A Primer On The Fundamentals And Applications Of Current Sensors
The measurement of current in an electric system is vital for a number of both safety and performance purposes. AC current in modern systems is commonly measured using a hall effect sensor, and most commercially-available AC current sensors make use of this method.
What is a Hall Effect Sensor?
Hall effect sensors make use of the hall effect, an electromagnetic property discovered by Edwin Hall in the late 1800s. By using the hall effect, hall effect sensors are able to convert fluctuations in a magnetic field into very small voltage differences. Hall effect sensors have a wide variety of applications, but they are useful as current sensors since current produces a magnetic field that the hall effect sensor can measure. Since the output voltage of the hall effect sensor is generally very small, most have built-in amplifiers to increase the voltage to a usable level.
Indirect Current Sensing with Hall Effect Sensors
Since the hall effect is measuring a produced magnetic field, hall effect sensors are a form of indirect current measurement. The primary advantage of this type of measurement is that it does not induce a voltage drop or loss of power in the circuit. This makes indirect measuring especially useful in higher voltage applications, where such issues can potentially be significant.
Since the hall effect is converting the strength of a magnetic field into voltage, current sensors which use a hall effect sensor as their core produce voltage proportional to the current. This allows them to function as current sensors, providing a known voltage output that can be interpreted as current.
Closed vs. Open Loop Sensors
The two most common types of magnetic current sensors are open- and closed-loop hall effect sensors. While the details of these sensor types differ quite a bit, from the perspective of the user the primary trade-off is cost vs. accuracy. Open-loop sensors tend to have a major price advantage over closed-loop alternatives, but they tend to offer less accuracy. Their design leads to issues with drift, or a change in voltage output despite the source current remaining constant. In practice, this drift tends to be minor for applications where extremely high accuracy is not necessary and a well-designed circuit can compensate for some of it.
Closed-loop sensors, on the other hand, offer less competitive prices in exchange for much higher accuracy. For applications where high current-sensing accuracy is required, closed-loop sensors will almost always be the correct choice. Additionally, closed-loop sensors offer protection from errors induced by noise on the output line, which can be especially useful in applications where the output is not well isolated.
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