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Week 5 “Low, High & Precise Resistance Measurements”
a) Voltmeter & Ammeter Methods
b) Wheatstone Bridge
c) Low-Resistance Measurements
d) High-Resistance Measurements
e) Low-Resistance Measuring Instruments
f) High-Resistance Measuring Instruments
Measurement Of Resistance
Resistance Is One Of The Most Basic Elements Encountered In Electrical And Electronics Engineering. The Value Of Resistance In Engineering Varies From Very Small Value Like, Resistance Of A Transformer Winding, To Very High Values Like, Insulation Resistance Of That Same Transformer Winding. Although A Multimeter Works Quite Well If We Need A Rough Value Of Resistance, But For Accurate Values And That Too At Very Low And Very High Values We Need Specific Methods. In This Article We Will Discuss Various Methods Of Resistance Measurement. For This Purpose We Categories The Resistance Into Three Classes-
Measurement Of Low Resistance (<1Ω) The Major Problem In Measurement Of Low Resistance Values Is The Contact Resistance Or Lead Resistance Of The Measuring Instruments, Though Being Small In Value Is Comparable To The Resistance Being Measured And Hence Causes Serious Error. Thus To Eliminate This Issue Small Valued Resistance Are Constructed With Four Terminals. Two Terminals Are Current Terminals And Other Two Are Potential Terminals. Figure Below Shows The Construction Of Low Resistance.
The Contact Resistance Between The Unknown Resistance And The Standard Resistance, Whose Effect We Need To Eliminate. For Measurement We Make The Ratio P/Q Equal To P/Q And Hence A Balanced Wheatstone Bridge Is Formed Leading To Null Deflection In The Galvanometer. Hence For A Balanced Bridge We Can Write
Hence We See That By Using Balanced Double Arms We Can Eliminate The Contact Resistance Completely And Hence Error Due To It. To Eliminate Another Error Caused Due To Thermo- Electric Emf, We Take Another Reading With Battery Connection Reversed And Finally Take Average Of The Two Readings. This Bridge Is Useful For Resistances In Range Of 0.1μΩ To 1.0 Ω. Measurement Of Medium Resistance (1Ω – 100kΩ) Following Are The Methods Employed For Measuring A Resistance Whose Value Is In The Range 1Ω – 100kΩ – Ammeter-Voltmeter Method Wheatstone Bridge Method
Ammeter Voltmeter Method
This Is The Most Crude And Simplest Method Of Measuring Resistance. It Uses One Ammeter To Measure Current, I And One Voltmeter To Measure Voltage, V And We Get The Value Of Resistance As Now We Can Have Two Possible Connections Of Ammeter And Voltmeter, Shown In The Figure Below.
A Bridge Circuit Always Works On The Principle Of Null Detection, I.E. We Vary A Parameter Until The Detector Shows Zero And Then Use A Mathematical Relation To Determine The Unknown In Terms Of Varying Parameter And Other Constants. Here Also The Standard Resistance, S Is Varied In Order To Obtain Null Deflection In The Galvanometer. This Null Deflection Implies No Current From Point C To D, Which Implies That Potential Of Point C And D Is Same. Hence
Measurement Of High Resistance (>100kΩ)
Megger
Definition : The Megger Is The Instrument Uses For Measuring The Resistance Of The Insulation. It Works On The Principle Of Comparison, I.E., The Resistance Of The Insulation Is Compared With The Known Value Of Resistance. If The Resistance Of The Insulation Is High, The Pointer Of The Moving Coil Deflects Towards The Infinity, And If It Is Low, Then The Pointer Indicates Zero Resistance. The Accuracy Of The Megger Is High As Compared To Other Instruments. Construction Of Megger The Construction Of The Megger Is Shown In The Figure Below. The Megger Has One Current Coil And The Two Voltage Coils V 1 And V 2. The Voltage Coil V 1 Is Passed Over The Magnet Connected To The Generator. When The Pointer Of The Pmmc Instrument Deflects Towards Infinity, It Means That The Voltage Coil Remains In The Weak Magnetic Field And Thus Experienced The Very Little Torque.
The Instrument Has Voltage Selector Switch Which Is Used For Selecting The Voltage Range Of The Instrument. The Voltage Range Is Controlled By Selecting The Varying Resistance R Connected In Series With The Current Coil. The Voltage Is Generated By Connecting The Hand Driven Generator. Working Of Megger The Testing Voltage Is Usually 500, 1000 Or 2500 V Which Is Generated By The Hand Driven Generator. The Generator Has Centrifugal Clutch Due To Which The Generator Supplied The Constant For The Insulation Test. The Constant Voltage Is Used For Testing The Insulation Having Low Resistance. The Megger Has Three Coils Two Pressure Coils And One Current Coil. The Pressure Coil Rotates The Moving Coil In The Anticlockwise Direction, Whereas The Current Coil Rotates It In The Clockwise Direction. When The Unknown Resistance Is Connected In The Circuit, The Pointer Of The Moving Coil Becomes Stable. The Pressure Coil And The Current Coil Balance The Pointer And Set It In The Middle Of The Scale. The Deflection Of The Pointer Is Directly Proportional To The Voltage Applied To The External Circuit. When The Testing Circuit Is Applied Across The Megger, And If There Is No Shorting Throughout The Insulation Then The Pointer Deflects Towards The Infinity. Which Shows That The Resistance Has High Insulation. For Low Resistance, The Pointer Moves Towards Zero.
