Basic Characteristics of Ladder Logic Programming

Ladder circuits have some common characteristics. They are:

1. Power bar on the far left
2. Current flow from the switches combined to allow flow
3. One or more relay coils at the right
4. Neutral or negative power bar on the right

The following circuit shows a typical ladder rung with switches and relays on the left and the relay on the right:

Sample Ladder Diagram – Single Rung

The control voltage for the system usually is fed from a control transformer that is fed from the power portion of the drawing or from a separate source. It is important to identify the source of power for each circuit. In many instances, more than one source exists and the power may not be off to all circuits when checking a problem with a specific circuit. Rails usually are protected from over-current conditions with a circuit breaker of fuse. Ladder drawings may continue from page to page. The rail is shown as continuing from one page to the next usually with an arrow or number with corresponding number on the succeeding page, same position, at the top of the rail feeding the succeeding page.

Example of Ladder Logic

Figure above shows a ladder rung. When the pushbutton (PB) is pushed, the circuit is completed and current flows energizing the coil (CR1).