How Four-Wire Kelvin Sensing Eliminates Lead Resistance
Four-wire Kelvin resistance sensing eliminates lead wire resistance by separating the current-carrying circuit from the voltage-measuring circuit. When measuring low-value resistors in the milliohm range, standard two-wire multimeters fail because the resistance of the test leads is often higher than the component being measured. By utilizing separate force leads to supply a known current and high-impedance sense leads to measure the exact voltage drop directly across the target component, no current flows through the sensing wires. This completely removes the voltage drop of the test leads from the calculation, ensuring highly accurate milliohm measurements.
The Problem with Two-Wire Resistance Measurement
In a standard two-wire resistance measurement, a multimeter uses a single pair of leads to both inject a test current and measure the resulting voltage drop. The meter calculates resistance using Ohm's Law (\(R = V / I\)).
The flaw in this method is that the test leads themselves have resistance, typically ranging from 0.1 to 1 ohm. Because the current flows through the leads and the resistor in series, the meter measures the total voltage drop across the entire loop. When testing a 10 kilohm resistor, a 0.5-ohm lead resistance is negligible. However, when measuring a 10 milliohm (0.01 ohm) current-sense resistor, that same 0.5-ohm lead resistance introduces a 5000% error, rendering the measurement useless.
How Separate Force and Sense Leads Work
Four-wire Kelvin sensing solves this problem by splitting the measurement system into two independent pairs of wires:
- Force Leads (Source): This pair of outer leads connects a precision current source to the resistor under test. They carry the main test current. Just like in a two-wire setup, these wires experience a voltage drop due to their inherent resistance, as well as the contact resistance at the test points.
- Sense Leads (Measure): This pair of inner leads connects a voltmeter directly across the leads of the target resistor, positioned just inside the force connections. They are responsible solely for measuring the voltage drop across the component.
The Role of High Input Impedance
The secret to why the sense leads do not introduce their own resistance error lies in the properties of the voltmeter. The voltmeter connected to the sense leads has an extremely high input impedance, typically in the megaohm or gigaohm range.
Because of this massive impedance, virtually zero current flows into the sense leads. According to Ohm's Law (\(V = I \times R\)), if the current (\(I\)) is zero, the voltage drop (\(V\)) across the sense leads and their contact points must also be zero, regardless of the wire's resistance (\(R\)).
With no voltage lost in the sense wires, the voltmeter reads the exact voltage present at the terminals of the milliohm resistor. The measurement instrument then takes this pure voltage reading and divides it by the known current supplied by the force leads, calculating the exact resistance of the component without any interference from the test setup.