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Bulk Metal Foil resistors tweaked for batteries in avionics

Posted: 02 Sep 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Vishay Precision Group? bulk metal foil? resistors? Li-ion? batteries?

Vishay Precision Group recently completed tests showing that its Bulk Metal Foil current sensing resistors meet the accuracy and stability required by its aircraft customers for charging of Li-ion batteries in avionics applications. VFR resistors provide a variety of device configurations and ultra-high-precision specifications to help ensure the efficient and safe charging of Li-ion batteries. They include the surface-mount Z-Foil VCS1610Z and VCS1625ZP and the high-power VCS331Z and CSNG.

Precision current sensing resistors are required to charge and monitor the Li-ion battery at a known current level until it reaches its final voltage. At this point, the charger circuitry switches over to constant voltage mode, providing the current necessary to hold the battery at this final voltage. For both phases, Bulk Metal Foil resistors provide higher accuracy than other technologies, such as thin film, thick film, and wirewound resistors.

"Li-ion batteries are well established in portable consumer electronics and are now becoming more popular in military and advanced civilian avionics. As the scope of their applications has expanded, so have the stringent requirements they must meet, especially in terms of cell safety and lifetime. For designers, the main challenge in meeting these requirements is in finding the optimal point between undercharging and overcharging through continuous accurate and stable monitoring of input and output current," said Yuval Hernik, VFR's senior director of application engineering.

VFR's Bulk Metal Foil current sense resistors offer a four-terminal Kelvin connection for increased accuracy. They feature an ultra-low TCR of 0.05 ppm/C typical (0C to 60C) and 0.2ppm/C typical (-55C to 125C, 25C ref.). The surface-mount VCS1610Z and VCS1625ZP offer load-life stability of 0.015per cent at 70C for 2,000 hours at rated power while the VCS331Z power resistor features load-life stability down to 0.005per cent (50ppm) typical at 3W and 25C for 2,000 hours (mounted on a heatsink) and thermal EMF of 0.05?V/C typical.

"The need for safety was emphasized in the latest near-catastrophic incident involving the charging of Li-ion batteries, in which a Dreamliner caught fire while parked at Heathrow. Luckily, no passengers were on board. In our opinion, designers in the avionics industry have every motivation to look for more precise and reliable commercial-off-the-shelf (COTS) resistors to accurately monitor the battery voltage when connected to the charger. Bulk Metal Foil current sensing resistors provide the low TCR and thermal EMF required to meet this need. They also provide wide resistance ranges, and unlike most COTS resistors, any resistance value within their given range is available with no additional cost or lead time effect," added Hernik.

Featuring a non-inductive (rms per volt of applied voltage (

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