Low OHMIC resistors


The worldwide significance of electronic energy meters is constantly increasing due to their comparatively small size, the absence of moveable mechanical components, their high precision and vastly higher flexibility. In comparison, the percentage of electromechanical inductiontype (Ferraris) meters is decreasing rapidly.

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Avariety of new requirements demanded by the power supply companies that cannot be implemented using conventional kWh meters (or only with enormous effort) can be met with the new generation of meters. Specially developed ASIC energy metering solutions are available on the market that support modern meter capabilities such as AMR, prepayment metering, data capture or choice of tariff. The precision of electronic meters is essentially defined by the current sensors used. In the past, current transformers were used primarily in conventional electromagnetic meters. In addition to offering loss-free consumer electricity metering, current transformers also offer automatic voltage-free metering of consumer electricity.

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Single phase meters require no galvanic isolation – the use of cost-efficient and space saving resistors has become widely accepted and is now state-of-theart. The problem with three phase meters is more complex. However, the first devices, each with a low cost IC and a measurement resistor per phase, are already in use. Galvanic isolation is achieved using an opto-coupler in the output signals. The precision resistor, BKW, developed by Isabellenhütte especially for use in energy meters, combines superior electrical properties with the capability to create physically optimised shunts.

The BKW is available in resistance values of 300 µW, 400 µW and 500 µW. The resistor is designed as a four-terminal measuring resistor. The voltage connectors are arranged as multifunctional contacts, i.e. they can be connected using the conventional method of soldering sense cables to the circuit board, as well as by direct soldering into the circuit board. The voltage conductors to the metering IC are then implemented via the conductor path. This saves assembly costs and prevents additional fluctuations in the measurement signal due to long conductors.

Resistor material prudently used

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The electrical parameters of the shunt are essentially defined by the characteristics of the precision resistance material Manganin®. This alloy efficiently fulfils the demand for low TC values (marginal temperature drift even under heavy loading), extremely good long-term stability and minimised power loss. The resistor material used is limited to the measuring area, which results in a reduction in power loss and, simultaneously, the largest possible measuring signal.

Made from punched and formed composite material, its total resistance is marginally above the nominal value, i.e. 300 µW, which not only results in low total power loss but also in very low internal heat resistance, thus keeping the self heating low. With regard to time stability and precision, the combination of these constructive advantages with the electrical properties of Manganin results in an unbeatable shunt solution for the energy meter. Solutions can still be found on the market which use a shunt made, for example, from Manganin sheet, a welded or hard-soldered combination of copper connections and resistor material.

The disadvantages of such a solution, alongside increased material consumption and manufacturing costs, are above all the increased power loss of performance caused by the distinctly higher total resistance in comparison to the nominal resistance value. Furthermore, there is a risk that the resistor material is wetted during soldering, thereby considerably influencing the TC value and the long-term stability. These disadvantages can be avoided with the Isabellenhütte precision shunts, which are manufactured as a punched and formed part from electron beam-welded Cu- Manganin-Cu strips.