A Basic Introduction to Ladder Diagrams for PLC Programming

The ladder diagram which is a special type of wiring diagram consists of two power rails, which are placed vertically on each side of the diagram, and rungs, which are placed horizontally between the power rails. The power rails are the source of power in the circuit (ac or dc), where the left rail is the “hot” side (voltage) and the right rail is neutral (ac) or ground (dc). Thus, each rung is connected across the voltage source and is an independent circuit.

A rung normally comprises of at least one set of switch or relay contacts and typically only one load such as a relay coil or a motor. When the contacts in a particular rung close to make a continuous path, then that rung becomes active, and its load is energized. For instance, the top rung in Fig 1.0 contains two switches and a pilot light in series. For the rung to become active, both the two switches must close (S1 & S2), which then apply voltage to the light. The middle rung has two switches in parallel (S3, S4) so only one of these switches must be closed to make the rung active. The load in this rung is a relay coil i.e. Relay X. The bottom run contains a set of NO contacts from Relay X, and the load is a motor. Subsequently, when either S3 or S4 of the middle rung closes, Relay X coil is energized and the motor (in the bottom rung) starts.

Also read:

• Basic Features of Programmable Logic Controllers (PLCs)
• The Basics of Ladder Diagrams for Programming PLCs
• What are PLC-Based Water Treatment Plants?
• Basic Features of Distributed Control Systems (DCS)
• Supervisory Control and Data Acquisition in Power Systems