Design News, February 2013

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S e n S o r S/ m ac h i n e v i S i o n two battery terminals, these battery systems typically have two diagnostic signals: a single-wire data line for battery health, and a single-wire thermistor output for battery temperature monitoring. These diagnostics are referenced to the negative terminal of the battery. When using a sense resistor in this application a design engineer must take various error terms into account. First, the resistor will dissipate energy from the battery as heat, making the system inefficient. Second, the voltage, VSENSE, developed across the sense resistor will be superpositioned on the thermistor voltage, VTHERM, such that the voltage seen by the charge controller is VSENSE + VTHERM, resulting in an error in the monitored battery temperature (figure 3). This could affect the charging control of the battery system and eventually hamper battery life. The sense resistor approach yields reduced accuracy when using lower sense resistor values, as discussed earlier in this article. In the case of the integrated Halleffect sensor, the conductor resistance is lower (as low as 0.6 mΩ), which greatly reduces power dissipation. There is practically no voltage developed across the conductor loop terminals, which helps maintain the integrity of the thermistor's diagnostic signal. Lastly, since the sensing is based on magnetic coupling, there is no dependence between conductor resistance and accuracy, simplifying the design. control circuits for appliances such as refrigerators, washing machines, and air conditioners (referred to as "white goods"). Integrated Hall-effect current sensors can provide compact currentsensing solutions with inherent high voltage isolation. Power consumption from current sensing can also be reduced significantly with integrated Hall ICs since they have very low primary conductor resistance. This helps meet energy-efficiency requirements such as Energy Star. These sensor ICs also provide a means of rapid detection of fault conditions that can prevent expensive system damage, improving reliability. Current Sensing for Motor Control in White Goods Energy efficiency, safety, reliability, and low-cost are required in motor Alihusain Sirohiwala is a systems engineer at Allegro MicroSystems Inc. For more information, go to DISTANCE OR LIQUID LEVEL MEASUREMENTS Non-Contact Measurement Range of 4 inches to 30 feet M3 Wireless Sensor •Zigbee Radio on Board •Battery Life: 3-5 Years •Digi International Network •Kynar Housing M-300 Wired Sensor •Low Cost •Rugged (IP67) •mAmp or V Output •RS485/232 PC Interface MASSA PRODUCTS CORPORATION 280 Lincoln St., Hingham, MA 02043 U.S.A. 781-749-4800 T12 Tre nd waT c h : sens o r s / m a c hi n e v i s i o n / a s u p p l e me nT To de s ign ne ws f eb r uary 2013 [www.designne ws .com]

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