Introduction to PLC control circuits and ladder diagrams

Compared with general computer languages, the software programming language of PLC has obvious characteristics. It is different from both high-level languages ​​and general assembly languages, and must meet the requirements of easy writing and debugging.

Early PLCs only supported ladder diagram programming language and instruction list programming language. Now, according to the International Electrotechnical Commission, five languages ​​​​have been developed to support PLC programming, namely: ladder diagram Delete (LD), instruction list Delete (IL), function block diagram Delete (FBD), sequential function flow chart Delete (SFC), structured text Delete (ST), etc. Today I will share with you some PLC control circuits and ladder diagrams, which are relatively basic and practical parts. Let's take a look!

01PLC circuit and ladder diagram for start, self-locking and stop control

Start, self-lock and stop control can be achieved using drive instructions (OUT) or set instructions (SET, RST).

1. Use coil drive instructions to achieve start, self-locking and stop control

Circuit and ladder diagram description:

When the start button SB1 is clicked, the start contact X000 in the ladder diagram program inside the PLC is closed, the output coil Y000 is energized, the internal hard contact of the output terminal Y0 is closed, the Y0 terminal and the COM terminal are internally connected, the contactor coil KM is energized, the KM main contact in the main circuit is closed, and the motor is energized to start.

When the stop button SB2 is clicked, the stop contact X001 in the ladder diagram program inside the PLC is disconnected, the output coil Y000 loses power, the internal hard contact between the Y0 and COM terminals is disconnected, the contactor coil KM loses power, the KM main contact in the main circuit is disconnected, and the motor loses power and stops.

2. Use set-reset instructions to achieve start, self-lock and stop control

Its PLC wiring diagram is similar to the above.

Circuit and ladder diagram description:

When the start button SB1 is clicked, the start contact X000 in the ladder diagram is closed, and the [SET Y000] instruction is executed. The result of the instruction execution sets the output relay coil Y000 to 1, which is equivalent to energizing the coil Y000, connecting the internal hard contact between the Y0 and COM terminals, energizing the contactor coil KM, closing the KM main contact in the main circuit, and energizing the motor to start.

When the stop button SB2 is clicked, the stop contact X001 in the ladder diagram program is closed, and the [RST Y000] instruction is executed. The instruction execution result resets the output coil Y000, which is equivalent to the coil Y000 losing power, the internal hard contact between the Y0 and COM terminals being disconnected, the contactor coil KM losing power, the KM main contact in the main circuit being disconnected, and the motor losing power and stopping.

02PLC circuit and ladder diagram for forward and reverse interlocking control

The circuit and ladder diagram are described as follows:

1) Forward interlock control

Click the forward button SB1 → the forward contact X000 in the ladder diagram program closes → the coil Y000 is energized → the Y000 self-locking contact closes, the Y000 interlocking contact opens, and the internal hard contact between the Y0 terminal and the COM terminal closes → the Y000 self-locking contact closes, so that the coil Y000 can still be energized after the X000 contact opens; the Y000 interlocking contact opens, so that the coil Y001 cannot be energized even when the X001 contact is closed (caused by misoperation SB2), thus realizing interlocking control; the internal hard contact between the Y0 terminal and the COM terminal closes, the contactor KM1 coil is energized, the KM1 main contact in the main circuit closes, and the motor is energized for forward rotation.

2) Reverse interlock control
Click the reverse button SB2 → the reverse contact X001 in the ladder diagram program is closed → the coil Y001 is energized → the Y001 self-locking contact is closed, the Y001 interlocking contact is disconnected, and the internal hard contact between the Y1 terminal and the COM terminal is closed → the Y001 self-locking contact is closed, so that the coil Y001 continues to be energized after the X001 contact is disconnected; the Y001 interlocking contact is disconnected, so that the coil Y000 cannot be energized even when the X000 contact is closed (caused by misoperation SB1), realizing interlock control; the internal hard contact between the Y1 terminal and the COM terminal is closed, the contactor KM2 coil is energized, the KM2 main contact in the main circuit is closed, and the motor is energized and reversed.

3) Stop control
Click the stop button SB3 → both stop contacts X002 in the ladder diagram program are disconnected → coils Y000 and Y001 are de-energized → contactor KM1 and KM2 coils are de-energized → main contacts KM1 and KM2 in the main circuit are disconnected, and the motor stops.

