Question

What are the methods to measure the resistance of a resistor? Explain​

Answer 1

Answer:

Electrical resistance is measured by either of two methods: constant current or constant voltage.

The constant current technique sources a known current through an unknown resistance and the resulting voltage is measured. This technique is generally used for resistance values below 200M ohms. This approach is used most often by Digital Multimeters and other resistance measuring instrumentation. Examples of just some of the Keithley products which use this technique are 2000, 2001, 2002, 2010 and 2700.

The constant voltage technique sources a known voltage across an unknown resistance and measures the resulting current. This approach is used for high resistance (1e8 to 1e16) measurement applications. Resistance of this magnitude is normally a measurement of leakage, such as capacitor leakage, insulator resistance or relay contact isolation. The advantage of the constant voltage method lies in measuring the unknown resistance with various constant values of test voltage. This helps characterize the resistance with a parameter known as voltage coefficient, which is a measure of how the resistance reacts to various magnitudes of test voltage. Examples of just some of the Keithley products which can measure resistance with this method are Electrometers and SourceMeter Instruments.

Answer 2

Answer:

Electrical resistance is measured by either of two methods: constant current or constant voltage.

The constant current technique sources a known current through an unknown resistance and the resulting voltage is measured.

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.

$\blue {Measurement\: of \:Low \:Resistance (<1Ω)}$

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.

The current is flown through current terminals C1 and C2 while the potential drop is measured across potential terminals V1 and V2. Hence we can find out the value of resistance under experiment in terms of V and I as indicated in the above figure. This method helps us to exclude the contact resistance due to current terminals and though contact resistance of potential terminals still comes into picture, it is very small fraction of high resistance potential circuit and hence induces negligible error.

The methods employed for measurement of low resistances are:-

$\sf \purple \implies \blue {Kelvin’s \:Double\: Bridge\: Method}$

$\sf \green \implies \red {Potentiometer \:Method}$

$\sf \orange \implies \pink{Ducter\: Ohmmeter.}$

$\sf \red :\implies \orange {Kelvin’s \:Double\: \:Bridge}$

Kelvin’s double bridge is a modification of simple Wheatstone bridge. Figure below shows the circuit diagram of Kelvin’s double bridge.

It is an electromechanical instrument used for measurement of low resistances. It comprises of a permanent magnet similar to that of a PMMC instrument and two coils in between the magnetic field created by the poles of the magnet. The two coils are at right angles to each other and are free to rotate about the common axis. Figure below shows a Ducter Ohmmeter and the connections required to measure an unknown resistance R.

One of the coil called current coil, is connected to current terminals C1 and C2, while the other coil called, voltage coil is connected to potential terminals V1 and V2. Voltage coil carries current proportional of the voltage drop across R and so is its torque produced. Current coil carries current proportional to the current flowing through R and so is its torque too. Both the torque acts in opposite direction and the indicator come to halt when the two are equal. This instrument is useful for resistance in range 100µΩ to 5Ω.

Measurement of Medium Resistance (1Ω – 100kΩ)

Following are the methods employed for measuring a resistance whose value is in the range 1Ω – 100kΩ –

$\sf \purple \implies \pink {Ammeter-Voltmeter\: Method}$

$\sf \green \implies \blue{Wheatstone \:Bridge \:Method}$

$\sf \orange \implies \red{Substitution\: Method}$

$\sf \pink \implies \purple {Carey- Foster \:Bridge\: Method}$

$\sf \blue \implies \green{Ohmmeter \:Method}$

$\sf \red \implies \orange {Ammeter \:Voltmeter\: Method}$

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

$\sf \green \implies \pink {R = \frac{V}{I}}$