Measuring Instruments

Chapter - 4 Questions

Chapter 4 : Ohmmeters

Complete the following :-

1- The ohmmeter consists of : a ) ...... b ) ...... c ) ...... d ) ......

2- In series type ohmmeter , oo corresponds to ...... deflection .

3- The ohmmeter is divided into ...... types .

4- In shunt type ohmmeter the 0 [ zero resistance value ] corresponds to...... deflection

5- The ohmmeter has ...... control resistance connected either in series or shunt with meter .

6- Th ohm’s scale for the shunt type ohmmeter is …………………………… .

0cc767e14695328692b2ab030efe0875.doc 1 Switching section Chapter - 4 Questions

Show True or False : -

1- The ohmmeter has zero adjust control resistance connected either series or parallel with meter . ( )

2- The error in ohmmeter measuring due to battery aging . ( )

3- The accuracy of shunt OHMS ADJUST control is less than the Series OHMS ADJUST control in the series type ohmmeter. ( )

4- The ohms scale for the shunt type ohmmeter is backwards . ( )

5- The ohms scale for the series type ohmmeter is backwards . ( )

6- The zero value for the series type ohmmeter located at the left of the scale .( )

7- To get good accuracy of resistance measurements select the scale that cause deflection of pointer at upscale. ( )

Choose the correct answer :-

1 ) The scale of ohmmeter is : a ) linear scale . b ) log scale . c ) non-linear .

2 ) To get good accuracy for resistance measurement : a ) Select the lower scale . b ) Select the scale that cause deflection of pointer at mid scale position c ) Select the higher scale .

3 ) To measure resistance : a ) First short circuit the leads and adjust zero . b ) Open circuit and connect the Rx . c ) Select the suitable scale . d ) All the above .

4 ) When the deflection “ D “ of the series ohmmeter is equal to 62.5 % this means : a ) Rx = 0 b ) Rx = RT c ) Rx = 0.6 RT d ) None of the above.

Solve The following Problems

1 ) The following series type ohmmeter circuit shown in figure is used to measure the value of RX ( IFS = 50 µA and Rm = 2 K Ω , R1 = 10 K Ω ) Find: a ) The value of R2 required to zero the ohmmeter .

b ) What the ohmmeter will read ( measure ) if Rx = 15 K Ω ( i.e. Deflection “ D “ )

c ) Repeat ( a ) and ( b ) if the battery aging down to 1.2 volt .

2 ) a ) Complete the following table which represents the calibration of the series type ohmmeter face scale .

b ) Draw the face scale of ohmmeter ,

c ) Repeat step ( b ) if RT = 10 K ohm

D % RX = ( 1 – D / D ) / RT

3 ) a ) Complete the following table which represents the calibration of the shunt type ohmmeter face scale .

b ) Draw the face scale of ohmmeter ,

c ) Repeat step ( b ) if RP = 10 K ohm

D % RX = ( D / 1 – D ) / RP

4 ) a) The figure shows a circuit of an Ohmmeter .

Complete the following:

1) The circuit is a ………………… - type Ohmmeter.

2) The adjust control in this circuit is …………….. zero ohm adjust.

3) The function of R2 is to compensate the battery …………… b) For a new battery V = 1.5 Volt, the value of R2 required to zero the Ohmmeter is = 6 K Ω.

1) What is the total resistance ( RT ) ?

2) What the Ohmmeter will read if Rx = 10 K Ω ( Same as “4 “ ,but more simple and less of requirements)

5 ) A 50 μA, 2 K Ω movement is used in the circuit shown in figure.

Determine:

a) The value of RZ to zero the Ohmmeter.

b) The Ohmmeter reading if Rx = 15 K Ω

c) If the battery ages down to 1.2 V, i - What is the value of RZ required to zero the Ohmmeter ? ii - What will be the indication of the Ohmmeter ?

0cc767e14695328692b2ab030efe0875.doc 8 Switching section