RER70FR191RCSL

FAQFrequently Asked Questions

• What are the key design considerations when integrating the RER70FR191RCSL into a high-current power distribution circuit? The RER70FR191RCSL is a 191 mOhm, 20W wirewound chassis mount resistor rated for military applications (MIL-PRF-39009). When used in power distribution, thermal management is critical—at rated power, the resistor will dissipate significant heat. The aluminum housing with flanges facilitates direct mounting to a heatsink or chassis for convection cooling. Design the circuit to ensure that current through the RER70FR191RCSL does not exceed approximately 10.2A continuous (√(20W / 0.191Ω)), and verify that the surrounding PCB or enclosure layout allows adequate airflow around the mounting flanges. The ±1% tolerance and ±100 ppm/°C temperature coefficient ensure predictable current sensing or load-sharing performance across military temperature ranges (-55°C to 250°C).
• Can the RER70FR191RCSL replace older carbon-film or thin-film current-sense resistors in legacy military or aerospace designs? The RER70FR191RCSL is a wirewound military-grade resistor (MIL-PRF-39009 qualified) with characteristics distinct from carbon-film or thin-film alternatives. Its low temperature coefficient (±100 ppm/°C) and tight ±1% tolerance make it suitable for precision current sensing where carbon-film resistors may drift significantly over temperature. However, the RER70FR191RCSL's physical size (26.97mm × 27.43mm footprint, 0.561" seated height) is larger than many thin-film alternatives, so space constraints in legacy designs may limit retrofit feasibility. If replacing an older part, verify that the solder-lug lead style matches your assembly process, and confirm that the chassis-mount flange geometry fits your existing mounting holes. The wirewound construction also exhibits slightly higher parasitic inductance than thin-film resistors, which may affect high-frequency circuit performance if the original design operated at RF or switching frequencies above 1 MHz.
• How does the RER70FR191RCSL perform in extended temperature cycling between -55°C and 250°C in industrial or military field environments? The RER70FR191RCSL is rated for operation across -55°C to 250°C, with a failure rate of R (0.01%) per MIL-PRF-39009 military standards, indicating reliable performance in harsh environments. The wirewound construction and aluminum housing provide robust mechanical and thermal stability. However, repeated thermal cycling induces stress on the solder joints at the lug connections; in applications with frequent thermal transients (e.g., switching on/off in extreme climates), inspect solder joints periodically and consider conformal coating for moisture and corrosion resistance. The ±100 ppm/°C temperature coefficient means the RER70FR191RCSL's resistance will vary approximately 0.0191 mOhm per degree Celsius—negligible for many applications but measurable in precision current-sense circuits where 0.1% accuracy is required across the full temperature range.
• What are the electrical and thermal tradeoffs when selecting the RER70FR191RCSL versus a paralleled array of lower-wattage resistors? A single RER70FR191RCSL resistor at 20W and 191 mOhm offers simplified design, reduced board space, and single-point mounting compared to paralleling multiple lower-wattage parts. However, paralleling offers design flexibility: if a single RER70FR191RCSL fails open due to excessive current transient, redundant paths remain active. The RER70FR191RCSL's wirewound construction and military qualification reduce failure risk in controlled environments. Paralleled arrays introduce resistance matching challenges—unless each element has ±1% tolerance like the RER70FR191RCSL, current will distribute unevenly, creating localized hot spots. For applications where single-point failure is unacceptable, paralleling may be justified despite added complexity; otherwise, the RER70FR191RCSL's integrated design and proven reliability favor the monolithic approach.
• Is the RER70FR191RCSL suitable for DC or AC current-sense applications, and are there frequency limitations? The RER70FR191RCSL is primarily designed for DC current sensing and load-sharing applications, as evidenced by its wirewound construction and chassis-mount form factor. At low AC frequencies (DC to approximately 100 kHz), the resistor behaves as a pure resistance with negligible reactive effects. However, wirewound resistors exhibit parasitic series inductance—typically on the order of tens of nanohenries—which becomes significant at higher frequencies (RF, switching converters at MHz range). If the RER70FR191RCSL is used in a switch-mode power supply operating at 500 kHz or higher, the inductance may cause impedance rise and alter circuit response. For AC current sensing at high frequencies, consider thin-film or thick-film alternatives with lower inductance. At standard power-line frequencies (50–60 Hz) or DC applications, the RER70FR191RCSL performs without frequency-related constraints.
