EKT112/4 Principles of Measurement and Instrumentation (Sem.2 2012/13)

Prepared by: Sabarina / Shahadah

Tutorial 5

(Bridge Measurements)

1. Calculate the value of unknown resistance, Rx in a Wheatstone bridge in Figure 1 if R1 = 20k,

R2 = 40k and R3 = 16k.

Figure 1 Wheatstone bridge

2. The Wheatstone bridge has supply voltage is 20 volts, R1 = R2 = R3 = 1 k. R4 is a thermistor which its resistance value is very sensitive to temperature and has positive temperature

coefficient. The temperature coefficient of thermistor is 5 /C. At 50C the resistance of the

thermistor is 1 k. Find the value of temperature if the Thevenin voltage of bridge is 50 mV.

3. An ac bridge is driven by a 2 kHz sinusoidal source. Is the bridge balanced if at this frequency

the bridge arms have the following impedances?

(a) Z1 = 4 + j4 ; Z2 = 1 + j1 ; Z3 = 4 ; Z4 = 1. (b) Z1 = 4 + j4 ; Z2 = 1 - j1 ; Z3 = 4 ; Z4 = 1.

4. An ac bridge is driven by a 2 kHz sinusoidal source. Given Z1 is 10 k, Z2 is 50 k, Z3 is series combination of a resistor of value 100 k and a capacitor of value 100 F. Z4 is series combination of an unknown resistor and unknown reactive element. Find the value of the

unknown reactive element.

5. Find the series inductance Ls resistance Rs and Q of an inductive element using Maxwell

Bridge. Given that Z1 is parallel combination of a resistor of value 470 and a capacitor value 0.22 F. Z2 and Z3 are 1 k. The bridge is driven by a 2 kHz sinusoidal source.

6. The Schering bridge (Figure 2) balances under the following conditions; R1=10 k, R2=1 k, C1=100pF, C3=400pF. The bridge is driven by a 1 kHz sinusoidal source.

(a) Find the values of Rx,and Cx and D.

(b) If the series combination of Rx,and Cx is to be replaced by a parallel combination Rp and Cp, find their values.

Figure 2 Schering bridge Figure 3 Wien bridge

7. Find the equivalent series resistance and capacitance Rx and Cx in a Wien bridge in Figure 3 at balance when f = 2.5 kHz, R4=100k, R2=25k, R3=12k, C3 = 20pF.