Fault inspection and treatment of transmitter 4-20mA circuit under different power supply modes

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Fault inspection and treatment of transmitter 4-20mA circuit under different power supply modes [Copy link]

Two-wire transmitters and three-wire transmitters are widely used in industrial sites, and 4-20mA loop failures occur from time to time. This article shares the experience of fault inspection and handling of 4-20mA loops of two-wire transmitters and three-wire transmitters under different power supply modes. 1. Inspection and handling of 4-20mA loops of conventional two-wire transmitters Figures 1 and 2 are conventional loops of two-wire 4-20mA signals, with Figure 1 being powered by a DCS card and Figure 2 being powered by a DC power supply. When judging the fault, you can use the DC voltage block of the multimeter to measure the voltage value of the V+ terminal to the ground or to the negative terminal of the power supply. The transmitter can work normally when it is around 21-24V; when the circuit in the transmitter is open, the measured voltage will be slightly higher than 24V. If the measured voltage is 0V, it may be that the power supply of the transmitter is interrupted. If the measured voltage is very low, lower than 12V (analog transmitter) or 18V (intelligent transmitter), it may be that the circuit is short-circuited. For further judgment, you can disconnect the V+ terminal and measure the voltage. If it is still 0V, it may be that the power supply is interrupted or the wiring is broken, or the positive line is grounded, or the positive and negative lines are short-circuited. If the voltage is still very low, it means that there is a short circuit fault in the output circuit. Figure 1: 4-20mA circuit of two-wire transmitter powered by DCS card Figure 2: 4-20mA circuit of two-wire transmitter powered by DC power supply Measuring the voltage together with measuring the loop current can make the fault judgment more accurate, that is, connect a multimeter in series at the I+ terminal to measure whether the transmitter output current is within the range of 4-20mA. You can also indirectly judge whether the transmitter is normal by measuring whether the voltage across the 250Ω resistor is within the range of 1-5VDC. If the measured current is within the range of 4-20mA, it means that the transmitter can work normally; if the measured current is ≥20mA, it may be that the load is short-circuited or there is a ground fault in the output circuit. If the measured current is 0, it may be that the power supply of the transmitter is interrupted or the wiring is broken, or the fuse in the DCS card is open, or the power supply protection circuit of the card is activated. If the measured current is ≤4mA or ≥20mA, it may be that the transmitter is faulty, and there is grounding in the output circuit, causing current shunting or parallel flow. Sometimes it may be difficult to determine the fault by measuring the current on only one line. For example, I have encountered on site that the output current value of the transmitter is consistent with the display value on the handheld operator, but the DCS screen shows a bad pixel. I checked that the DCS card is normal. The current measured by connecting to the V+ terminal is 16mA (see Figure 1), which is consistent with the display on the transmitter head. However, the current measured at the I+ terminal of the DCS card is only 3mA, which is the input signal of the DCS. Since it is less than 4mA, the DCS screen shows a bad pixel. Where did the 13mA current go? The current from the Ic terminal to the I+ terminal is shunted. The conclusion is that only one signal negative line is leaking. The negative line is indeed leaking with a megohmmeter. After the insulation treatment of the wire, the DCS screen display is consistent with the transmitter display. When judging whether the signal line has a short circuit, open circuit, or leakage grounding fault, it is still necessary to measure the current at the transmitter end and the DCS card end respectively. Fault inspection and processing of the transmitter 4-20mA circuit under different power supply modesyunrun.com.cn/tech/2139.html 2. Inspection and processing of the circuit with safety barrier Figures 3 and 4 are circuits of two-wire 4-20mA signal with safety barrier. Figure 3 is the power supply of DCS card (the safety barrier is a passive safety barrier that obtains energy on the output side. For the functions and characteristics of this safety barrier, please move to Changhui Instrument Network to learn more ), and Figure 4 is several forms of safety barrier power supply. Fault judgment can refer to the method of conventional circuits, and it is also necessary to judge whether the safety barrier has a fault. The safety barrier can judge whether it is normal by measuring the current at the input and output ends, but some safety barriers can only judge the fault by measuring the voltage at both ends of the 250Ω resistor. You can also observe whether the fault light of the safety barrier is always on. If the red light is always on, there may be a fault in the input circuit, such as a broken wire, loose wiring, or a problem with the power supply voltage of the safety barrier. If the two sets of signals of the one-input and two-output safety barrier are inconsistent, first determine whether there is a problem with the safety barrier or the card. If the two sets of signals of the safety barrier are consistent, first check whether the card is normal. You can try to switch the output signal of the safety barrier to an empty AI channel to observe and judge. If the two sets of signals are still inconsistent after the replacement, you should check whether the settings are correct, whether the ranges of the two channels are consistent, and whether the line settings of the card are mixed up. Figure 3 Two-wire 4-20mA circuit of the DCS card power supply equipped with an output side energy acquisition safety barrier Figure 4 Two-wire transmitter 4-20mA circuit powered by a safety barrier 3. Inspection and processing of the three-wire transmitter 4-20mA circuit Figures 5 and 6 are three-wire 4-20mA signal circuits. The power supply of the three-wire transmitter is separately supplied, and the negative end of the power supply and the negative end of the signal share a wire. Safety barriers should be used in situations where explosion-proof requirements are required or where a common ground is not possible. The power supply is mostly provided by the safety barriers, as shown in Figure 6. Fault judgment can refer to the above output circuit method. Figure 5 4-20mA circuit of a three-wire transmitter powered by a DCS card Figure 6 4-20mA circuit of a three-wire transmitter powered by a safety barrier