Current sensing resistors convert the current flowing through them to a voltage drop. Monitoring or measuring this voltage drop allows the current through the resistor to be measured. With current sensing resistors, materials of construction and configuration style are important parameters to consider. In terms of materials, choices include carbon composition, carbon film, ceramic composition, metal alloy, metal film, thick film (chip), thin film (chip) and wire wound. For configuration style, choices include single resistor, resistor network, and resistor chip array. Single current sensing resistors have a single resistance value. Current sensing resistor networks feature a pin layout with multiple resistors connected in series. By contrast, current sensing resistor chip arrays connect multiple resistors in parallel.
Current sensing resistors carry specifications such as number of terminals, resistance range, tolerance, power rating, continuous operating voltage, temperature coefficient parameter (TCP), and operating temperature. Most current resistors have either two or four terminals. Resistance range is measured in ohms. Tolerance is specified as percentage. Power rating is the maximum power level that a current sensing resistor supports. The continuous operating voltage is measured in volts. The temperature coefficient parameter (TCP) measures the rate at which the nominal resistance value changes as a function of temperature. Typically, TCP is expressed as parts-per-million per degree Celsius (ppm/C). Operating temperature is an important environmental parameter to consider, especially for current sensing resistors which are exposed to flow, reflow, or wave soldering.
Lead style and mounting style are important considerations when selecting current sensing resistors. There are six basic types of leads: axial leads, gull-wing leads, J-leads, radial leads, screw terminals and tab terminals. Current sensing resistors without leads are also available. In terms of mounting style, some products are designed to bolt-on to a chassis. By attaching solidly to a metal surface, these chassis-mounted resistors provide maximum heat dissipation. Surface mount technology (SMT) adds current resistors to a printed circuit board (PCB) by soldering component leads or terminals to the top side of the board. Typically, the PCB pad is coated with a paste-like formulation of solder and flux. Current sensing resistors that use through-hole technology (THT) are mounted on a PCB by inserting component leads through holes in the board, and then soldering the leads in place on the opposite side of the board.
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