The burgeoning market for portable electronics has opened up a huge demand for current-sensing resistors, and manufacturers are responding with lower resistance levels. Current-sensing resistors are also finding increased usage in motor and motion controls as well as in the huge automotive market.
The use of resistors to monitor battery charging in portable electronics, and to protect motion controls and additional features in vehicles, is fueling solid growth for these passive components. A recent Dataquest Inc. analysis of the Apple iMac computer underscores the importance of passives: 443 of the 532 electronic components in the iMac were passives, though not all were resistors.
Designers shopping for current-sensing resistors can pick from many technologies, mainly thick and thin films, with some wire products and other options. They can also choose alternative current-sensing techniques, but most designers find that current-sensing resistors pair the lowest cost with the highest accuracy.
Component developers have been lowering resistance values to minimize the impact of the circuit and to keep heat down. Changes in resistive materials and substrates are the pri-mary fixes, though even minor tweaks in termination techniques can play a big role in reducing resistance levels by a milliohm or two.
Among other applications, carmakers use current-sensing resistors to turn single-speed motors into dual-speed ones. Putting in a resistor is a simpler and cheaper way to produce a dual-speed fan or fuel pump than creating a motor that has two speeds.
IRC Wire and Film Technologies Division (Boone, N.C.) is addressing the automotive segment of this market with wire products that eschew the conventional wirewound approach. Instead, a serpentine ribbon of wire gives the 4LPW and similar products resistance ranges down to 0.005 ohm. At that level, resistors can handle 3 to 5 watts at 25degreesC, with maximum current ratings of 10 A. The 4LPW has two current leads and two sense leads.
Though IRC is using a different approach, con-ventional wirewounds can also be used for current sensing. Ohmite (Skokie, Ill.) has broadened its 5-W TWW line to address current sensing. The parts go down to 0.035 ohms, with plus/minus5 percent tolerances. Vertical mounting con-serves board space and helps dissipate heat.
Current-sensing re-sistors at Caddock Electronics Inc. (Riverside, Calif.) are designed for motors, motor drives, power supplies and some battery chargers. To serve these markets, Caddock's SR line of four-wire current-sense resistors goes down to 0.08 ohms and has a non-inductive design.
Tolerances are typically plus/minus1 percent, with temperature coefficients ranging from -50 to 100 ppm/degreesC for resistors with values up to 1 ohms. For those with resistance levels of 8 to 24 milliohms, the TC goes from 0 to 200 ppm/degreesC. These resistors save board space by standing upright, rising only 0.37 inch above the circuit board. They take up only 0.365 x 0.09 inch.
Caddock also offers chip resistors that can be used to sense currents. The CC line comes in flip-chip configurations for those who want to put them on boards and a wire-bond model for designers putting them into IGBTs or other modules where most of the other components are attached using wire bonding. Flip-chip models have 1- and 1.5-W ratings, while the wire-bond resistors have 9- and 15-W capability. The two-lead parts have resistance values as low as 0.020 ohms, with tolerances down to plus/minus1 percent.
Another chip product comes from AVX Corp. (Myrtle Beach, S.C.).
The LR50 thick-film line works with up to 1 W of power, offering resistive values as low as 0.050 ohms. Standard tolerances for the 1020-size devices are plus/minus1 percent.
Another IRC operation, the Advanced Film Division (Corpus Christi, Texas), has come out with both two- and four-lead models. Among its four-lead, Kelvin-style offerings is the LRK family of thick-film resistors with values down to 0.003 ohms, a level designed for current sensing in portable products like laptops.
Unlike many four-lead parts, they can be surface-mounted to the board, with two terminals at each end of the resistor. Tolerances are plus/minus1 percent. The two-lead PLR line has power ratings to 2 W, higher than many other products. Resistance values go down to 0.01 ohms.
Taking aim at the precision segment of the current-sensing market, Isotek Inc. (Swansea, Mass.) has unveiled the PMC line, with resistance values of 1 to 3 milliohms and tolerances of plus/minus1 percent at 3 W. With thermal resistance of only 5degreesC per watt, PMC parts can hold their 3-W continuous power rating across a temperature range of -55degrees to 125degreesC. Those features are achieved by using a manganin foil resistor element attached to an aluminum substrate. The four-terminal parts have pulse power ratings of 10 times the rated power. Inductance is less than 10 nanohenries.
Meanwhile, Precision Resistor Co. Inc. (Largo, Fla.) is offering four-terminal current-sensing shunts that come in either through-hole or surface-mount formats. The PLVand LVS lines have ohmic values down to 0.001 ohms, with resistance tolerances as low as plus/minus0.005 percent. TCR ratings are typically 15 ppm.
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