Analysis Of Two And Four Lead Resistance Measurements
Analysis of two and four-lead resistance measurements
Johnny Greavu
University of Minnesota-Twin Cities
September 29, 2014
Introduction
We measured the resistance per foot of a wire using setups of both two and four test leads in tandem with a digital voltmeter (DVM) and compared their accuracy. While resistance can be determined by measuring voltage and current separately, it can also be measured more simply and directly using the resistance measuring feature available to some digital voltmeters, such as the Agilent 34401A we employ here. Plugging two test leads into the DVM and placing them in series with the device under test (DUT) can be sufficient, but only if the DUT has a resistance of at least an order of magnitude larger than the internal and contact resistances within the leads. If one forgets about these resistances, they may be led to believe that two leads are always sufficient for accurate resistance measurements. A four-lead setup adds two more test leads that divert current flow from the original two, therefore eliminating the voltage drops caused by the leads’ internal resistances and their contact resistances with the DUT. Here, both setups were tried and their accuracy and practicality compared. A two-foot long 22 American Wire Gauge copper wire’s resistance per foot was measured to be 49.2 m/ft (206% from the accepted value of 16.1/ft) and 16.6 m/f (3.11% from the accepted value) using the two-wire and four-wire techniques, respectively.
Theory
The resistance between two points and is given by Ohm’s law,
where is really , the difference in electric potential between the points (), and is the current. When analyzing complex circuits we often assume that wires and test leads have no inherent resistance, but in reality this isn’t the case. As will be demonstrated, the assumption fails when the resistance of the DUT isn’t significantly higher than the sum of the leads’, contact, and any other resistances around the circuit.
Kirchoff’s
Johnny Greavu
University of Minnesota-Twin Cities
September 29, 2014
Introduction
We measured the resistance per foot of a wire using setups of both two and four test leads in tandem with a digital voltmeter (DVM) and compared their accuracy. While resistance can be determined by measuring voltage and current separately, it can also be measured more simply and directly using the resistance measuring feature available to some digital voltmeters, such as the Agilent 34401A we employ here. Plugging two test leads into the DVM and placing them in series with the device under test (DUT) can be sufficient, but only if the DUT has a resistance of at least an order of magnitude larger than the internal and contact resistances within the leads. If one forgets about these resistances, they may be led to believe that two leads are always sufficient for accurate resistance measurements. A four-lead setup adds two more test leads that divert current flow from the original two, therefore eliminating the voltage drops caused by the leads’ internal resistances and their contact resistances with the DUT. Here, both setups were tried and their accuracy and practicality compared. A two-foot long 22 American Wire Gauge copper wire’s resistance per foot was measured to be 49.2 m/ft (206% from the accepted value of 16.1/ft) and 16.6 m/f (3.11% from the accepted value) using the two-wire and four-wire techniques, respectively.
Theory
The resistance between two points and is given by Ohm’s law,
where is really , the difference in electric potential between the points (), and is the current. When analyzing complex circuits we often assume that wires and test leads have no inherent resistance, but in reality this isn’t the case. As will be demonstrated, the assumption fails when the resistance of the DUT isn’t significantly higher than the sum of the leads’, contact, and any other resistances around the circuit.
Kirchoff’s