2 Ladder Basics
Understanding Electrical Ladder Drawings
Before discussing or understanding the Programmable Logic Controller, we must first understand the devices the PLC was invented to replace. That was the ladder diagram. The ladder diagram or electrical schematic or elementary diagram can be divided into two distinct portions. The first is the power portion and the second is the control. Showing flow of power to a motor or other device in a factory environment is the primary focus of the power portion.
Showing control of that motor is the focus of the control portion. While power flow is important, the focus is to dissect the control portion. Items such as fuses and disconnect devices may appear in both power and control circuits. The use of symbols is important for both power and control. The types of symbols vary and are somewhat arbitrary based on where you are located and, in some cases, the company you work for. This is contrary to the symbols used in the PLC which must be fixed or uniform. The use of symbols is a descriptive description of the device being used. A list of some symbols and the function of the device is given below:
Some of the symbols are used in examples on the following pages. Many devices follow the pattern of how the device actually works. For example, notice how the foot switch resembles an actual foot switch or how a flow or sail switch resembles a sail on a sailboat. Also, note that different combinations of open and closed contacts can be furnished on the same device to give multiple combinations of functions to the control circuit either through hard-wired circuits or to the PLC and then through its logic to the appropriate output.
Many times, the input devices are said to be either normally open (NO) or normally closed (NC). The normally open or closed status refers to the shelf state of the device. If a device is normally open, a resistance check of the device with a digital multi-meter will give a reading of OL. If the device is normally closed, a resistance check will give a reading of 0.0.
The normally open and normally closed state of the devices is not labeled on the ladder drawing. Rather, the reader must recognize the symbol. A helpful hint in trying to determine whether the contacts are open or closed is to think of them in terms of gravity. If gravity is acting on the device, its normal state is as shown in the drawing. An exception to this concept is found in devices that contain springs. For instance, in the drawing of a normally open pushbutton, it appears that the pushbutton should be falling down and closing. However, there is a spring in the pushbutton that holds the contacts in the open (up) position.
Components of the Ladder Diagram
Ladder circuits have some common characteristics. They are:
- Power bar on the far left
- Current flow from the switches combined to allow flow
- One or more relay coils at the right
- Neutral or negative power bar on the right
The following circuit shows a typica