PLC PROGRAMMING - LADDER LOGIC TUTORIAL-3

PLC PROGRAMMING - LADDER LOGIC 

 

The use and understanding of PLC programming depends on the following factories

1. Knowledge of the process to be controlled
2. Understanding of electrical schematics
3. An appreciation for the logic operation and for various types of logic and relay device

Ladder Diagram

• Ladder diagram is a traditional method of describing relay logic of the control circuit

• A ladder diagram reflects a conventional wiring diagram of the physical arrangement of the various component (switch, relays, motor, valve,) and their interconnection. It is used by an electrician (field maintenance engineer ) to do the actual wiring of the panel.

Ladder Programming 

• There are few basic ladder programming symbols that are used to express the meaning and purpose of a control circuit

Plc programming base Upon two basic operation :

1.            Normally Open – No current flow
2.            Normally Close- current flow

What is Normally?

• It is simply stated that the contact is in when something is not affecting it.

• If it is a relay then it is not energized, if it is a switch then 

• it is off and if it is a high limit switch as a level alarm then it is below the limit

Normally open :

• In PLC programming it is generally defined as NO(normally open)

• This means that contact does not allow to flow any current in its normal state. No current will flow in this state.

• For  emerging it, contact should be close state to allow current flow

• Example: A (NO) push-button connected with bulb.

• Initially push button in the open state, no current flow in rung thus bulb can't be blow
  
  
  

• as push-button pressed or energize current will flow through rung and bulb will blow.

• 

• Both operation shown in the above picture  

Normally close:

• In PLC programming it is generally defined as NC ( Normally Close )

• Which means that contact allows to flow current in its normal state. The current will flow in this state.

• To de-energized it, contact should be open state to does not allow to current flow

• Example: A (NC) push-button connected with bulb.

• Initially push button in the close state, current flow in rung thus bulb can be blow

• as push-button pressed or de-energize No current will flow through rung and bulb will not blow.

•  Both operation shown in the above picture