Types and Characteristics of Current Sensors

What methods do you think of when you hear the phrase “measuring the amount of current”? Perhaps many people imagine a method of calculating the current value from the voltage value that flows through a known resistor by passing the current to be measured.

Such a current detection method is generally called a “shunt method (current shunt method).” This method is suitable for detecting a small current in a system driven by a relatively low voltage without isolation. There are many inconveniences in handling in applications where high voltage of 100V or higher and large current of several to several hundred amperes flow, such as industrial equipment and residential equipment.

Disadvantage

  • The resistor generates a large amount of heat, and the size of the substrate and peripheral components increase for heat dissipation.
  • The system is hard to build due to the voltage drop and back electromotive force generated by the resistor.
  • The photocoupler for insulation requires power supply on both high voltage side (primary side) and low voltage side (secondary side), which increases the number of components, cost, and mounting area (Figure 1).
  • Each time the current range to be measured changes, it is necessary to reselect resistors and redesign peripheral circuits accordingly.
  • Users must correct the characteristic fluctuation (temperature drift) due to temperature change by themselves.
Figure 1. Schematic Diagram of Shunt Method

About Current Sensor

In order to solve these problems, a so-called “current sensor” was brought to the market.

The principle of the current sensor is shown in Figure 2.
The current to be measured generates a magnetic field around the current path. By detecting this generated magnetic field using a magnetic sensor, the amount of current can be measured.

Unlike the shunt method, the current sensor uses a conductor with a comparatively low resistance value as a current path. There is also no need to change the resistance depending on the amount of current. The current sensor will solve the inconveniences of the shunt method in the following ways:

Figure 2. Schematic Diagram of Magnetic Field Generated by Current

Advantage

  • Heat generation is small so that the substrate size can be reduced.
  • The voltage drop and the back electromotive force are small compared to the shunt method, so it is easier to control.
  • It requires only a single, low voltage (secondary) power source with high isolation, allowing the number of components, cost, and mounting area to be reduced.
  • Even if the current range to be measured changes, there is no need to redesign peripheral circuits.
  • The temperature compensation function is built into the internal IC so that the temperature drift is very small.

Trend

In late 1980s, current sensors of various kinds (e.g. cored current sensor, module-type current sensor, and current transducer) were brought to the market and were used in many applications (Figure 3).

In late 2000s, the downsizing of industrial equipment accelerated and concern for the environment has led to the requirement of high efficiency and energy saving. This market trend revealed the following issues about current sensors in certain applications:

Figure 3. Schematic Diagram of Cored Current Sensors

Issues

  • The device size is large due to the magnetic core.
  • The magnetic core has hysteresis and causes an inaccurate measurement of the current.
  • The magnetic loss of the core causes energy loss.
  • It’s expensive.

To remedy these problems, a “coreless current sensor” - a current sensor without a magnetic core - was developed (Figure 4).

The coreless current sensor solves the problems caused by the core while also maintaining the advantages of the cored current sensors.

Since the early 2010s, coreless current sensors have been used in many applications such as low voltage drives, AC servo motor drives, robot controllers, and air conditioners.

Figure 4. Diagram of a Coreless Current Sensor

Summary of This Page

Types and Characteristics of Current Sensors

'Currentier' series are open-type current sensors which have the world's smallest package, high accuracy, high S/N ratio, and the world's fastest response time.
With these advantages, we will offer the best current sensing solution to wide-ranging applications, such as inverter control, overcurrent detection, and electrical power detection.

* Currentier is a trademark of Asahi Kasei Microdevices Corporation.