resistance measurements

General Form of The AC Bridge

Z

1

Z

4 

Z

2

Z

3   1    4    2    3

Comparison Bridges

• • • Capacitance comparison bridge Equating the real terms :

R x



R s R

2

R

1 Equating the imaginary terms :

C x



C s R

1

R

2 • Inductance comparison bridge

L x



L s R

2

R

1

R x



R s R

2

R

1

• • • Inductance comparison bridge with extended measurement range With the switch in position 1 :

R x

 

R s



r



R

2

R

1 With the switch in position 2 :

R x



R s R

2

R

1 

r