PLC control circuit diagram and principle explanation

The most frequently used PLC programming languages ​​are structured text and ladder diagram. For technicians with little basic knowledge, it is quickest to start learning PLC programming from ladder diagram. Regardless of the brand of PLC, the structure of its ladder diagram is similar to the actual electrical control circuit. Below, we recommend several of the most commonly used control circuits.

1. Start, hold and stop circuits

Four ladder diagrams for starting, holding and stopping Y10 are shown in the following figure. These ladder diagrams can all realize the functions of starting, holding and stopping. x0 is the start signal and X1 is the stop signal. Figures a and c use the normally open contact of Y10 to realize self-locking holding, while figures b and d use the SET, RST instructions to realize self-locking holding.

2. Multi-location control circuit

The following figure is a program for controlling a relay coil in two places. X0 and X1 are the start and stop control buttons of one place, and X2 and X3 are the start and stop control buttons of another place.

3. Interlock control circuit

The following figure is the interlocking circuit of three output coils. Among them, X0, X1 and X2 are start buttons, and X3 is the stop button. Since only one of Y0, Y1, and Y2 can be connected at a time, the normally closed contacts of Y0, Y1, and Y2 are connected in series to the control circuits of the other two coils.

4. Sequential start control circuit

As shown in the figure, the normally open contact of Y0 is connected in series in the control circuit of Y1, and the connection of Y1 is conditional on the connection of Y0. In this way, Y1 is allowed to be connected only when Y0 is connected. After Y0 is turned off, Y1 is also turned off and stopped, and under the condition that Y0 is turned on, Y1 can be turned on and stopped automatically. X0 and X2 are start buttons, and X1 and X3 are stop buttons.

5. Motor forward and reverse control circuit

6. Centralized and decentralized control circuits

In an automatic line composed of multiple single machines, there is an interlocking of centralized control on the main operation console and decentralized control on the single machine operation console. The ladder diagram of centralized and decentralized control is shown in the figure below. x2 is a selection switch, and its contacts are interlocking contacts for centralized control and decentralized control. When X2 is ON, it is a single machine decentralized start control; when x2 is OFF, it is a centralized total start control. In both cases, both the single machine and the main operation console can issue a stop command.