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They are called "ladder" diagrams because they resemble a ladder, with two vertical rails supply power and as many "rungs" horizontal lines as there are control circuits to represent.
If we wanted to draw a simple ladder diagram showing a lamp that is controlled by a hand switch, it would look like this: L1 is the "hot" conductor, and L2 is the grounded "neutral" conductor. These designations have nothing to do with inductors, just to make things confusing.
The actual transformer or generator supplying power to this circuit is omitted for simplicity. In reality, the circuit looks something like this: Typically in industrial relay logic circuits, but not always, the operating voltage for the switch contacts and relay coils will be volts AC.
Lower voltage AC and even DC systems are sometimes built and documented according to "ladder" diagrams: So long as the switch contacts and relay coils are all adequately rated, it really doesn't matter what level of voltage is chosen for the system to operate with. Note the number "1" on the wire between the switch and the lamp.
In the real world, that wire would be labeled with that number, using heat shrink or adhesive tags, wherever it was convenient to identify.
Wires leading to the switch would be labeled "L1" and "1," respectively. Wires leading to the lamp would be labeled "1" and "L2," respectively. These wire numbers make assembly and maintenance very easy.
Each conductor has its own unique wire number for the control system that it's used in. Wire numbers do not change at any junction or node, even if wire size, color, or length changes going into or out of a connection point.
Of course, it is preferable to maintain consistent wire colors, but this is not always practical. What matters is that any one, electrically continuous point in a control circuit possesses the same wire number. Take this circuit section, for example, with wire 25 as a single, electrically continuous point threading to many different devices: In ladder diagrams, the load device lamp, relay coil, solenoid coil, etc.
While it doesn't matter electrically where the relay coil is located within the rung, it does matter which end of the ladder's power supply is grounded, for reliable operation.
Take for instance this circuit: Here, the lamp load is located on the right-hand side of the rung, and so is the ground connection for the power source. This is no accident or coincidence; rather, it is a purposeful element of good design practice.
Suppose that wire 1 were to accidently come in contact with ground, the insulation of that wire having been rubbed off so that the bare conductor came in contact with grounded, metal conduit. Our circuit would now function like this:If you’re familiar with PC programming in languages like Java or C#, then you’ll probably know about the books Design Patterns: Elements of Reusable Object-Oriented Software and Patterns of Enterprise Application regardbouddhiste.com books are about Software Design Patterns.
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