LCD Display Inverter

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On the suppression method of electromagnetic induction line series mode interference

The closed loop formed by the electromagnetic induction circuit will be induced in this changing magnetic field, causing the connecting wires between the signal source and the instrument and the wiring inside the instrument to cause interference in the circuit through magnetic coupling. This electromagnetic induction potential is connected in series with the useful signal. When the signal source is far away from the Display instrument, the interference is more prominent. In addition, high-frequency generators, motors with commutators and other equipment will also produce high-frequency interference.

1. The generation of interference

In production, the measured parameter is often converted into a weak low-level voltage signal, which is superimposed on the signal line over a long distance and enters the instrument.

1. The closed loop formed by the electromagnetic induction circuit will be induced in this changing magnetic field, so that the connecting wires between the signal source and the instrument and the wiring inside the instrument will cause interference in the circuit through magnetic coupling. This electromagnetic induction potential is connected in series with the useful signal. When the signal source is far away from the display instrument, the interference is more prominent. In addition, high-frequency generators, motors with commutators and other equipment will also produce high-frequency interference.

2. Electrostatic induction. Electrostatic induction is the result of the interaction of two electric fields. In the two opposite wires, if the potential of one changes, the potential of the other wire also changes due to the change of the capacitance between the wires, and the interference source forms interference in the loop through capacitive coupling.

3. Additional thermoelectric potential and chemical potential. Due to the thermoelectric potential generated by different metals and the chemical potential generated by metal corrosion, direct current electrical interference is formed in the circuit loop.

4. Vibration. In a strong vibration environment, the wire generates an induced electric potential due to its motion in the magnetic field. This interference is connected in series with the signal and enters the instrumentation in the form of series mode interference.

5. Interference introduced by different ground potentials. In the vicinity of high-power electrical equipment, when the insulation performance of the equipment is poor, the introduction of the potential difference of different ground potentials will cause interference, and in the use of the meter, there are often more than two connection points at the input end intentionally or unintentionally. In this way, the potential difference of different grounding points will be introduced to the instrument in the form of common mode interference, which appears on the two signal lines at the same time.

On the suppression method of electromagnetic induction line series mode interference

6. The signal source is an unbalanced bridge. When the bridge power supply is grounded, interference is caused by removing the unbalanced voltage on the diagonal of the bridge.

7. In addition to acting on analog circuits, some pulse-like interference voltages can sometimes directly enter digital circuits to give interference. The sources of these interference voltages are inductive loads such as switches, motors, relays, and discharge-generating machines.

2. Interference suppression

The formation of the interference problem is due to the existence of interference sources, which affect the instrumentation through certain coupling channels. In order to reduce these effects, the interference suppression problem should be considered when designing the instrument, and its anti-interference ability should be improved as much as possible. In practical applications, it is necessary to find out and combine twisting, shielding, grounding, balancing, filtering, isolation and other methods to cut off the coupling channel to suppress interference. At the same time, the display instrument is required to have high temperature resistance, low temperature, high pressure, corrosion, high viscosity and other properties and good dynamic characteristics to reduce the measurement error of the measured parameter.

1. Suppression method of serial mode interference

Serial mode interference may occur at the signal source, or it may be induced or received from the lead. Since serial mode interference is in the same position as the signal under test, once the serial mode interference occurs, its harmful effects are often not easily eliminated, so it should be prevented first.

The area enclosed by the signal loop can be greatly reduced, and the series mode interference potential difference of the electric field entering the loop through the inductive coupling on the two signal lines is greatly reduced.

Wrap it up, and then put a layer of insulation on the outer layer or directly use shielded cables for the signal wires, and the shielding layer is grounded. Since the non-magnetic shielding layer has no effect on the 50 Hz magnetic field, if necessary, the signal wire can be inserted into the iron pipe to make the signal wire magnetically shielded. After electrostatic shielding, the induced potential can be reduced to 1/100~1/1000 of the original.

Filtering: For DC signals that change very slowly, a filter circuit is added to the input end of the instrument to attenuate the interference mixed with the effective signal to. Two to three-stage RC filter circuits are often added before the input stage, and it is better to use a double T-type filter with lower internal resistance.

Cancellation: digital instruments such as double integral type and pulse width modulation type can perform A/D conversion on the average value of the input signal instead of the instantaneous value, which can average out some serial mode interference.

Try to lay the signal wires separately from the power wires. Reasonable wiring, under permitted conditions, reverse the current flow of the wires to reduce the interference of the magnetic field generated by each other; signal wires and power wires are not allowed to be laid parallel to each other, nor should they enter the instrument through the same wire hole Inside. Low-level signal wires should be connected to adjacent positions of signal terminals with as short untwisted wires as possible to reduce the area of ​​inductive interference. It is forbidden to use the same cable for power wires and signal wires. Do not use the same wiring plug-in for high-level and low-level lines. When necessary, separate the high-level and low-level lines next to the connector, with ground terminals and spare terminals in the middle.

2. Suppression of common mode interference to cut off the leakage path of common mode interference voltage so that interference cannot enter. In the low-level test, the signal line should only have a point of grounding and the shielding layer of the signal line must also have a point of grounding. Regardless of the signal line and instrumentation, it needs to be shielded. The grounding and shielding should be properly combined and used. Solve most of the interference problems. When an ungrounded signal source is connected to a grounded amplifier, the signal wire shielding layer should be connected to the common end of the amplifier. When a grounded signal source is connected to an ungrounded amplifier, even if the signal source is not connected to the earth, the signal wire shielding layer should be connected to the common end of the signal source to keep it at zero potential, which can effectively cut off the leakage current of the potential. , Improve the anti-interference ability of the measurement signal, which is a commonly used method in the measurement system.

The instrument adopts double-layer shielding floating protection technology: in order to improve the anti-common-mode interference capability of the instrument, while the amplifier input part is floating, the instrument adopts double-layer shielding and floating protection. In addition to using the watch case as a layer of shielding, an inner shielding cover is used to shield the input part of the amplifier in the instrumentation. No electrical connection is made between the two shielding layers, between the amplifier input part and the inner shielding layer. The inner shielding layer should not be connected to the instrument shell, but a separate wire should be drawn out as the protective shielding end to connect with the shielding layer of the signal line, so that the protective shielding extends to the full length of the signal line, and the signal line shielding is at the signal source Ground at one point, so that the input protection shield and signal shield of the instrument and meter are stable to the signal source and are in an equipotential state. Therefore, the shield can be used to reduce the common mode voltage coupled to the wire.

Application of balance circuit: The stability of a system depends on the balance of the signal source, signal leads, load, and other stray distribution parameters. In order to improve the anti-common-mode interference capability of instruments and meters, balance measures are adopted to make the voltages converted on the two lines equal, so as to reduce the part of the common-mode voltage coupled to the load.

Suppression of interference introduced by the power supply: The main interference inside the instrumentation comes from the leakage current generated by the low-power transformer. In order to prevent leakage current interference, the primary winding of the transformer can be placed in the shielding layer and the shielding layer can be grounded. At this time, the phase voltage on the Then flow into the amplifier, measuring circuit and signal source to produce interference. In order to prevent interference from the power transformer, a three-layer shielding structure is adopted, that is, the primary shielding layer of the power transformer is directly grounded to the case, the secondary winding of the power supply device is connected to all shielding layers, and the secondary winding shielding layer of the amplifier power supply is at the same level as the amplifier ground. Potential state. The pulse-shaped interference caused by the power supply has a greater impact on the digital circuit. A high-frequency filter should be installed on the power supply line. The filter should be installed in an iron shielded box with input and output leads filtered by a through-core capacitor. .

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