Working principle and circuit diagram of the isolation safety barrier at the detection end

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http://yunrun.com.cn/upload/201901/02/201901021500599827.png The detection terminal isolation barrier serves as a link between the field two-wire transmitters and the instruments and power supplies in the control room. It provides power to the two-wire transmitters, while simultaneously transmitting the signal current from these transmitters to the instruments in the control room via an isolation transformer on a 1:1 basis. During the aforementioned transmission process, a dual voltage and current limiting circuit is used to ensure that the voltage and current supplied to hazardous areas do not exceed 30V and 30mA (DC) under any circumstances, thereby ensuring the safety of those areas. http://yunrun.com.cn/upload/201901/02/201901021425282476.png Figure 1: Block diagram of the principle of the isolation safety barrier at the detection end. Figure 1 shows the block diagram of the isolation safety barrier used at the detection end in a certain factory. A 24V DC power supply is converted into an 8kHz AC voltage using a DC-AC converter; this AC voltage is then transmitted through transformer T1. One path of this voltage is processed through rectification, filtering, as well as voltage and current limiting circuits to supply power to the transmitter (still in the form of 24V DC), while another path is processed through rectification and filtering to supply power to the demodulation amplifier. The 4-20mA signal current obtained from the transmitter passes through a voltage and current limiting circuit before reaching the modulator, where it is modulated into an alternating current. This alternating current is then coupled to the demodulation amplifier via transformer T2; after demodulation, it is restored to a 4-20mA direct current signal, which is sent to the instruments in the control room. Therefore, from the perspective of signal transmission, the detection-side isolation barrier acts as a transmitter with a transfer coefficient of 1; the signal to be transmitted is sent out unchanged after going through the processes of modulation → transformer coupling → demodulation. Here, apart from the flux connection, the power supply, transmitters, and control room instruments are electrically isolated from one another. Figure 2 shows a simplified schematic diagram of the isolation safety barrier at the detection end. Figure 2 is a simplified principle diagram of this type of isolation safety barrier at the detection end; the various components are described below with reference to Figure 1. The power supply DC-AC converter is composed of transistors VT1 and VT2, diodes VD1-VD4, and transformer T1, among other components. This is a magnetically coupled self-oscillating multivibrator. For reliability in the detection terminal isolation barrier of circuit diagram 2 for transistor-based voltage and current limiting, two identical sets of voltage and current limiting circuits are used in series; one set consists of transistors VT3, VT4, and zener diode VD15, while the other set consists of transistors VT5, VT6, and zener diode VD16. Figure 3 Voltage and current limiting circuit. For ease of explanation, one such circuit is shown in Figure 3; transistor VT4 and the transmitter are connected in series to perform voltage and current limiting functions. The base circuit of VT4 is controlled by transistor VT3; under normal operation, VT3 is off. VT4 receives sufficient base current through resistor R7 from the rectified and filtered voltage across capacitor C3, resulting in it being in a saturated conducting state. As a result, the 4-20mA signal current from the transmitter can flow smoothly through the voltage and current limiting circuit. By examining the base-emitter circuit of transistor VT3, it can be seen that if the voltage drop across resistors R5 and R6 exceeds 0.6V, VT3 will begin to conduct, thereby reducing the base current of transistor VT4. If the current through VT3 is high, most or all of the current passing through R7 will flow through VT3 rather than reaching the base of VT4, causing transistor VT4 to exit saturation and enter the amplification or cut-off region. There are two possible reasons for this situation in the circuit: ① Overvoltage at the power supply: The breakdown voltage of the Zener diode VD15 in Figure 3 is approximately 30V. If the rectified voltage across the filtering capacitor C4 exceeds 30V, the Zener diode VD15 conducts, providing current to the base of transistor VT3 via resistor R4. This causes VT3 to conduct and to draw away the base current from VT4, resulting in VT4 turning off. As a result, the voltage UAB supplied to the field decreases, thereby serving to limit the voltage. ②Overcurrent in the transmitter: When the signal current across resistor R6 is within the normal range of 20mA, the voltage drop does not exceed 0.6V. Additionally, due to the presence of resistor R5 (18kΩ), even if the voltage drop across R6 exceeds 0.6V slightly, VT3 will not conduct sufficiently. When the transmitter current exceeds around 25mA, the voltage drop across R6 will gradually cause VT3 to conduct fully, thereby taking away the base current from VT4 and enabling VT4 to function in order to limit the current flowing into the field to within 30mA. Figure 4 Pressure and current limiting characteristic curves The characteristics of the aforementioned pressure and current limiting circuit are shown in Figure 4. When the rectified voltage UC4 across the filtering capacitor C4 is less than 30V, the output voltage UAB = UC4, and the transistor VT4 does not play any role in voltage limiting. However, when UC4 > 30V, VT4 quickly tends to turn off, and as UC4 increases, UAB quickly drops to zero. Similarly, the current-limiting function of the circuit is also achieved by reducing the output voltage UAB through transistor VT4. It should be noted here that the voltage rating of the voltage-limiting and current-limiting transistors VT4 and VT6 in Figure 2 must be high enough. Because when there is an overvoltage on the power supply, VT4 and VT6 are both in the off state, so the entire overvoltage is applied to these two transistors. Currently, the supply voltage for isolation safety barriers in mainstream detection systems is DC24V. Finally, let’s discuss the modulation and demodulation amplifiers. The principle circuit of this part can be drawn separately as shown in Figure 5. The power supply for the two-wire transmitter is provided by full-wave rectification through diode VD9, and the modulation-demodulation amplification circuit in Figure 5 via diodes VD10, VD13, and VD14. Since VD13 and VD14 operate alternately during the positive and negative half-cycles of the power supply, by connecting the upper and lower halves of the primary coil of transformer B2 to these two diode circuits respectively, the 4-20mA DC signal current from the transmitter will flow alternately into the upper and lower parts of the transformer’s primary coil, resulting in a square-wave voltage at its secondary side. Here, transformer T2 operates in the mode of a current transformer, with its secondary load impedance being very small. Thus, when the turn ratio of the primary and secondary coils is 1:1, the amplitude of the secondary square wave current is equal to the primary current. Since the signal current is unidirectional, the demodulation problem is simple; it only requires full-wave rectification of the secondary current of current transformer T2. To produce a constant current output, a common-base circuit is used here for rectifying and amplifying. Given that in a common-base amplification circuit, the larger the β of the transistor, the closer the ratio of the input current (emitter current) to the output current (collector current) is to 1, composite transistors composed of VT7, VT8 and VT9, VT10 are used in the demodulation amplification circuit to increase the equivalent β value and improve operational accuracy. In Figure 5, the square-wave current at the secondary side of current transformer T2 serves as the input current for the composite transistor. After passing through the common-base amplification circuit, two half-wave constant-current outputs are generated; by adding these together, a 4-20 mA DC current equal to the original signal current is obtained. This current can be supplied directly to the instruments in the control room, or it can be converted into a 1-5V voltage output via resistor R13 (250Ω). The Zener diode VD19 is a freewheeling diode at the current output terminal, with a breakdown voltage of 6–7V. It does not operate when a normal load is connected to the current output terminal; once the external load circuit is disconnected, VD19 takes over automatically to ensure that the output circuit remains connected. The circuit diagram of the detection-side isolation barrier shown as an example in the text is not that of any advanced technological product; it is intended solely for explaining the principle of detection-side isolation barriers, and should not be used as a reference. Source: Changhui Instruments http://yunrun.com.cn/