CHAPTER 2 - LADDER DIAGRAM

Ladder (or line or elementary) diagram is a schematic representation of an electrical circuit and it is not a physical representation. It is an important concept not only at electro-pneumatic systems, but also at PLC. Each ladder diagram is constructed from input switches and out relays that lies between the positive and negative power lines. The order of elements in a ladder diagram, shown as below:

As shown by the figure, the positive power line of the supply is on the extreme left of the ladder. While the negative line of the supply (ground here) is on the extreme right. Between the power lines come the inputs and outputs. Each line the ladder is called a "rung". Each rung may contain only one output unless the outputs are in parallel.

Outputs in a ladder diagram are always at the right just before the negative line of the power supply. Mainly we want the current to flow from the positive line to the negative line through the output in order for it to be actuated. Outputs in ladder diagrams are mainly relays in operation.

As an introduction to ladder diagrams, consider the simple wiring diagram for an electrical circuit in Figure 11.1a. The diagram shows the circuit for switching on or off an electric motor. We can redraw this diagram in a different way, using two vertical lines to represent the input power rails and stringing the rest of the circuit between them. Figure 11.1b shows the result. Both circuits have the switch in series with the motor and supplied with electrical power when the switch is closed. The circuit shown in Figure 11.1b is termed a ladder diagram.

    

References:

Petruzella, Frank. 1996. Industrial Electronics. Mc-Graw Hill. Singapore

Maxfield, Clive. FPGAs: World Class Designs: World Class Designs. http://books.google.co.id/books?id=kQuOKBSOz5QC&pg=PA455&lpg=PA455&dq=IEI&redir_esc=y#v=onepage&q&f=false 20 April 2014

Laboratory, Automation. Electro-Pneumatic Circuits. http://www.msalah.com/PCL/Session%202.pdf 20 April 2014