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VPG unveils high-precision Z-Foil resistors

Posted: 19 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:sensing resistors? heat sink? thermal stabilisation?

Vishay Precision Group (VPG) has revealed that its Vishay Foil Resistors brand (VFR) now has ultra-high-precision Z-Foil power current sensing resistors for applications where rapid R stabilisation and resistance stability under transient power conditions are needed. Geared to deliver optimal performance when mounted on a chassis or cooled heat sink, the VCS331Z, VCS332Z, VFP4Z and CSNG Series feature low TCR of 0.2 ppm/C from -55C to 125C, 25C ref., PCR (R due to self-heating) of 4ppm/W typical, and tolerances to 0.01 per cent, detailed the firm.

For high-power applications, the devices offer power ratings up to 10W (on heatsink) and a four-terminal Kelvin configuration in order to remove the unwanted influence of lead resistance and lead sensitivity to temperature, detailed the company.

The VCS331Z, VCS332Z, VFP4Z, and CSNG claim to offer excellent load-life stability to 0.005 per cent typical (50ppm) at 3W (heat sink) and 25C for 2,000 hours, a thermal stabilisation time of Many instrumentation circuits depend upon current sense resistors for precision measurements and system controls. The devices can also be used in lithium-ion battery circuits for alternative energy applications where energy that cannot be harvested must be stored. In modern battery management circuits, Bulk Metal Foil resistors can be used in the measurement of the "state-of-charge" (SOC) of the battery cells. The characteristics of Li-ion cells complicate SOC measurement, so precise ICs and precision current sense resistors with improved stability and low thermal EMF help to extend battery lifetime.

The Bulk Metal Foil resistor is based on the planar construction of a cold-rolled Ni-Cr foil bonded to a flat ceramic substrate. The monolithic foil, as opposed to the agglomeration of particles in thin film resistors, has a strength and substance independent of the substrate, which gives the metal foil its own specific thermal linear coefficient of expansion (LCE). The foil also has a known and repeatable independent positive temperature coefficient of resistance (TCR). When the foil is bonded to the substrate, it attempts to respond to temperature changes in accordance with its own LCE but is constrained to follow the lower LCE of the ceramic substrate. The resistance change due to the differential LCEs counterbalances the metal's positive TCR for a near-zero net TCR of the resistor. The composite formation with a planar foil structure makes it possible to very accurately etch a pattern into the resistor that is non-inductive and non-capacitive, with low noise and low thermal EMF.

The VCS331Z, VCS332Z and VFP4Z are differentiated by power rating, thermal EMF and configuration. The CSNG is a custom version of these resistors that can be uniquely designed and tailor-made to the customer's specific applications.

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