are based on proven foil resistor technologies with a 50-year track record of delivering ultra-precision performance while continuing to evolve and improve to serve the needs of the market.
Many monitoring and control applications utilize a 4 mA to 20 mA closed-loop circuit, consisting of a DC power supply to power the loop, a sensor/transducer/transmitter that inserts the sensor-related current, and a meter or control device at the other end of the loop. Generally, analog sensors with just microvolt-level outputs continuously detect changes in any significant stage in the process, said Yuval Hernik, senior director of application engineering for VFR.
The sense resistors used in these circuits, typically 250 Ω, are critical for system operation and must offer high stability and reliability. Since voltage across the resistor is fed directly to the meter, any error could indicate that the sensor is reading a non-existent condition. The process control correction made on this reading would then cause the system to operate at less than optimal conditions and reduce its efficiency, which could result in increased operating costs, environmental pollution, hazardous spills, associated fines, and poor product quality. Prudent designers consider all of the possible problems that could affect the performance and reliability of their monitoring and control circuits, and select the sense resistor that best protects the performance of their equipment. And with the menaces of early obsolescence and counterfeiting ever more evident in the electronics industry, buyers are more closely than ever scrutinizing not just devices but their manufacturers as well to avoid problems down the road.
VFR's Bulk Metal Foil resistors for control and monitoring applications provide the high reliability and long-term stability required for 4 mA to 20 mA circuits. The surface-mount VSMP Series and through-hole Z Series devices offer ultra-low TCR of ±0.05 ppm/°C typical (0°C to +60°C) and ±0.2 ppm/°C typical (-55°C to +125°C, +25°C ref.), while the secondary standard VHP100 resistor offers an essentially zero TCR and is available with tight tolerances down to ±0.005% (50 ppm). The devices feature exceptional load-life stability of ±0.005% at 70°C for 2,000 hours at rated power and ±0.01% at 70°C for 10,000 hours. The resistors are virtually immune to extraordinary incidents such as radiation exposure and withstand ESD spikes to at least 25 kV.
Featuring a non-inductive (<0.08 μH), non-capacitive design, Bulk Metal Foil devices offer a rise time of 1.0 ns, with effectively no ringing, a thermal stabilization time of <1 s (nominal value achieved within 10 ppm of steady state value), current noise below 0.010 µVrms per volt of applied voltage (<-40 dB), voltage coefficient of <0.1 ppm/V, and thermal EMF of 0.05 μV/°C typical. The devices provide wide resistance ranges, with any resistance value within their given range available at any tolerance with no additional cost or lead-time effect.
An often overlooked reliability concern in 4 mA to 20 mA DC current loops is moisture, which can open thin film resistors in just a few hours of operation or cause a more gradual increase in resistance over a longer period of time. In addition to the thicker resistance grid of Bulk Metal Foil resistors, devices such as the VHP100 feature a hermetically sealed metal and glass enclosure to protect the resistive element against humidity, and are oil filled to improve the thermal dissipation path to the outside. In industrial control and monitoring applications, environmental pollution is also a constant concern, including high concentrations of sulfur vapor, which can penetrate conformal coatings and cause damage to resistors. Surface-mount Bulk Metal Foil chip resistors like the VSMP Series feature a special coating and construction that is reliable in these extreme environments.
Samples and production quantities of the foil resistors are available now, with lead times of five days for prototype samples and from two days to five weeks for standard orders at the catalog houses, depending on their shelf availability.