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

• 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

1

23 RR

RRx

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 :

• The Thevenin resistance :

• The galvanometer current :

42

2

31

1

RRR

RRR

EEcd

42

42

31

31

RRRR

RRRR

RTH

gTH

THg RR

EI

Kelvin Bridge

• Wheatstone bridge circuit, showing resistance Ry of the lead from point m to point n

• Basic Kelvin double bridge circuit --------------------------

32

1 RRR

Rx

2

13 RR

RRx

Loop Tests with The Portable Test Set

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

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

• (b)

BABll x

2

22

1R

X

Guarded Wheatstone Bridge

• Used for high resistance measurements

General Form of The AC Bridge

3241 ZZZZ

3241

Comparison Bridges

• Capacitance comparison bridge• Equating the real terms :

• Equating the imaginary terms :

• Inductance comparison bridge

1

2

RR

RR sx

2

1

RR

CC sx

1

2

RR

LL sx

1

2

RR

RR sx

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

• With the switch in position 2 :

1

2

RR

rRR sx

rRR

RR sx 1

2