WHAT IS KELVIN CONNECTION?
Comprehensive Guide to Kelvin Connections: High-Accuracy Low Resistance Measurements
Introduction to Kelvin Connections and Precision Measurements
Kelvin connections, also known as "four-terminal sensing" or "Kelvin Sensing," are essential techniques in precision electrical and electronics measurements. This method is specifically designed to enhance the accuracy of resistance measurements, particularly when dealing with low-resistance values.
In this article, we will explore the fundamentals of Kelvin connections, their benefits, and practical applications in various measurement setups.
This technique plays a crucial role in minimizing errors associated with lead and contact resistances, ensuring high-precision measurements.
Understanding Two-Wire Connections
In a traditional two-wire measurement setup, the test current flows through the same leads used to measure the voltage drop. This configuration introduces inaccuracies due to the resistance of the test leads and contact resistances.
Limitations of Two-Wire Method: When measuring low resistance values (typically 1Ω or less), the lead resistance causes a significant voltage drop in addition to the component's voltage drop, resulting in inaccurate readings.
Example: Measuring a 0.1Ω resistor with a 1A current source:
-
- UUT voltage drop = 0.1V
- Combined lead + contact resistance drop = 0.1V
- DMM sees a total voltage drop of 0.2V
- Calculated resistance = 0.2Ω (twice the actual value)
As the resistance of the Unit Under Test (UUT) increases, the impact of lead and contact resistances decreases. However, these error sources are always present and can affect accuracy.
Advantages of Four-Wire (Kelvin) Connections
The four-wire method, or Kelvin connection, overcomes the limitations of the two-wire approach by using separate leads for current and voltage measurements.
How It Works:
- Current leads force a constant current through the UUT.
- Sense leads measure the voltage drop directly across the UUT, eliminating the effect of lead resistances on the voltage measurement.
- Minimal current flows through the sense leads, so their resistance and contact resistance do not affect accuracy.
Example: Measuring a 0.1Ω resistor with a 1A current source:
- UUT voltage drop = 0.1V
- The sense path has negligible current; hence, no additional voltage drop.
- DMM sees only the voltage drop across the UUT, resulting in an accurate reading of 0.1Ω.
This method is highly effective for low-resistance measurements, providing precision unmatched by the two-wire technique.
Semi-Kelvin Connections
In practice, many testing setups are semi-Kelvin connections, where the Kelvin configuration is approximated using simpler spring probes or similar methods. Many testing setups use semi-Kelvin connections, which approximate the true Kelvin method using spring probes or other simplified configurations.
These probes may provide some improvement over the traditional two-wire method by reducing the lead resistance effects, however, they do not fully eliminate contact resistance. Semi-Kelvin connections are often acceptable if contact resistance is low enough and does not significantly impact measurement accuracy.
It can be seen that the spring probe does not provide a true Kelvin connection, as the four wires are terminated at the probe receptacle, not at the point of contact to the UUT.This will remove the effect of the wire resistance, but not remove any contact resistance.
If the contact resistance is low enough this may be an acceptable compromise.
Additional factors such as physical placement, pin separation and topology may make a semi kelvin solution acceptable for use. In order to be 'true' Kelvin, each 'power' and 'sense' lead needs to make the connection directly to the test component lead and as close as possible to the test component itself.
Benefits: Semi-Kelvin setups can reduce lead resistance effects but may still suffer from contact resistance issues. They are suitable when contact resistance is low enough to not significantly affect measurement accuracy.
Limitations: The four wires terminate at the probe receptacle rather than directly at the UUT, which means contact resistance is not entirely eliminated.
True Kelvin Connections for High-Precision Measurements
For resistances typically less than 1Ω, true Kelvin connections offer the most precise measurements. Each power and sense lead must be connected directly to the component under test to minimize errors from both lead and contact resistances.
Practical Considerations:
- Spring probes may offer easier construction and longer lifespan but introduce voltage drops due to current flow through the probe.
- True Kelvin connections are preferred for high-precision applications but may require careful design to ensure mechanical reliability.
Enhancing Accuracy with LCR Meters and AT Testers
Most LCR meters (and the Voltech AT testers) allow you to perform Short and Open compensation to further remove the effect of leads on a measurement. It would appear at first that such compensations would remove the effect of Lead and contact resistance for you.
However, it is important to realize that contact resistance can vary widely between EACH test unit connection.
This would in reality, be different and unrepeatable, and of the order of 20 mOhms to 150 mOhms between each separate fit of a component even with seemingly “good” contacts
4 Wire / Semi Kelvin
The problem with any fixed “one time” Short compensation is that it will only remove the contact/lead resistances seen at time of compensation. As this will change with every subsequent UUT insertion it will only ever remove a fixed offset from your real measurements.
4 Wire / True Kelvin
As explained above, the contact resistance in the Sense line changes on each insertion
In True Kelvin, however, the zero current flow in the Sense path means that, regardless of the instantaneous level of constant resistance, the associated contact resistance voltage drop, will never be seen by the high impedance voltmeter.
Conclusion: Why Kelvin Connections Matter
Key Advantages of Kelvin Connections:
High Accuracy: Eliminates lead and contact resistances, ensuring accurate low-resistance measurements.
Essential for Low-Resistance Testing: Critical for applications where measuring resistances below 1Ω is required.
Improved Repeatability: Provides consistent results, which are crucial in quality control and manufacturing processes.
Whether using true Kelvin connections or semi-Kelvin methods, understanding this technique is vital for achieving precision in electrical engineering and quality control testing.
By leveraging Kelvin connections, especially with advanced equipment like the AT5600, you can significantly enhance the accuracy of your electrical measurements, making it an indispensable technique for professionals in the field of precision electronics and electrical testing.
Kelvin Connections in the AT5600: Enhancing Measurement Accuracy
The Voltech AT5600 is a highly versatile automated transformer tester designed to deliver precise electrical measurements, especially for applications requiring low-resistance testing. One of the key features that sets the AT5600 apart is its support for Kelvin connections, ensuring high-accuracy results in critical testing scenarios.
How Kelvin Connections Improve AT5600 Testing
The AT5600 utilizes four-wire Kelvin connections to measure the electrical characteristics of components, particularly those with low resistances. By separating the current-carrying leads from the voltage-sensing leads, the AT5600 can accurately measure resistances as low as a few milliohms, which is essential for quality control in manufacturing and R&D environments.
- Eliminates Lead and Contact Resistance: The four-wire method eliminates the influence of lead and contact resistances on the measurement, allowing the AT5600 to capture the true resistance of the Unit Under Test (UUT). This is particularly crucial when testing transformers, inductors, and other components where precise resistance values are required.
- Enhanced Accuracy for Low Resistance Components: The AT5600 is specifically designed to handle low-resistance measurements with precision, making it ideal for applications like transformer winding resistance testing, where values can be as low as a few milliohms. Using Kelvin connections, the AT5600 achieves highly accurate and repeatable results, ensuring compliance with industry standards.