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PLC in the industrial field of protection solutions

          PLC that Programmable logic Controller, meaning programmable logic controller, which is a digital computing operation of the electronic control system, designed for industrial applications designed with high anti-interference ability and reliability. It uses a class of programmable memories for user-oriented instructions such as storing programs internally, executing logic operations and timing counts, and controlling various types of production operations through digital or analog inputs / outputs.

          PLC is a significant feature is the I / O and more, which means that PLC adaptability, even if the user because of product replacement and other factors, resulting in control flow chart changed, usually only need to modify the control flow chart, without modifying the PLC hardware Wiring, so PLC in the industrial control field has been a great application.

          From the composition of the structure of the PLC can be divided into two categories: one is the integrated monolithic PLC, which is characterized by power, central processing unit, I / O interfaces are integrated in a chassis. The other is a standard template-based structured PLC, which features a power module, the central processing unit template, I / O modules are structurally independent of each other, according to the specific application requirements, select the appropriate template installed in Fixed rack or rail to form a complete PLC application system.

          PLC in the actual use of industrial sites in the process, due to the harsh industrial environment, wiring workload is large, there will inevitably be wiring errors, often the power I / O port will be limited, resulting in over-current port burned Therefore, it is necessary for each I / O port of the PLC to design overcurrent protection measures and place the self-recovery fuse PPTC at each I / O port to avoid the accidental connection of a short circuit accident. In this way, if the debugger accidentally shorted the I / O port, the self-recovery fuse PPTC will immediately start protection, limiting the operating current of the entire loop of the I / O port, protecting the PLC's motherboard. When troubleshooting, self-recovery fuse PPTC quickly and automatically return to a low-resistance state, PLC and can work properly.

          PLC use, I / O port sub-digital and analog port, for the input port, is divided into digital input port and analog input port. For PLC current analog input port, also need to pay attention to over-voltage protection. Because the current analog I / O port, there is a sampling resistor, usually high-precision 250 ohms, when the PLC input current, such as 4 ~ 20mA current signal, the current through the I / O port 250 ohm resistor, within the PLC Form 1 ~ 5V voltage signal. Because the PLC's DC power is usually 24V, PLC testing or commissioning in the field, the debugger can easily connect 24V power supply to PLC's analog input I / O port. If there is no protection, 250 ohm sampling resistance is connected for 24V Power, the power consumption in the sampling resistance will be a substantial increase (determined by the formula V2 / R), so that the sampling resistor will be damaged. A possible protection solution is to add a Zener tube (or TVS tube) to the circuit in parallel with the sampling resistor, Zener tube to protect the sampling resistance. But the attendant problem is that Zener tubes (or TVS tubes) are transient voltage suppressors that have good protection characteristics for transient voltages but are also powerless for long-term overvoltage protection. TE Connectivity PPTC + Zener or PolyZen can provide a complete solution.

          TE's PolyZen devices incorporate Zener diode precision overvoltage protection components that work in concert with the internal PPTC overcurrent protection components. The PPTC in the PolyZen device senses the temperature of the Zener tube due to the seamless thermal coupling between the PPTC and Zener devices in the event of a sustained high-power state (overvoltage event). When the Zener tube is over-pressed while As the temperature increases, the temperature of the PPTC increases. Since the PPTC is a positive temperature coefficient temperature sensitive resistor, the PPTC resistance rapidly changes its temperature from a low resistance state to a high resistance state, limiting the current and protecting the Zener tube.

This article comes from:http://www.relay-protector.com/technical/847.html.Please refer to the source!

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