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Failure of 4-20mA Pressure Transducer in Different Power Supply

For 4-20mA pressure transducer, two-wire and three-wire are used more in industrial production. The 4-20mA circuit failure often occur. This article will share the troubleshooting and processing experience for the 4-20mA circuit in the two-wire pressure transducer and the 4-20mA circuit in the three-wire pressure transducer in different power supply modes.

1. Inspection and treatment for 4-20mA circuit of conventional two-wire pressure transducer

Figure 1 and Figure 2 show the conventional circuit of the 4-20 mA signal for a two-wire pressure transducer. Figure 1 shows the DCS card power supply, and Figure 2 shows the DC power supply. When judging the fault, the voltage value of the V+ terminal to the ground or the negative terminal of the power supply can be measured by the DC voltage file of the multimeter. The pressure transducer can work normally when the voltage is in the range of 21-24V. When the circuit inside the pressure transducer is open, the measured voltage will be slightly higher than 24V. If the measured voltage is 0V, it may be because the power supply of the pressure transducer is interrupted. If the measured voltage is low and below 12V (analog transducer) or 18V (smart transducer) it may be a short circuit in the circuit. For further judgment, the V+ terminal can be disconnected and measure the voltage. If the voltage is still 0V, it may be a power interruption or a broken wiring. It is also possible that the positive line is grounded or the positive and negative lines are shorted. The voltage has been low, indicating a short circuit fault in the output circuit.
4-20mA circuit of two-wire pressure transducer powered by DCS

4-20mA circuit of two-wire pressure transducer powered by DC power supply
When measuring the voltage, the current measurement to the measuring circuit can be matched to make the fault judgment more accurate. The specific approach are as follows. Connect the multimeter at the I+ terminal and measure whether the output current of the pressure transducer is within the range of 4-20 mA. The measured current ≥ 20mA may be due to a short circuit in the load or a ground fault in the output circuit. If the measured current is 0mA, it may be because the power supply of the pressure transducer is interrupted or the pressure transducer wiring is broken. It is also possible that the insurance circuit in the DCS card is open, or the power supply of the card has a protection circuit action. If the measured current is ≤ 4mA or ≥ 20mA, the pressure transducer may be faulty. The presence of ground in the output circuit results in current shunting or cocurrent. If the measured current is in the range of 4-20 mA, the pressure transducer can work normally. In addition, users also can measure that whether the voltage at both ends of the 250Ω resistor are within the range of 1-5VDC. It can indirectly determine whether the pressure transducer is normal.
In some cases, it may be difficult to determine the fault by only measuring the current on one line. For example, the output current value of the pressure transducer is consistent with the displayed value on the handheld device, but the DCS screen is displayed as a dead pixel. At this time, check if the DCS card is normal. The current measured at the V+ terminal is 16mA (see Figure 1), which is consistent with the pressure transducer head display. However, the current measured at the I+ terminal of the DCS card is only 3 mA. This current is the input signal of the DCS. Since the current is less than 4 mA, the DCS screen appears as a dead pixel. So where is the current of 13mA? The current from the I- to the I+ is shunted. As a result, only one signal negative line has leakage. After detecting the negative line with a megohmmeter, it is found that the leakage is indeed. After the wire is insulated, the DCS screen display is consistent with the pressure transducer display. When judging whether the signal line has short circuit, open circuit, electric leakage or ground fault, it is necessary to measure the current of the pressure transducer end and the DCS card end respectively.

2. Inspection and treatment for safety barrier circuits in use with pressure transducers

Figure 3 and Figure 4 are the circuit of the two-wire pressure transducer 4-20mA signal with safety barrier. Figure 3 shows the DCS card power supply and Figure 4 shows the safety barrier power supply. For the fault judgment, in addition to the method of referring to the conventional circuit, it is also necessary to judge whether the safety barrier is faulty. It is possible to judge whether the safety barrier is normal by measuring the current at the input and output. However, some safety barriers can only judge the fault by measuring the voltage at both sides of the 250Ω resistor. It can also be observed whether the fault light of the safety barrier is always bright. The red light may be because the input circuit is faulty, such as disconnection, loose wiring, and problem with the safety barrier supply voltage. If the two sets of signals of the safety barrier with one input and two exit are inconsistent, first determine whether there is a problem with the safety barrier or the card. The output signal of the barrier can be switched to an empty AI channel for observation and judgment. If the two groups of signals are still inconsistent after replacement, check whether the settings are correct, whether the two-channel range is consistent, and whether the card's wire system settings are confusing.
4-20mA circuit of two-wire pressure transducer powered by DCS with safety barrier

4-20mA circuit of two-wire pressure transducer powered by safety barrier
3. Inspection and treatment for 4-20mA circuit of three-wire pressure transducer

Figure 5 and Figure 6 are the circuit of the three-wire pressure transducer 4-20mA signal. The power supply of the pressure transducer are separately powered. The negative terminal of the power supply and the negative terminal of the signal share a single line. Safety barriers should be used where explosion-proof or non-communication is required. Most of its power supply is provided by the safety barrier. As shown in Figure 6, the fault determination can be performed by referring to the method of the output circuit above. 4-20mA circuit of three-wire pressure transducer powered by DCS

4-20mA circuit of three-wire pressure transducer powered by safety barrier

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