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## Advantages of 2 Wire Resistance Measurement | Disadvantages of Two Wire resistance Measurement

### What is Two Wire Resistance ?

SMUs (Source Measurement Units) and DMMs (Digital Multimeters) are used for two wire measurement. DMM usually performs resistance measurement using constant current method. In this method, resistance is calculated by ratio of measured voltage by source current.

Usually DMM has very high internal input impedance. Following table lists out test currents for various resistance ranges. This test current are taken for the resistance measurement by constant current method.

Resistance Range Test current
100 Ohm 1 mA
1 KOhm 1 mA
10 KOhm 100 µA
100 KOhm 10 µA
1 MOhm 1 µA
10 MOhm 0.1 µA
100 MOhm 0.1 µA

The figure-1 depicts two wire resistance measurement schematic. It uses constant current method for resistance measurement. Following labels are used in the 2-wire schematic.
VM = Voltage measured by meter
VR = Voltage across resistor
➨Measured resistance = VM/ I = R + (2 * RLEAD)

### Benefits or Advantages of 2 Wire Resistance Measurement

Following are the benefits or advantages of 2 Wire Resistance Measurement:
➨It is simple measurement technique compare to 4-wire.
➨It requires less number of lead resistors and only 2 wires for measurement.

### Issues or disadvantages of 2 Wire Resistance Measurement

➨ The main drawback or disadvantage or issue with 2 wire resistance is found with low resistance measurement. Here total lead resistance is added to measurement. As test current 'I' causes significant voltage drop across lead resistances, measured voltage and voltage across 'R' is not equal. Typical range of lead resistance is from 10mΩ to 1Ω. Hence it is difficult to measure accurate two wire resistance for RUT (Resistance Under Test) below 100Ω. Lead resistance used are dominant source of error in two wire resistance measurement method in addition to instrument error.
➨It is less accurate method compare to 4-wire resistance measurement method. Also refer 4-Wire resistance measurement and advantages of four wire resistance measurement technique.