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Why Are Signal Isolators Necessary for a Control System?

In the production process, whether it is the control system or the monitoring system, it is possible to face the interference of complex factors, such as short circuit, overvoltage, unknown pulse and so on.

Therefore, the signal isolator becomes an integral part of the control system. It has the functions of protecting the lower control loop, weakening the influence of environmental noise, suppressing grounding or equipment interference, limiting the voltage and current, protecting the input and output of the control system and the communication interface.

Ⅰ. The principle of signal isolators

The principle of the signal isolator is to convert the signal of the transmitter or instrument through the modulation and conversion of the semiconductor device, and then realize the isolation conversion through the light-sensing or magnetic-sensing device, and finally implement the demodulation and convert it back to the original signal before isolation. At the same time, the isolator isolates the power supply of the isolated signal to ensure absolute independence between the converted signal, power supply and ground.

At the same time, the signal isolator filters the interference signal superimposed on the measured value, and matches the signal according to the input and output requirements of the control system. Therefore, isolation, amplification, filtering and matching are the functions of the signal isolator.

Some DCS modules have their own photoelectric isolation function, which can fully meet the normal working requirements in some systems. However, it is impossible for the system to be in normal working condition all the time. In the event of an accident causing device damage, replacing the isolator is more time-saving and cost-effective than replacing the DCS module.

Ⅱ. The classification of signal isolators

Signal isolators are divided into active signal isolators and passive signal isolators.

1. Active signal isolators

Active signal isolators are powered by an independent power supply to ensure that the signal isolators work well. Modules require active signals on the input side, and on the output side they provide filtered and amplified signals, with isolation between input/output and power supply depending on the application. Active signal isolators include three-terminal isolation, input terminal isolation and output terminal isolation.

2. Passive signal isolators

Passive signal isolators provide an additional and substantial convenience by eliminating the need for additional power supplies. The working power of the module is provided through the input or output loop, and the current consumed by its internal circuit is extremely small, which does not affect the correct transmission of the signal. According to the power supply mode of the signal isolator, it is divided into input side power supply, output side power supply, passive feeding, etc.

3. How to choose signal isolators

The isolator is located between the two system channels. When selecting an isolator, the input and output functions must first be determined, and the input and output modes of the isolator (voltage type, current type, loop supply type, etc.) must be adapted to the 1:3 mode of the front and rear channels.

In addition, there are many important parameters such as accuracy, power consumption, noise, dielectric strength, bus communication function, etc. that will affect the performance of the product. For example: noise is related to accuracy, power consumption and heat are related to reliability, users should be careful when choosing. What's more, choosing qualified signal isolator suppliers are important. To sum up, suitability, reliability, and cost-effective products are the main principles for choosing an isolator.

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