Related discussions on PLC equivalent circuit

 PLC (programmable controller) is widely used in the modern automation industry. PLC has been developed and applied to this day. Based on the knowledge I have learned, I compiled this article to explain the relevant knowledge of PLC equivalent circuit . Post it and share it with everyone.

From the comparison between PLC control system and electrical control system, it can be seen that the PLC user program (software) replaces the relay control circuit (hardware). Therefore, for users, PLC can be equivalent to a collection of many various "soft relays" and "soft wiring", and the user program uses "soft wiring" to combine "soft relays" and "soft wiring". A "control circuit" whose "contacts" are connected according to certain requirements.

In order to better understand this equivalence relationship, an example is provided below. Figure 1 shows the electrical control system for one-way starting operation of a three-phase asynchronous motor. Among them, the contacts of the input devices SB1, SB2 and FR constitute the input part of the system, and the output device KM constitutes the output part of the system.

Figure 1 Electrical control system for one-way operation of three-phase asynchronous motor

In the figure, the input devices SB1, SB2, and FR correspond to the "coils" of the "soft relays" X0, X1, and X2 inside the PLC. The input devices control the status of the corresponding "soft relays", that is, through these "soft relays" Change the state of the external input device into the state inside the PLC. This type of "soft relay" is called an input relay; similarly, the output device KM corresponds to the "soft relay" Y0 inside the PLC, and the corresponding state is controlled by the "soft relay" Y0 The status of the output device KM is to output the internal status of the PLC through these "soft relays" to control external output devices. This type of "soft relay" is called an output relay.

Therefore, what the PLC user program needs to realize is: how to use the input relays X0, X1, and X2 to control the output relay Y0. When the control requirements are complex, other types of "soft relays" inside the PLC, such as auxiliary relays, timers, counters, etc., must be used in the program to meet the control requirements.

Figure 2 Equivalent circuit of PLC

a) Main circuit b) Control circuit

If a PLC is used to control this three-phase asynchronous motor to form a PLC control system, according to the above analysis, the main circuit of the system remains unchanged. As long as the contacts of the input devices SB1, SB2, and FR are connected to the input terminals of the PLC, the output device The KM coil is connected to the output end of the PLC to form the input and output hardware circuit of the PLC control system . The function of the control part is realized by the user program of PLC , and its equivalent circuit is shown in Figure 2.