Electricity: power supply or regulation systems – External or operator controlled – Using an impedance as the final control device
Patent
1996-11-18
1999-02-02
Sterrett, Jeffrey
Electricity: power supply or regulation systems
External or operator controlled
Using an impedance as the final control device
G05F 308
Patent
active
058670181
DESCRIPTION:
BRIEF SUMMARY
RELATED APPLICATIONS
This application claims the priority of New Zealand Patent Application No. 260012, filed Mar. 2, 1994; New Zealand Patent Application No. 270054, filed Dec. 2, 1994 and International Patent Application No. PCT/NZ95/00022, filed Mar. 2, 1995, which are incorporated herein by reference.
FIELD OF THE INVENTION
This invention comprises resistance network which provides a range of accurate resistance values, as may be used for the evaluation of the accuracy and/or linearity of electrical resistance measuring instruments.
BACKGROUND
Resistance networks exhibiting useful properties are used in many aspects of electrical metrology for: the realisation of electrical measurement standards; the evaluation of those standards; and the evaluation of electrical measuring equipment. One such family of networks in common use is the 4-terminal series-parallel build-up resistors known as Hamon resistors. The basic design of the Hamon resistor was proposed by Hamon in 1954--see B V Hamon, "A 1-100 ohm build-up resistor for calibration of standard resistors", J Sci Instru, vol 31, pp 450-453, December 1954. Three of the four key principles that Hamon brought together in his resistor network were known for some time. The principles exploited by Hamon resistor are:
The Four-Terminal Resistor: Electrical resistances drawn in most engineering texts have two terminals. In such a resistor it is impossible to electrically discriminate between the resistor itself and the resistance in the two lead wires connected to it. This severely limits the attainable accuracy in many applications. For high accuracy work four-terminal resistors are used. These resistors have two terminals through which the current is passed, and two terminals where the voltage is measured. The ratio of the voltage to current, the resistance, is independent of all of the four lead resistances. This principle is probably due to Lord Kelvin.
Equal Resistors Connected in Series and Parallel to Realise Accurate Resistance Ratios: It was known that the resistance of N nominally equal value resistors in series is equal to N squared times the resistance of the N resistors in parallel. Further, if the values of the N resistors are adjusted to equality within a given tolerance then the accuracy of the ratio of the series to parallel resistances is accurate to the square of the tolerance. For example, 10 nominally equal 10 ohm resistors matched to within 1 part in 1000 (0.1%) may be connected in series to make a 100 ohm resistance, or in parallel to make a 1 ohm resistance. It can be shown that the ratio of the two resistances so made is accurate to 1 part in 1 000 000 (0.00001%). This principle is probably due to Lord Rayleigh.
The Four-Terminal Junction: Prior to the publication of Hamon's paper, Rayleigh's series-parallel principle proved difficult to exploit. For low values of resistance the effects of lead and contact resistances could not be ignored and the resulting errors limited the accuracy of the measurements. Hamon discovered a way to realise and exploit a four-terminal resistor so that when a current is passed through any pair of leads there is zero voltage produced across the other two leads. Hamon discovered that a four-terminal junction can be realised simply by ensuring that three of the four leads to a junction block are symmetrical with respect to the fourth. Several successful geometries for the manufacture of the junction are known. U.S. Pat. No. 3,252,091 discloses one such geometry.
A four-terminal junction can be used to connect each of the (say, 10) resistors permanently in series in such a way that: firstly, each resistor is a well-defined four-terminal resistance; secondly, the resistance of the (say, 10) in series is exactly equal to the sum (of the 10) because the variable effects of the contact resistances are eliminated; and thirdly, with the use of a combining network as outlined below, it is possible to connect the (10) resistors in a parallel network such that the effects of the lead resistances is negligible.
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Jones Keith
White David Rodney
Industrial Research Limited
Sterrett Jeffrey
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