Kelvin-Varley voltage divider

In electrical circuit technology, the Kelvin-Varley voltage divider is a form of voltage divider which can be set in steps using switches and consists of resistor networks with fixed resistance values . The type of construction allows a repeatable setting of the divider values ​​of the voltage ratio, which is not possible when using potentiometers . Applications are in electrical measurement technology and in devices that electronically control the switches of the voltage divider. Commercially available Kelvin-Varley voltage dividers for manual use have a decadic division for the individual stages, a binary division is common for digitally controlled electronic measuring systems.

construction

Simplified circuit of a Kelvin-Varley decimal voltage divider with four decimal places

The Kelvin-Varley voltage divider is a cascaded form of individual voltage dividers, whereby each stage represents exactly one point of the voltage divider ratio and can thus be set independently of the other points in a reproducible manner using a step switch . The figure shows a Kelvin-Varley decimal voltage divider with four decimal places ( ) with a set voltage divider value ${\displaystyle n=4}$

${\displaystyle k={\frac {\mathrm {U_{OUT}} }{\mathrm {U_{IN}} }}=0{,}2073}$

shown. The divider value applies when the output is loaded with a high resistance and the current drawn is negligibly small compared to the cross-current in the voltage divider. ${\displaystyle \mathrm {U_{OUT}} }$

Each stage consists of several fixed resistors of the same value which have taps between the resistors that are used for switching. In the decadal version, the -th stage, apart from the last stage, consists of 11 resistors, the last stage has only 10 resistors. The ratio of the resistance values between two adjacent stages and with is in the decade version the value 5, i.e. This means that the resistance values ​​increase by the factor per step ${\displaystyle i}$ ${\displaystyle N}$ ${\displaystyle R_{i-1}}$ ${\displaystyle R_{i}}$ ${\displaystyle i=1\ldots n}$

${\displaystyle R_{i-1}={\frac {R_{i}}{N}}={\frac {R_{i}}{5}}}$

from. The input resistance has the value 100 kΩ in the adjacent circuit. ${\displaystyle R_{in}=10\cdot R_{n}}$

The difficulty in the circuit design is that all resistors of a stage must have the same value and the adjustment of 11 or 10 resistors to one value is impractical. Practically structured Kelvin-Varley voltage dividers therefore have additional adjustment elements in the form of individual potentiometers that can be adjusted for calibration purposes per stage and a topology adapted to them.

literature

• Rupert Patzelt, Hans W. Fürst (Ed.): Electrical measurement technology . Springer, Vienna 1993, ISBN 3-211-82442-1 .