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Transmitter principle and troubleshooting

by:Sure     2021-08-07
1 Testing of sensor components When the sensor is faulty, it is generally not possible to repair it on site, only to replace it. If no phenomena such as damage to the isolation diaphragm or oil leakage are found, the sensor assembly can be tested according to the following steps: 1. Carefully pull out the lead-out socket of the sensor assembly from the plug socket. 2. Check the forward and reverse biases of the internal diode circuit: one circuit is a test of the red wire and yellow 1 sensor assembly. When the sensor is faulty, it is generally not repaired on site, only replaced. If no phenomena such as damage to the isolation diaphragm or oil leakage are found, the sensor assembly can be tested according to the following steps: 1. Carefully pull out the lead-out socket of the sensor assembly from the plug socket. 2. Check the positive and reverse biases of the internal diode circuit: one circuit is the red wire and the yellow wire, and the other circuit is the green wire and the blue wire. Connect the positive pole of the multimeter to the red wire and the negative pole to the yellow wire. The series circuits D1, D2, D5 The sum of resistance values u200bu200bof, D6 and R4 should be close to or equal to the sum of resistance values u200bu200bof the series circuit D3, D4, D7, D8 and R5 of the green line and the blue line. 3. Check the resistance of the sensor assembly shell and the four-wire, that is, check the resistance between the capacitor plate and the sensing diaphragm connected to the shell, and the resistance should be greater than 10MΩ. 2. Troubleshooting The fault locations of capacitive transmitters are generally divided into two categories: sensor faults and electronic detection and amplification conversion faults. Sensors are generally not prone to problems, and there is generally no doubt under the normal conditions of detection according to the above methods. Should focus on the detection circuit part. The failure phenomena of 1151 capacitive transmitter are mainly as follows: 1. The output is too large. In the absence of pressure (differential pressure), the output current (mA) of the transmitter is too large, sometimes exceeding the range, adjust the zero point and The range potentiometer does not work. There are many parts of the circuit that have this fault, and it is the rear pole of the circuit that is often damaged, that is, the current control amplifier to the current control output part. IC3 is a voltage amplifier converted to mA output by V17. Measure the potential value of IC3 pin 3 with a multimeter DC voltage block, adjust the range or zero point, the voltage of pin 3 should change, indicating that the front pole circuit is normal. At the same time, the output of pin 6 should have an amplified voltage signal output. In order to judge whether IC3 and the peripheral bias loop are normal. If the voltage of pin 6 is always high (close to the power supply voltage), it can be judged that IC3 is damaged, just replace the same model LM308. If IC3 is normal, use a multimeter to measure whether V17 and V18 are broken down. In actual testing, V17 and V18 breakdowns are often encountered and the output is too large. 2. The output is too small or no output. Check whether the voltage applied to both ends of the transformer is normal; whether the voltage at both ends of the integrated operational amplifier is normal; whether the oscillation circuit is vibrated; whether the range, zero point, and potentiometer adjustment voltage change and whether the load resistance of the 15Ω range is normal damage. Use the method of detecting that the output is too large to detect the rear pole circuit and judge whether the rear pole circuit is normal. 3. The output is unstable. Check whether the transformer has intermittent short circuit, open circuit and multiple grounding phenomena; check whether the voltage applied to the transformer is stable and normal; whether the voltage stabilizing circuit is normal; check each zener diode and test whether the oscillation frequency is stable; Whether the circuit board is soldered or not. 4. Output non-linearity. This type of fault occurs relatively more and is difficult to detect. The deterioration of various parts of the circuit may cause this type of fault. But it mainly appears in the non-linear adjustment circuit part. Use a multimeter to electrically block to detect whether the diodes V9, V10, linear potentiometer R24, resistors R23, R22 and C3, C6, C4 are normal, and adjust the output of R24 to see if there is a corresponding change. The damage of the above components often causes poor linearity. In the long-term operation of capacitive pressure and differential pressure transmitters, various faults have occurred. When dealing with faults, careful observation and analysis should be used to determine the general location of the fault and the steps to solve it, and then debug and test by hand, so that you can get twice the result with half the effort. In addition, the user is reminded here that the instrument must be adjusted and qualified after maintenance before it can be put into normal use.
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