Figure 1 Schematic diagram of an AC current transmitter.
As shown in Figure 1, the system primary current I1 is converted by the current transformer into a standard secondary current I2 (5A, 1A). I2 enters the transmitter through the input end of the transmitter, first flows through the bridge rectifier circuit composed of diodes D1 - D4 to realize AC / DC conversion, wherein the varistor RV protects the input signal from overvoltage. The rectified DC signal is filtered by the capacitor C1 and then generated by the sampling resistor RL to generate a 0 - 5V DC voltage signal, and then filtered by the DH420 transmission module to a 4- 20mA DC current signal after being filtered by the capacitor C2. The output of the output is input to the dcs /plc system to measure the primary current I1 of the system. Among them, the multi-turn adjustable potentiometers RP1 and RP2 respectively realize the setting function of the full scale 20 mA and the zero position 4 mA; the diode D6 realizes the polarity protection function of the output signal; the transient protection tube TVS takes the transient induction of the output signal end Overvoltage suppression protection of lightning and surge currents.
It is worth noting that the TVS diode is a diode designed to suppress overvoltage. It is based on the common Zener diode. Compared with ordinary Zener diodes, TVS diodes have a silicon PN junction with a large cross-sectional area. When the PN junction operates in an avalanche state, the pulse absorption capability is stronger than that of a normal diode.
TVS diodes are available in both unipolar and bipolar. The unipolar TVS diode protects only the impulse voltage in one direction, which is equivalent to a Zener diode, which is clamped by its volt-ampere characteristics. The working principle of the bipolar TVS diode is equivalent to the reverse series connection of two Zener diodes, which can protect the impact voltage of both positive and negative poles. TVS diodes have low clamping voltages (down to 12V), fast response times (up to 10-12 s), and low inter-electrode capacitance for signal and data line protection. However, due to its small current capacity (101 ~ 102A), it is generally not suitable for primary and secondary protection of power supplies.
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