You can sense current using a series resistor and measuring the voltage drop across it. According to Ohm’s law, the current through the resistor is then the voltage drop divided by the resistance value. That makes the voltage drop proportional to the value of the resistance and the current flowing through it. The disadvantage is obvious – to prevent the voltage drop from affecting the circuit parameters, one needs a very low value resistor when the current involved is high. Additionally, as the current reduces, so does the voltage drop. That involves amplification of the voltage drop, creating additional circuit complexity.
Ideally, current sensors should not use any power when detecting the current in the circuit. However, real current sensors do require a part of the energy from the circuit for providing the information. For sensing AC currents, current sense transformers are typically useful. A single wire from the circuit acts as the primary of the transformer or the primary may be a single turn winding on the transformer.
The AC current sense transformer develops a current in the secondary, proportional to the sensed primary current. The secondary current is allowed to flow through the terminating resistor to produce an output voltage. As the turns ratio of the transformer decides the secondary current, a low turns ratio (pri/sec << 1) minimizes the current through the terminating resistor. A balance of the transformer ratio and low-enough current through the terminating resistor ensures adequate output voltage. You select the appropriate AC current sensor based on the frequency range and current rating of the sensor for the conditions of your application. The highest flux density to prevent saturation of the sensor core will then depend on the worst-case current and frequency conditions in the circuit. The requirement is to generate a voltage output from the sensor that will vary linearly with the current being sensed. If the core saturates, the output becomes non-linear, and the output voltage is no longer strictly a representative of the input current. Sensors come in surface mount or through hole types, with different turns ration and overall dimensions. As noted earlier, you can have a sensor only type, which has a conductor integral to the application serving as the primary. The other is a current transformer type, where the primary is an included winding. Current transformer manufacturers offer online selection tools for selecting the right current sensor for the specific application. Initially, the user selects either an SMT sensor or a leaded type of sensor. The tool then requires the user to input the maximum sensed current expected, the input frequency, the duty cycle of the primary current waveform and the desired output voltage. The output voltage being the desired output voltage for the maximum input current the user expects. Based on the maximum input current, the number of secondary turns and the output voltage necessary, the tool suggests the required terminating resistor value. For this calculation, the tool assumes a single-turn primary. The tool also provides the maximum flux density based on the above parameters and the maximum operating frequency, making sure the value does not exceed 2K Gauss to ensure linearity.