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 R

RRRx

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

RR

R

RR

REEcd

42

42

31

31

RR

RR

RR

RRRTH

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 RR

RRx

2

13 R

RRRx

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)

BA

Bll x

2

22

1

RX

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

R

RRR sx

2

1

R

RCC sx

1

2

R

RLL sx

1

2

R

RRR sx

• Inductance comparison bridge with extended measurement range

• With the switch in position 1 :

• With the switch in position 2 :

1

2

R

RrRR sx

rR

RRR sx

1

2