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New Vishay Foil Resistors Video Demonstration Explores Power-Induced Thermal EMF Error Voltage in Current Sense Resistors
MALVERN, PENNSYLVANIA — March. 08, 2012 Vishay Precision Group, Inc (NYSE:VPG) today announced that its Vishay Foil Resistors division has released a new video demonstration of the thermal EMF error voltage generated by uneven heat distribution from power-induced self-heating in resistors. The short video compares the performance of two resistor technologies: metal band and Bulk Metal® Foil current sense resistors. The tests convincingly show how internal heat distribution from power dissipation generates large thermal EMF error voltages in the metal band current sense resistor, but causes virtually zero thermal EMF error in the Bulk Metal Foil resistor.
In addition to uneven heating of dissimilar metals from outside sources, resistors are subject to differentially distributed internal heating from their own power dissipation. This heat generation operates upon the resistor’s element-to-termination junctions to induce a net error voltage, which is equivalent to a battery inserted into the circuit. For the low resistance values typical of current sense resistors, the error voltage can be significant. Additionally, the presence of differential thermal EMF is an indicator of inefficient thermal management inside the resistor and, therefore, a predictor of greater long-term stability limitations.
Using a test that measures the thermal EMF error of a resistor, the demo video introduced today highlights the inherently low differential thermal EMF characteristics of Bulk Metal Foil resistors. The test system used in this demonstration consists of a precision voltmeter, laptop, thermal EMF test fixture, and 50 mΩ 2512 current sensing resistors.
In this demonstration, one watt is applied to each resistor for 15 seconds, during which the change in resistance due to self-heating power dissipation is monitored and displayed. After the power is turned off, the voltage drop of the resistor continues to be displayed for an additional 15 seconds. Both resistors show good PCR (power coefficient of resistance ) when power is applied. After the power is cut off, however, the metal band (strip) resistor generates its own residual voltage of over 100 μV. Conversely, the Bulk Metal Foil resistor generates no thermals and its voltage immediately drops to zero.
Due to their high thermal EMF characteristics, metal band, thick film, and thin film resistors are a poor choice for low-level measurement circuits. Bulk Metal Foil resistors, however, are comprised of carefully chosen materials to eliminate the consequential error voltage due to differential thermal EMF, and are thus the ideal device for low-level current sense circuits.
The differential thermal EMF demo is available now on the Vishay Foil Resistors website, at http://www.vishaypg.com/foil-resistors/videos/?video=26.
Further information about Vishay Foil Resistors products is available at www.vishayfoilresistors.com. Follow Vishay Foil Resistors at http://twitter.com/FoilResistor.
About Vishay Foil Resistors
Vishay Foil Resistors is a division of Vishay Precision Group, producing the most precise and stable resistors available. The portfolio comprises products in a variety of resistor and package configurations to meet the needs of a wide range of applications: discrete resistors and resistor networks in surface mount and through-hole (fixed-lead) configurations; customized chip resistor networks; precision trimming potentiometers; and discrete chips for use in hybrid circuits. The foil resistors and current sensors are used in applications requiring a high degree of precision and stability, such as in medical testing equipment, high performance audio equipment, precision measuring instruments, aerospace, and military applications.
About Vishay Precision Group
Vishay Precision Group (VPG) produces sensors based on precision resistive foil technology, and sensor-based systems. It provides vertically integrated products and solutions for multiple growing markets in the areas of stress, force, weight, pressure and current measurements. As a spin-off from Vishay Intertechnology, VPG’s decades-long track record encompasses innovations in precision foil resistors, current sensors, and strain gages, which have served as a foundation for VPG’s more recent expansion into strain gage instrumentation, load cells and transducers, weighing modules and complete systems for process control and on-board weighing. Vishay Precision Group may be found on the Internet at www.vishaypg.com.