Choosing the correct current sensor does not require guessing

● application area 

Current sensors detect AC or DC currents in wires or circuit wiring. They can be used to detect on/off/pulse current conditions or measure the current magnitude in wires or traces. This discussion is limited to AC current sensors.

An ideal current sensor does not use any power to detect current in wires or traces, but actual current sensors require some circuit energy to provide information. Current sensors are often used to measure and control load currents in power supplies, safety circuits, and various control circuits. In applications that require current control, such as in power supplies, accurately sensing the magnitude of the current is a fundamental requirement.

In pulse current applications or situations where only the conduction state needs to be detected (such as certain safety circuits), precise amplitude of the current may not be required. In other safety circuits, when the current exceeds the preset limit, the sensed current can be used to trigger shutdown. 

●technology

Resistors can be used to sense current by measuring the voltage drop across the resistor. According to Ohm's law, the sensing current I=V/R. The use of low value resistors in series with the measured current can minimize losses caused by voltage drop and dissipation. This sounds simple, but due to the low voltage drop across such a small resistor, it may be necessary to amplify the voltage to detect it, thereby increasing the complexity of the circuit. The shunt current sensor samples a small portion of the sensed current. The current is diverted through parallel resistors and the voltage drop is measured. Like a series resistor, the voltage drop is proportional to the sensed current. Current detection transformers are commonly used for AC current detection. These current sensing devices can use a single wire in the circuit as the primary of the transformer, or they can provide the primary winding.

These AC current detection transformers generate a current in the secondary that is proportional to the current detected in the primary. The secondary current is measured by the voltage drop across the terminal resistor (RT). By using low turn ratio current transformers (pri/sec<<1), the current passing through the terminal resistor can be minimized to the greatest extent possible. This will also reduce the voltage generated across the terminal resistor, and if the output voltage is too low, amplification may be necessary. The selection of transformer turns ratio and terminal resistance must balance the requirements of low current consumption and sufficient output voltage.

Choosing an appropriate current sensor/transformer requires determining the frequency range and current rating of the sensor based on your application conditions. The type of sensor, installation (surface mounting or through-hole), turns ratio, and overall size are additional factors to consider. The sensor type can be "sensor only", where the integrated conductor in the application is used as the primary, or it can be a current transformer that includes the primary.

The worst-case current and frequency determine the highest magnetic flux density that can be seen by the sensor or transformer. For most AC current sensors, exceeding 2000 Gauss means that the output is non-linear relative to the sensed current, and the output voltage is no longer strictly proportional to the input current. A higher number of secondary turns helps to keep the magnetic flux density below this limit. For through-hole current sensors, if the wire size and hole size allow, the turns ratio can be significantly reduced by adding additional turns (one turn through the hole is one turn). This allows the use of higher input current transformers to provide higher output voltage on the terminating resistors.

This tool calculates the required terminal resistance (RT) based on the maximum input current (Ipri), secondary turns (Nsec), and output voltage (Vout): RT=Nsec × Vout/Ipri (calculated based on 1). The tool also calculates based on the output voltage (Vout), duty cycle, and secondary turns

Calculate the maximum magnetic flux density of the secondary with frequency to ensure it does not exceed 2000 Gauss.

Conclusion:

Choosing a suitable current detection transformer requires knowledge of the expected maximum detection current, the frequency and duty cycle of the detection current, and the required output voltage corresponding to the expected maximum detection current.