• What mounting and thermal interface materials are recommended when installing the RER70FR191RCSL on a chassis or heatsink? The RER70FR191RCSL features aluminum housing with flanges and solder lugs, designed for direct mounting to a metal chassis or heatsink using the flange holes. To maximize thermal transfer and ensure long-term reliability, use a thermal interface material (TIM)—such as aluminum oxide-filled epoxy, mica washers with thermal grease, or phase-change pads—between the flange and mounting surface. Torque mounting screws evenly and moderately; over-tightening may crack the ceramic core or warp the aluminum housing, while under-tightening leaves air gaps that degrade thermal conductivity. Apply thermal compound sparingly to avoid migration into the solder-lug connections. The solder lugs should be soldered to heavy copper traces or bus bars with adequate thermal mass to prevent reflow damage during assembly. For vibration-prone military or aerospace applications, consider thread-locking compound on mounting fasteners to prevent loosening.
• How does RoHS non-compliance of the RER70FR191RCSL affect its use in modern commercial or EU-regulated applications? The RER70FR191RCSL is RoHS non-compliant, meaning it may contain lead or other restricted substances not permitted in EU or RoHS-regulated end products. This designation does not affect the resistor's electrical or thermal performance but creates supply-chain and regulatory constraints. If your end product is subject to EU RoHS Directive 2011/65/EU or equivalent regulations, the RER70FR191RCSL cannot be used unless a specific exemption applies (e.g., military applications often receive exemptions). For defense, aerospace, or legacy industrial designs not subject to RoHS, the RER70FR191RCSL remains compliant. If you are transitioning a design to a new commercial market requiring RoHS compliance, source a RoHS-compliant alternative wirewound chassis-mount resistor with equivalent resistance, tolerance, and power rating, and verify that thermal and mechanical properties match your design requirements.
• Can the RER70FR191RCSL withstand accidental overvoltage or current transients, and what protection measures are recommended? The RER70FR191RCSL is rated for 20W continuous power and designed to withstand military-grade transient environments (MIL-PRF-39009). However, its specifications do not detail peak power or pulse energy limits. A brief current spike—such as from an inductive load release or hot-swap event—that pushes momentary power above 20W will thermally stress the wirewound element and may degrade the resistor or cause open-circuit failure. To protect the RER70FR191RCSL in applications prone to transients, place a fast-acting fuse or current-limiting series element upstream. Additionally, if the circuit includes inductive components (inductors, relay coils), ensure that freewheeling diodes are installed to clamp overvoltage spikes. For sensitive measurement circuits, add RC filtering on the voltage across the RER70FR191RCSL to attenuate high-frequency transients that may temporarily exceed the resistor's thermal time constant.
• What is the moisture and corrosion resistance of the RER70FR191RCSL, and is additional sealing required for humid or corrosive environments? The RER70FR191RCSL features military-grade moisture resistance (Moisture Sensitivity Level MSL 1: Unlimited), indicating it can tolerate humidity and condensation without accelerated degradation during normal handling and storage. The aluminum housing and wirewound core resist salt-spray and humidity better than uncoated carbon-film resistors. However, the solder lugs and lead connections are the primary corrosion entry points. In highly corrosive environments (salt-fog, chemical exposure, or marine applications), apply conformal coating (e.g., acrylic, silicone, or urethane) to the solder joints and lug connections after assembly. Avoid coating the thermal interface between the flange and heatsink, as this degrades heat transfer. If the RER70FR191RCSL is installed in an outdoor or chemical processing facility, inspect solder joints periodically and consider potting or encapsulation of exposed connections for long-term reliability.
• Are there pin-compatible or drop-in alternatives to the RER70FR191RCSL with different tolerance, power, or form factor, and what are the design implications of switching? The RER70FR191RCSL is a specialized military wirewound chassis-mount part; direct drop-in replacements are limited. Pin compatibility depends on solder-lug spacing and flange geometry rather than lead-count. Vishay Dale offers other resistors in the RER70 series (MIL-PRF-39009) with different resistance values and power ratings; if you require a different resistance value, the RER70 series maintains identical mechanical and electrical performance across the lineup. For non-military applications, commercial wirewound resistors from manufacturers like Ohmite, Stackpole, or Arcol may offer similar 191 mOhm resistance and 20W power ratings with tighter tolerances (±0.5% or ±0.1%) but may lack military qualification or have different thermal coefficients. Before switching, verify that the alternative's tolerance, temperature coefficient, power rating, and physical footprint align with your circuit's requirements. If the replacement has higher inductance or different parasitic properties, re-qualify high-frequency or precision-sensing circuits to confirm performance.