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Demystifying the Kelvin-Varley voltage divider: Why use 11 resistors instead of 1000?
The Kelvin-Varley voltage divider, an innovative electronic circuit invented by William Thomson, 1st Baron Kelvin, and Cromwell Fleetwood Varley, focused on Generates an output voltage that is precisely proportional to the input voltage and is resolvable for decades. In a sense, a Kelvin-Varley voltage divider is an electrically precise digital-to-analog converter that is widely used in calibration and metrology laboratories to provide resolution, accuracy, and linearity up to 0.1 ppm.
The basic structure of this circuit is similar to the traditional Kelvin voltage divider, but its uniqueness lies in the use of an "iterative" architecture to overcome some limitations. Unlike conventional voltage dividers that require 1,000 precision resistors for high resolution, the Kelvin-Varley divider only uses 11 resistors to provide ten times the resolution of each stage. This usage not only eliminates Cost and complexity are also significantly reduced.
“The ingenious design of the Kelvin-Varley voltage divider allows us to achieve extremely high voltage division accuracy without adding too many resistors.”
The basic design structure of this circuit is a multi-stage series connection, and each stage is composed of equal resistors. For example, if the resistor value of the first stage is assumed to be Ri, the design will ensure that each stage has 11 resistors. Compared with the design of a conventional voltage divider, this The design significantly reduces the number of resistors required and ensures voltage division accuracy.
Application of Kelvin-Varley voltage divider
Full-accuracy implementation of the Kelvin-Varley voltage divider relies on no output current flowing because the output effective source impedance changes with external conditions. Typically this voltage divider is applied with a zero voltage detector to compare its output voltage to a known voltage standard such as a Weston cell. Again, drawing current from known voltage sources must be avoided during this process.
"To achieve the highest accuracy in measurement, Kelvin-Varley voltage dividers require precision design and good environmental management."
The design structure after each stage will further improve its input impedance, keeping the entire system in an efficient operating state. The resistance value at each stage should be moderately reduced in subsequent designs to maintain overall work efficiency.
Precision adjustment
For high-precision applications, it is particularly important to ensure that any tens-digit resistor has the same resistance value. This means that resistors must be selected with tight tolerances and may need to be individually adjusted to achieve a consistent resistance value. This process can be accomplished with the help of a Wheatstone bridge circuit and sensitive zero detection equipment, which can quickly confirm the difference in resistance of the two resistors.
Error source
In the process of circuit design and implementation, there are also some potential error factors. For example, the temperature coefficient of a resistor is a key factor affecting voltage accuracy. Ideally, a resistor should have a constant resistance value, but in fact, changes in the external environment will cause changes in resistance value. The influence of temperature on the resistance of different materials varies greatly. Choosing the appropriate resistor is the basis for stable measurement.
“For high-precision measurements, we need to minimize errors due to temperature changes in design.”
In addition, the self-heating phenomenon of the resistor under the influence of power will further affect the accuracy of the measurement. When performing high-voltage tests, special attention needs to be paid to the impact of self-heating on measurement accuracy. For a typical Kelvin-Varley voltage divider, the uppermost resistor will withstand 10W of power up to 1000V, which is a major challenge as the temperature rises.
Finally, thermal electromotive force is also an invisible factor that affects measurement. Different metal joints may produce weak voltage differences at different temperatures. This is also one of the sources of errors that need to be considered in high-precision measurements.
In short, the Kelvin-Varley voltage divider plays an important role in metrology with its unique structural design and high-precision advantages. Is there more unknown application potential hidden behind this design?< /p>
