In this article, we'll cover some of the most popular current sensors used by electrical engineers today, magnetic field sensors, and resistor sensors.
Principles of Current Sensing
There are different current sensing techniques with different fundamental principles behind them. The most familiar principles are Ohm's law of resistance, Faraday's law of induction, and magnetic field sensors.
Current sensors following Ohm's law of resistance are low cost, easy to manufacture, and reliable. Current sensors that employ the Faraday's law of induction technique makes current sensing easy in applications where electrical isolation is required. The method can be used to measure current in static magnetic fields.
Magnetic field current sensors are used to sense current in static and dynamic magnetic fields. There are two types of sensing configurations in this technique: the open-loop configuration and the closed-loop configuration. There is no compensation for measurement error in an open-loop arrangement, though it is simple to perform current measurements with it.
Engineers performing tests and instrumentation experiments. Image Credit: Teesside University.
Types of Current Sensors
Electrical engineers use the above-mentioned current sensing techniques to manufacture current sensors. Some examples of current sensors are explained below.
Hall-Effect sensors are an example of a type of magnetic field current sensor. The Hall-effect sensor is made up of a Hall element, which is a thin conductive material. When a Hall element is placed perpendicular to current flow in a magnetic field, a hall voltage which is proportional to the magnetic field strength is generated.
The hall element thickness is one of the factors that determine the sensitivity of a Hall-effect sensor. Chemical compounds such as Indium Antimonide (InSb), Indium Arsenide (InAs), and so on are used in the manufacturing of a hall-effect sensor.
The sensing configuration of Hall-effect sensors includes both open-loop and closed-loop configuration. The closed-loop configuration provides a high accuracy measurement. Hall-effect sensors are primarily used in current sensing applications. Also, its power dissipation is very low. The sensor finds applications in power conversion systems and motor drives.
Shunt Resistor Sensor
This type of sensor is based on the working principle of Ohm's law of resistance. It is straightforward to design. The proportional measure of current flow is determined by the equivalent voltage drop across the shunt resistor.
This sensor can measure both direct current (or DC) and alternating current (or AC). One disadvantage of using a shunt resistor sensor is its ability to generate an amount of power loss in the current conducting path. The sensor is not suitable for use in high current applications.
However, the measurement accuracy is high compared to the Hall-effect sensor and can perform efficiently in high-temperature applications. It is possible to measure transient current pulses with the use of high-performance coaxial shunt resistors.
The sense circuit and the current to be measured can be separated using isolation amplifiers when using a shunt resistor sensor in electrical circuits. This makes isolated current sensing possible in HEV/EV subsystems. It has a very high long-term stability and is low cost.
An illustration of the GMR effect current sensing technique for magnetic recording applications. The sensor is made of a spin valve. Image Credit: University of Nebraska-Lincoln.
This type of current sensing made up of magnetic core material is based on the principles of Faraday's law of induction. In a current transformer, the proportional current is measured by a proportional change in flux. It has high sensitivity thanks to the high permeability of the magnetic core material.
It is relatively low cost and finds applications in power conversions.
Magnetoresistance Effect Sensors
Like magnetic effect field sensors, these type of sensors is also used to measure static and dynamic magnetic fields. To make measurements, the resistance of the ferromagnetic materials used is dependent on the magnitude and direction of the magnetic field applied.
This type of measurement is used in anisotropic magnetoresistance (aka AMR) effect sensors. In AMR effect sensors, the angle of the current flow through ferromagnetic materials has a resistance that is a measure of the current flow direction and magnetization.
In practice, barber poles are used in AMR effect sensors in which the angle of the current flow can be made to be 45⁰ to the magnetic field direction. This helps to achieve low impedance and high sensitivity.
Disadvantages of GMR Sensors and Their Workarounds
Another example of a magnetoresistance effect sensor is the giant magnetoresistance (or GMR) effect sensor. Like AMR, it is also made up of ferromagnetic materials. It is cheap to produce as the sensor is small in size. This sensor can also be used to detect low current flow that AMR effect sensors can't identify.
One disadvantage associated with magnetoresistance effect sensors is that they exhibit high thermal drift. This can be compensated by constructing a bridge configuration for the resistors used in the sensors (a Wheatstone bridge configuration can be employed). On top of that, the measurements in this type of sensors are non-linear.
Magnetoresistance effect sensors can be used in Hall-effect sensor-based applications. Most notably, the sensors are used for magnetic recording.
All in all, electrical engineers have a wide range of current sensing techniques they can employ in test and measurement applications.