New Vishay Foil Resistors Video Demonstration Explores Power-Induced

Thermal EMF Error Voltage in Current Sense Resistors

MALVERN, Pa. — Mar. 9, 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, 1 W is applied to each resistor for 15 seconds, during which time 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 more than 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 poor choices 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 devices for low-level current sense circuits.

The differential thermal EMF demo is available now on the Vishay Foil Resistors website. For more information about the Vishay Foil Resistors and other products by Vishay Precision Group, Inc. visit Trendsetter Electronics at www.trendsetter.com or contact the company’s representatives toll-free on 1.800.310.3050.

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