How to Select Current Sensors?

To select an appropriate AC current sensor for an application, you must know the operational frequency range and the current rating the sensor will encounter. Additional considerations that you will need to decide are the type of the sensor, its mounting (through-hole or surface mount), turns ratio, and the overall dimensions.

Sensor type refers to a sensor only configuration, where a conductor integral to the application forms the primary. Another type could be a complete current transformer where the primary is included as a winding. Engineers typically use current sensors to measure and control the load current in control circuits, safety circuits, and power supplies. Power supplies usually require accurate control of current, and this requires sensing the magnitude of the current accurately.

Irrespective of whether you are using the sensor or transformer, the highest flux density handled is dependent on the worst-case current and frequency faced by the device. However, note that exceeding 2000 Gauss will mean most AC current sensors output will be non-linear. Therefore, the current through the sensor and its output voltage will no longer remain proportional, as the magnetic core of the sensor saturates at very high flux densities. To keep the flux density below the saturation limit, it is necessary to use higher secondary turns.

For instance, in wire-through-the-hole style of current transformers, looping additional primary turns through the hole can dramatically reduce the turns ratio, provided the wire diameter and the hole size permit. Increasing the primary turns allows the use of a higher input current transformer to provide higher output voltage across the terminating resistor on the secondary.

Manufacturers of current transformers offer online tools to help designers select the right current sensor or current transformer for specific application conditions. Initially, the user has to select the type of sensor—a transformer or a sensor only. The next selection is the preferred mounting style—SMT or Through-hole. The online tool also requires other parameters such as the maximum sensed current expected in amperes, the input frequency in kHz, the duty cycle of the primary current waveform as a percentage, and the desired output voltage corresponding to the expected maximum input current.

The tool then calculates the required terminating resistance based on the maximum input current, the number of secondary turns and the output voltage—basing the calculations on a single-turn primary. Next, the tool calculates the maximum flux density of the secondary, making sure it does not exceed 2000 Gauss. It does this by taking into account the output voltage, the duty cycle, secondary turns, and the frequency of operation.

The result lists all part numbers of the manufacturer that meet these input conditions, typically including a graph of the output voltage versus the sensed current for the calculated terminating resistance.

To select an appropriate current sense transformer for your application, you require knowledge of the maximum current, frequency, and duty cycle of the sensed current, including the output voltage you require. Using this information, the online selector tools will provide you with the appropriate terminating resistor value and a list of current sensors that meet the conditions of the application.