What Is a Current Sensor?
A current sensor is a device that measures the current flowing in a circuit.
Current sensors are built into current circuits, so they require the circuit to be disconnected and can be dangerous, but there are also types of current sensors that can measure current by simply clamping a portion of the circuit from the outside.
Uses of Current Sensors
Current sensors are used in situations where the value of the current flowing in a particular circuit is desired. Current sensors that are directly integrated into a circuit are often used to measure and record the value of the current that flows in a circuit on a regular basis. In this case, the information is used for self-maintenance of equipment in operation.
On the other hand, current sensors that clamp the circuit from the outside are useful in non-routine situations, such as pre- and post-checking of energization during electrical work or checking the energized part to investigate the cause when an electrical circuit fails.
Principle of Current Sensors
A current sensor directly connected to a circuit acts as a resistor and measures the actual current flowing through the circuit based on the voltage applied to the circuit. Clamp-type current sensors cannot measure the actual current flowing in a circuit because they are not directly connected to the circuit. Therefore, the magnetic field generated from the current flowing in the wires of the circuit is measured and output as a current equivalent value.
Specifically, there is a magnetic core in the clamping section, and the magnetic field is detected at that section and converted to a current value. Since the polarity of the magnetic field differs between AC and DC, the principle for observation also differs. Among the various measurement methods, the Hall element method is one that can detect both DC and AC.
In this method, a Hall element is incorporated in the magnetic core to measure the magnetic field generated in the circuit under test as a voltage, which is converted into a current value via a built-in amplifier.
How to Select a Current Sensor
There are two types of current sensors: direct-current circuits and alternating-current circuits. It is important to select an appropriate current sensor depending on the current component of the circuit you wish to measure and the accuracy you wish to measure.
Generally, if you do not require a large degree of accuracy, select a Hall element type that can handle both DC and AC circuits, but it may not clamp well depending on the installation position of the circuit.
For AC, select a Rogowski type current sensor without a core, which allows measurement while bending the clamping section.
Other Information on Current Sensors
1. Detection Method of Current Sensors
Current sensors have different applications depending on the characteristics of the detection method. Current detection methods can be broadly classified into two types: resistance detection type and magnetic field detection type.
Resistance Detection Type
The resistance detection type detects current by inserting a resistance called a shunt resistor at the point where the current to be measured flows. According to Ohm’s law, a voltage drop occurs when a current flows through the resistor.
By measuring this voltage value and dividing it by the resistance value, the current can be measured. Although it depends on the circuit design, it is common to select a small resistance value for the shunt resistor so that it does not affect the operation on the circuit.
However, when measuring large currents, heat generation and withstand voltage of the resistor value can be a problem. If the voltage drop is small, a current sense amplifier should be used to amplify the voltage to be detected.
Magnetic Field Detection Type
The magnetic field detection type measures the magnetic field created by the flowing current. According to Ampere’s law, when a current flows, a magnetic field is generated in the direction in which the right-hand thread turns when the direction of the current is the direction in which the right-hand thread moves.
This magnetic field is detected using a Hall element. A Hall element is an element that uses the Hall effect, a phenomenon in which an electromotive force appears in a direction orthogonal to both the current and the magnetic field, and is used to detect the magnetic field.
2. Circuit of a Current Sensor
The resistance detection type using a shunt resistor is used for overcurrent detection in power supplies such as DC/DC converters, LED drivers that are driven by current, and current control and circuit protection in motor drive circuits. The magnetic field detection type using Hall elements does not require an element to be placed on the circuit pattern where the current is to be measured, so it is also used in clamp ammeters that clamp wires to measure current and in motor position detection.
Current detection using resistance detection type with a shunt resistor or magnetic field detection type with a Hall element often produces a small output, so when a circuit is configured with discrete components, it is common to combine a current sense amplifier or operational amplifier in the circuit. Manufacturers also offer dedicated ICs with built-in amplifiers instead of discrete components, which can be selected according to the application and characteristics.