03PLC circuits and ladder diagrams for multi-site control

(1) Single person multi-location control

Start control at location A. When the start button SB1 is clicked at location A → the normally open contact of X000 closes → the coil Y000 is energized → the normally open self-locking contact of Y000 closes, the hard contact inside the Y0 terminal closes → the normally open self-locking contact of Y000 closes to lock the power supply of the Y000 coil, the hard contact inside the Y0 terminal closes to energize the contactor coil KM → the main contact of KM in the main circuit closes, and the motor is energized to run.

Stop control at location A. When the stop button SB2 is clicked at location A, the normally closed contact of X001 is disconnected, the coil Y000 loses power, the normally open self-locking contact of Y000 is disconnected, the hard contact inside the Y0 terminal is disconnected, the contactor coil KM loses power, the main contact of KM in the main circuit is disconnected, and the motor loses power and stops.

(2) Multiple people and multiple locations control

Start control. Press buttons SB1, SB3, and SB5 at locations A, B, and C at the same time → coil Y000 is energized → the normally open self-locking contact of Y000 is closed, the internal hard contact of the Y0 terminal is closed → the Y000 coil is powered and locked, the contactor coil KM is energized → the KM main contact in the main circuit is closed, and the motor is powered and runs.

Stop control. When one of the stop buttons SB2, SB4, and SB6 is clicked at the same time at locations A, B, and C, the coil Y000 loses power, the normally open self-locking contact of Y000 is disconnected, and the internal hard contact of the Y0 terminal is disconnected. The normally open self-locking contact of Y000 is disconnected, cutting off the power supply to the Y000 coil, and the internal hard contact of the Y0 terminal is disconnected, causing the contactor coil KM to lose power. The main contact of KM in the main circuit is disconnected, and the motor loses power and stops.

04 PLC circuit and ladder diagram for timing control

1. PLC circuit and ladder diagram for delayed start timing operation control

It can achieve the following: after pressing the start button for 3 seconds, the motor starts working and stops automatically after working for 5 seconds.

The PLC circuit and ladder diagram are described as follows:

2. PLC circuit and ladder diagram of multi-timer combination control

It can achieve the following: after clicking the start button, motor B starts running immediately, motor A starts working after 30 seconds, motor B stops after 70 seconds, and motor A stops after 100 seconds.

The PLC circuit and ladder diagram are described as follows:

05 PLC circuit and ladder diagram of timer and counter combination to extend timing control

The longest timing time of Mitsubishi FX series PLC is 3276.7s (about 54min). The timing time can be extended by using timers and counters.

The PLC circuit and ladder diagram are described as follows:

The timer T0 in the figure has a timing unit of 0.1s (100ms). When used with counter C0, its timing time T = 30000 × 0.1 seconds × 30000 = 90000000 seconds = 25000 hours. If you need to reset the timing, you can open switch QS2, close the normally closed contact [2]X000, and then execute the "RST C0" instruction. After that, counter C0 will be reset, and then QS2 will be closed again to restart the 250000 hour timing.

06PLC circuit and ladder diagram for multiple output control

The PLC circuit and ladder diagram are described as follows:

(1) Starting control

(2) Stop control

07PLC circuit and ladder diagram for overload alarm control

PLC circuit and ladder diagram description:

(1) Starting control

Click the start button SB1 → [1] X001 normally open contact closes → [SET Y001] instruction is executed → Y001 coil is set, that is, Y001 coil is energized → the internal hard contact of Y1 terminal is closed → contactor KM coil is energized → KM main contact is closed → the motor is energized and runs.

(2) Stop control

Click the stop button SB2 → [2] X002 normally open contact closes → [RST Y001] instruction is executed → Y001 coil is reset, that is, Y001 coil loses power → the internal hard contact of Y1 terminal is disconnected → contactor KM coil loses power → KM main contact is disconnected → the motor loses power and stops.

(3) Overload protection and alarm control

08 PLC circuit and ladder diagram for flashing control

Circuit and ladder diagram description:

Close the switch QS → the normally open contact of X000 closes → timer T0 starts timing for 3s → after 3s, timer T0 operates, the normally open contact of T0 closes → timer T1 starts timing for 3s, at the same time, Y000 is energized, the hard contact inside the Y0 terminal closes, and the lamp HL lights up → after 3s, timer T1 operates, the normally closed contact of T1 opens → timer T0 is reset, the normally open contact of T0 opens → the Y000 coil loses power, and the timer T1 is reset at the same time → the Y000 coil loses power and the lamp HL goes out; timer T1 is reset to close T1, because the switch QS is still in the closed state, the normally open contact of X000 is also closed, and timer T0 starts timing for 3s again.