A current sensor is a sensor that converts the measured current into a usable output signal. According to the detection principle, it can be divided into: resistive shunt current sensor, current transformer, Hall current sensor, Rogowski coil current sensor, fluxgate current sensor, fiber optic current sensor, etc.
Current sensors are mainly classified into
① Resistive shunt: Essentially, it's a resistor with a very small resistance. When current flows through it, Ohm's law generates a voltage across the resistor. The magnitude of the current is determined by measuring the voltage. Resistive shunts offer high accuracy, fast response, low cost, and ease of use. However, they are not inherently isolated and consume significant power when measuring large currents.
② Current Transformer (CT): A current transformer is a device that converts a large primary current into a small secondary current based on the principle of electromagnetic induction for measurement and protection. Its primary winding has very few turns and uses thick wire. The primary winding is connected in series in the circuit where the current needs to be measured, and the secondary winding must not be open-circuited. The advantage of a current transformer is its electrical isolation; the disadvantage is that it can only measure AC circuits.
③ Hall current sensor: This includes open-loop Hall current sensors and closed-loop Hall current sensors. A closed-loop Hall current sensor amplifies the output voltage of the Hall device, then amplifies it further by passing the current through a compensation coil. The magnetic field generated by the compensation coil is opposite in direction to the magnetic field generated by the measured current, compensating for the original magnetic field and gradually reducing the output of the Hall device, thus enabling the Hall device to operate at zero magnetic flux. An open-loop Hall current sensor generates a magnetic field in a toroidal core when the primary current IP flows through a long conductor. The magnitude of this magnetic field is proportional to the current flowing through the conductor. The generated magnetic field is concentrated within the magnetic ring and measured and amplified by a Hall element in the air gap of the magnetic ring. The output voltage proportionally reflects the primary current.
Closed-loop Hall current sensors offer higher measurement accuracy than open-loop sensors, but they also come at a higher cost.
From an application perspective, current transformers and Hall effect sensors both require a primary coil to generate a magnetic field. The differences lie in two aspects. First, current transformers require a changing magnetic field, while Hall effect sensors can have a constant magnetic field. Therefore, current transformers can only be used for alternating current (AC), while Hall effect sensors can be used for both AC and DC current. Second, current transformers have an iron core, while Hall effect sensors do not. The former is non-linear with respect to frequency, while the latter is linear. Therefore, current transformers have a narrower applicable frequency band, generally used in fixed frequency bands, while Hall effect sensors have a wider frequency band.
Open-loop Hall current sensor schematic diagram
Current Sensor Main Parameters and Selection
Standard Rating IPN: The rated value measured by the current sensor;
Accuracy: The error between the sensor's measured value and the actual value. When evaluating sensor accuracy, the effects of offset current and temperature drift must also be considered.
Linearity: Linearity determines how proportional the sensor's output signal is to the input signal within the measurement range;
Response time: Generally refers to the time difference between the output signal reaching 90% of its stable value and the input signal reaching 90% of its stable value;
Bandwidth: The current frequency that can be supported in current sensor applications;
Gain G_th: The ratio of output voltage to measured current, in mV/A.
When selecting a current transformer, factors such as measurement range, power supply, accuracy, cost, and installation method should be considered comprehensively.
Typical current acquisition circuit
Current transformer acquisition circuit
Hall current sensor acquisition circuit
