Five pitfalls in current transformer application circuit design
An advanced guide from "making it work" to "making it work well"
2026-07-14
In communication with numerous customers regarding current transformer applications, it has been observed that many engineers tend to follow "functional" solutions when designing current detection circuits. As long as the circuit works and the data roughly matches, the project is handed over for use. However, "functional" does not equate to "well-functioning" - often, just a few key details are missing between "functional" and "well-functioning", yet they can lead to qualitative differences in system accuracy, reliability, and dynamic range. This article addresses common misconceptions in current transformer selection and circuit design, outlines five design pitfalls that are easily overlooked, and provides corresponding solutions.
The first pitfall: incorrect placement of the sampling resistor. The current transformer outputs alternating current (AC). If a direct current (DC) voltage signal is required, rectification and current-to-voltage (I/V) conversion are involved. Many engineers are accustomed to first using a resistor to convert the AC current into an AC voltage, and then rectifying and filtering it into a DC voltage. This circuit can output a signal, but the accuracy is poor. The root cause lies in the distortion of the voltage-current relationship due to diode nonlinearity. The correct approach requires only minor modifications: placing the sampling resistor after the rectification circuit, utilizing the constant current characteristic of the current transformer, and allowing the transformer to automatically compensate for diode nonlinearity, thereby significantly improving detection accuracy. However, this method has certain requirements for the driving capability of the transformer. If the transformer output signal is weak, alternative solutions such as precision rectification are needed.
The second pitfall: improper load resistance matching. The load impedance of a current transformer directly affects its accuracy. When the load impedance exceeds the transformer's tolerance range, the excitation current increases sharply, and the magnetic core tends to saturate, leading to a significant increase in measurement error. Some engineers arbitrarily increase the sampling resistor value in order to obtain a higher ADC input voltage, resulting in the transformer's accuracy specifications being completely compromised. The correct approach is to consult the CT data sheet or consult the manufacturer before designing the sampling circuit, confirm its rated load range, and ensure that the sampling resistor value does not exceed this limit. Only in this way can the factory-stated accuracy level of the transformer be achieved.
The third pitfall: directly feeding the AC signal into a unipolar ADC. The AC signal output by the CT is a bipolar signal centered around 0V, while most microcontroller ADCs can only accept positive unipolar signals. Directly feeding the AC signal into the ADC will result in the negative half-cycle signal being unrecognized, leading to severe abnormalities in the sampled data. There are three common solutions to this problem: First, software can be used to only convert and process the positive half-cycle signal, which is suitable for applications where accuracy is not critical. Second, a rectifier circuit can be employed to convert the bipolar signal into a unipolar signal. Third, a DC bias voltage can be superimposed on the AC signal to raise the entire signal within the input range of the ADC, and then the original AC value can be restored through software. The third solution is the most commonly used, but attention must be paid to the accuracy and temperature drift issues of the bias circuit itself, which can be compensated through differential sampling or software calibration.
The fourth pitfall: Focusing solely on the transformation ratio while neglecting the accuracy class and saturation characteristics. Many engineers, when selecting current transformers (CTs), only pay attention to the transformation ratio, overlooking the differences in accuracy class and saturation characteristics. Measurement CTs maintain high accuracy within the rated operating current range but quickly saturate under high-current faults to protect downstream equipment. Protection CTs, on the other hand, need to maintain linear transformation even at currents several dozen times the rated current to ensure correct operation of the protection device. If a measurement-grade CT is used in a protection application, rapid saturation of the magnetic core can lead to protection malfunctions; if a protection-grade CT is used in a metering application, its accuracy under normal loads may not meet requirements. When selecting a CT, it is essential to consider the specific application scenario, comprehensively evaluate the accuracy class, rated load, and saturation characteristics, and choose the appropriate product.
The fifth trap: open circuit on the secondary side of the current transformer. This is the most dangerous safety hazard. When the CT operates normally, the secondary current exerts a demagnetizing effect on the primary current, resulting in a very low voltage at the secondary terminal. Once the secondary side is open-circuited, the demagnetizing effect disappears, and the primary current becomes entirely excitation current. The magnetic core quickly saturates deeply, and due to the large number of turns in the secondary winding, it may induce a high voltage of thousands of volts, which is sufficient to break down insulation and endanger life. Therefore, it isabsolutely impermissible to install a fuse in the secondary circuit of the current transformer, nor is it allowed to switch the circuit arbitrarily during operation. If switching is indeed necessary, reliable measures to prevent open circuits must be taken in advance.
Upon reviewing the aforementioned five issues, it becomes evident that they share a common ground: the circuit "functions," yet the designer lacks a genuine comprehension of the fundamental characteristics of the current transformer as a "current source." Additionally, there is insufficient attention paid to crucial aspects such as load matching, signal conditioning, selection and adaptation, as well as safety standards. The gap between "functionality" and "optimal performance" lies precisely in the oversight of these finer details
Shenzhen Deheng Technology Co., Ltd. has been deeply involved in the field of precision current sensors for many years. Its product line covers Hall effect current sensors, fluxgate current sensors, precision current transformers, current transmitters, and combined current and voltage transformers, with accuracies up to 0.01%. The company not only provides high-quality precision current transformer products but has also provided complete current transformer application solutions for customers in numerous industries, including new energy vehicles, charging piles, power distribution equipment, communication power supplies, IoT, instrumentation, smart homes, power systems, photovoltaic power generation, energy storage equipment, and rail transit. From product selection and load matching to sampling circuit design and signal processing, Deheng helps users overcome the hurdle from simply "using" a device to "using it effectively." If you encounter problems such as poor accuracy, poor linearity, insufficient range, or selection difficulties when designing current detection circuits, Deheng Technology's technical support team can provide you with targeted application solutions and suggestions.
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