In order for an electronic device to be controlled with any level of accuracy, the underlying circuit must be equipped with the appropriate elements to facilitate whatever action is required. One of the most critical needs of both control and monitoring applications is current sensing. This measurement is crucial as both a standalone reading and when used in conjunction with other automation components. Current sensing may be completed through a number of different mechanisms, each producing varying levels of accuracy and isolation. In this blog, we will discuss everything you need to know about current sensing and what tools can be used to achieve it.
In order to understand current sensing techniques, it is first necessary to review a physical definition of how current is derived. According to Ohm's law, current (I) can be calculated using I=V/R, where "V" is the electrical potential across two points on a conductor and "R" is the resistance. This equation can be rearranged mathematically to produce V=I*R. Therefore, if one were to keep the resistance constant using a resistor, then the current can easily be determined by measuring the voltage drop across the conductor. Based on this observation, the overwhelming majority of current-sensing techniques implement resistor components with a set resistance level.
The optimal current sensor would have high sensitivity for any changes in voltage, high accuracy, and wide bandwidth. It would also have a low temperature drift, power consumption, and price. Additionally, the resistance value of the current sensor should be low enough so as not to interfere with the device's functionality. Shunt resistors fit the majority of these descriptors, making them the most commonly used type of current sensor across all applications. In particular, shunt resistors have a very low resistance value when compared to power capacity, and they also have a broad operating temperature range. When choosing a shunt resistor, it is important to select a composition material with a lower temperature coefficient of resistance. The materials with the lowest coefficients and operating temperatures are typically alloys such as manganin.
Shunt resistors vary immensely in their price and quality. Although there are several factors that go into determining the price of a particular resistor, the one with the greatest impact is commonly the tolerance. Compared to regular resistors, shunt resistors operate with much lower tolerances. However, current sensing applications tend to require more accuracy, making the tolerance range of shunt resistors between (0.1-1%). Since price tracks closely with the tolerance, it is important to consider the demands of the application in question.
Another popular current sensing tool is the Hall effect sensor, which uses the relationship between a magnetic field and current to produce an accurate output. These components contain a conductive material that creates a magnetic field perpendicular to the current's direction. When the conductive strip is magnetized, a voltage difference is established between the two opposing ends. This difference is directly proportional to the strength of the magnetic field, allowing for current calculations without the need for a resistor.
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