Programmable Logic Controllers (PLCs) play a vital role in today's industrial automation. As a high-performance industrial control device, PLC has become the core of many automation systems with its powerful functions and flexible configurations. However, to ensure the efficient operation of the PLC control system, it is not enough to rely on advanced programming technology, but also need to pay enough attention to the wiring of the system. Although the wiring work in the PLC control system accounts for a small proportion, but it is actually the basis of the entire programming design. Only when the wiring is correct, the subsequent programming work can be carried out smoothly.
I. the analysis of DC input circuit
DC input circuit is a common form of circuitry in the PLC, as shown in the figure, a typical wiring for DC input circuit. In this wiring method, the switch state of the external line directly affects the PLC internal optocoupler operating state. When the switch of the external line is closed, the light-emitting diode of the PLC internal photocoupler will be lit, thus saturating the phototransistor conduction. This conduction signal is transmitted to the processor, whereby the CPU determines that there is a signal input to the line. On the contrary, when the external switch is disconnected, the light-emitting diode in the optocoupler is extinguished, the photosensitive transistor is cut off, and the CPU assumes that there is no signal in the circuit. This simple wiring makes the DC input circuit widely used in PLCs.
II. Characteristics of AC Input Circuit
Unlike DC input circuits, AC input circuits have their own unique applications in PLC. The main difference between the AC input circuit and the DC input circuit is the addition of a rectifier link. In the AC input circuit, the input voltage is usually AC120V or 230V, which will be converted to DC through a bridge rectifier after current limiting by resistor R and isolation by capacitor C. The operation principle is the same as that of DC input circuit. Thereafter the principle of operation will be the same as the DC input circuit.
III. Delay and Reliability of the Input Circuit
Due to the addition of current limiting, isolation and rectification in the AC input circuit, which makes its input signal delay time is relatively long. This is the shortcoming of AC input circuits. However, it should also be noted that it is the high voltage at its input that makes the input signal reliability of AC input circuits higher than that of DC input circuits. This high reliability gives AC input circuits an irreplaceable advantage in certain specific application scenarios.