Module – 3:
Introduction of DC and AC Bridges:
Wheatstone Bridge, Kelvin Double Bridge, Maxwell’s Bridge, and Hay’s Bridge.
Sources of errors in Bridges and their elimination by shielding and grounding
Module – 3:
1.
State and obtain the balance conditions for ac bridges.
2.
In what respects an ac bridge differs from a dc bridge? What is bridge
sensitivity?
3.
Explain Maxwell’s bridge for the measurement of the unknown inductance
of a coil. Mention its suitability and advantages.
4.
Explain why:
(a) Maxwell’s bridge is not suitable for high Q-coils?
(b) Maxwell’s bridge is not suitable for low Q-coil?
5.
The arms of an ac Maxwell’s bridge are arranged as followsAB is a non-reactive resistor of 1000Ω in parallel with a condenser of
capacitance 0.5μF;
BC is a non-reactive resistor of 600Ω;
CD is an unknown inductor, and
DA is a non-reactive resistor of 400Ω
If balance is obtained under thee conditions find the values of resistor
& inductance in CD. What is the value of Q at 1000Hz.
(Ans, 240Ω, 0.124,
3.14)
6.
In inductance comparison bridge measurement range can be extanded by
slight modification-Explain how?
7.
Maxwell bridge t balance has C1=.01μF, R1=470KΩ, R2=100KΩ,
R3=5.1KΩ. Find series equivalent resistance and inductor.
(Ans. Qx=1.09K, Lx=5.1H)
8.
Impedance of an ac bridge are as followsZ1=100Ω<800(Inductive Impedance)
Z2=250Ω<00(Pure resistance)
Z3=400Ω<300(Inductive Impedance)
Z4=unknown
Determine the impedance of the unknown arm.
9.
An ac bridge is in balance with following constants:
Arm AB=450 Ω, arm BC=300 Ω in series with
L=15.9mH. The source frequency is 1KHz. Find the constants of arm CD.
10.
Prove that for a high Q-Coil.
Lx =R2 R3 C1
11.
Obtain the operation of Hay’s bridge for the measurement of inductance of
a coil. Explain its stability. Explain why Hay’s bridge becomes independent
of applied frequency.
12.
The four arms of Hay’s bridge are arranged as follows
AB contains a coil of unknown inductance;
BC is a non-reactive resistor of 1000 Ω;
CD is a non-reactive resistor of 883 Ω in series with a standard capacitor
of 1.38 Ω;
DA is a non-reactive resistor of 16800 Ω.
If the applied freq. is 50C/S and bridge is balanced, determine Lx & Rx
after coil.
(Ans. 20.4H, 2440 Ω)
13.
Hay’s bridge is fed from a source of 500Hz .Balance is obtained with
C1=0.35μF, R1=64.5 Ω, R2=2410 Ω, R3=750 Ω Series resistance of
capacitor is 0.4 Ω. Calculate R and L after choke connected in the
unknown arm.
14.
Describe how you can measure an unknown capacitance with an ac
bridge. Describe its writs.
15.
Describe an ac Schering bridge. How will you measure the following with
such a bridge:
1) unknown capacitance
2) leakage resistance of the capacitor
3) power factor
4) dissipation factor and loss angle
16.
In a Schering bridge a sheet of bakelite 4.5 mm thick is tested at 50 HZ.
The bridge employs a standard air capacitor C3 of 106 pF, a nonconductive resistance of 1000/π ohm in parallel with a variable
capacitor and a non-reactive variable resistor. Balance is obtained with
C1 = 0.5μ F
C2 = 260 Ohm
Calculate the capacitance, power factor and relative permittivity of sheet.
[Cx = 130 pF, rx = 1.23 x 108 Ohm, pf = 0.05, €r
= 5.9]
16.
A condenser bushing forms arm AB of a Schering bridge and a standard
capacitor of 500 pF and a negligible loss forms arm AD. Arms be
consists of 300 Ohm resistance. When the bridge is with a cap of
0.148 μF, The supply freq. is 50 HZ. Calculate capacitance and
dielectric loss of capacitor.
18.
Describe the operation of Wein bridge. Obtain an expression for applied
freq. Show that in such a bridge.
F
=
1/2πR
Explain balancing problems C
in a Wein bridge.
19.
An ac bridge has the following constant :
Arm BC=1000 ohm in parallel with C= 0.159 μF,
arm BC = 1000 ohm, CD = 500 Ω, DA = 0.636 μF is series with an
unknown resist. Find the freq. for which this bridge is in balance and
determine the value of unknown resistance in arm DA to produce this
balance.
20.
Explain shielding and grounding of bridges and Wagner ground. Explain
sources of errors in bridge circuit.