Shunt resistors are connected in parallel with an instrument or component to divert electrical current. They provide an alternate path for current in case of failure, and can be used to accomplish a reduction in input sensitivity from the input line to ground. Current shunt resistors are low resistance, passive electronic devices used to measure alternating current (AC) or direct current (DC) by the voltage drop those currents create across the resistance. Electrical specifications for shunt resistors include ohms, resistance tolerance, current rating, power rating, temperature coefficient of resistance, and power coefficient of resistance. Ohms (O) measure a material’s opposition to the flow of electricity in a circuit. Temperature coefficient of resistance (TCR) refers to the change in resistance with change in temperature. Power coefficient of resistance (PCR) is the temperature rise caused by self-heating. For current-sensing shunts, resistances typically range from 100 µO to 500 mO. Low TCR and PCR are important. For shunt resistors used in current conversion applications, high precision may be required.
Physical specifications for shunt resistors include mounting style, lead type, and resistor material. Shunt resistors can be bolted, chassis mounted, surface mounted, or through-hole mounted. Chassis-mounted resistors attach to a metal surface for maximum heat dissipation. Surface mount technology (SMT) and through-hole technology (THT) are other common mounting styles. Lead types include: axial leads, gull-wing leads, J-leads, radial leads, screw terminals and tab terminals. Shunt resistors without leads are also available. Choices for shunt resistor material include: carbon film, ceramic, metal alloy, metal film, metal oxide, thick film, thin film, and wire wound. Carbon shunt resistors are made of a mixture of finely-ground carbon and insulating material held together by a resin binder. Ceramic shunt resistors are made of solid, high-temperature, ceramic, resistive materials with bonded metal contacts. Metal alloy shunt resistors contain two or more metallic elements. Wire wound shunt resistors are made by winding thin wire onto a ceramic rod.
Shunt resistors differ in term of packing method. Some passive electronic components are packed in tape reel assemblies that include a carrier tape with embossed cavities for storing individual components. Others are packed in trays (rails) are made of carbon-powder or fiber materials and molded into rectangular outlines that contain matrices of uniformly spaced pockets. These containers protect components during shipping and provide proper component location and orientation for use with industry-standard, pick-and-place board assembly equipment. Shunt resistors that are packed in shipping tubes, stick magazines, or bulk packs are also available.
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