Current transformers are typically used as control, circuit protection and monitoring devices in applications such as switching power supplies, motor current load sensing, lighting and instrumentation. With the current transformer spot growing, how to choose a suitable current mutual inductor transformer need to consider many factors, this article describes a simple selection method, this method in many applications for the selection of suitable cost-effective device is very helpful. While spot devices are inexpensive and straightforward, there are some functional limitations in their use, and some applications may require special products that may even need to be fully customizable. The choice of current transformer transformers to consider a variety of factors, such as size, frequency, function and current range. Input Current First of all, the choice of current transformer must be clear and verify a number of indicators, such as size, frequency, function and sampling current range. Its accuracy and efficiency actually depend on these parameters. In addition to the trade-offs that can be made in the accuracy of a current transformer, if the current transformer transformer exceeds the manufacturer's rated current rating, its operating temperature will continue to rise and become uncontrollable resulting in circuit failure. In addition, if a current transformer is rated much more than its "sample current," the size of the device will inevitably be large and too expensive for its application. In general, it is a good idea to select a current transformer transformer approximately 30% above the maximum expected value of its "sample current". Turns ratio Common current transformer transformer turns ratio range from 1:10 to 1: 1000 range. The higher the turns ratio (r = Nsec / Npri), the higher the current measurement resolution. However, it is noteworthy that an excessively high turns ratio will result in an increase in distribution capacitance and leakage inductance, thereby reducing the accuracy of current transformer transformers and operating performance at high frequencies due to self-resonance. However, if the turns ratio is too low (low inductance), then the output signal may be distorted or "droop" (single-stage input signal must be skewed), causing the control circuit to be unstable and the measurement inaccurate. Inductance and excitation current The secondary inductance of the current transformer determines the fidelity of the output signal. The value of the inductance is inversely proportional to the excitation current, which is commonly known as the "induced current". In order to ensure the maximum fault tolerance of the current transformer, the excitation current should be several times smaller than the magnitude of the sampled current. For most applications such as switching power supply, take the sampling current of 10% as the maximum excitation current is more ideal. For example, if a circuit must guarantee a maximum of 10% loss for a sample current of 1 to 20 A at 100 kHz, then the maximum excitation current must be set at 100 mA (10% of the minimum sample current). A sample current of 1 A will produce a 10% error and a 20A sample current will produce a 0.5% error. If the manufacturer's data sheet does not indicate the excitation current, then it can be calculated by the following formula: e = CLdI / dt | dI / dt | = e / L Where e is the device output voltage in V, L is the inductance in H, and dI / dt is the excitation current to time ratio in A / s. Output voltage and load resistance The output voltage (Vo) should be set as low as possible to reduce the insertion loss. Assuming the optimal secondary output voltage of a circuit is 0.5V and the output current is 20A, a current transformer of 1: 100 turn ratio will generate a secondary current of about 200mA. As shown in Figure 2, the load resistance should be: Ro = Vo / Is = 0.5 / 0.2 = 2.5Ω.
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