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6: Ladder Logic

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    922

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    • 6.1: “Ladder” Diagrams
      Ladder diagrams are specialized schematics commonly used to document industrial control logic systems. 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.
    • 6.2: Digital Logic Functions
      This page explains constructing logic functions for a lamp circuit using ladder diagrams, detailing the representations of OR, AND, NOT, NAND, and NOR gates through contact arrangements. It highlights parallel contacts for OR, series contacts for AND, and normally-closed contacts for NOT gates. Additionally, it describes creating combined functions like Exclusive-OR with series-parallel configurations and the use of relays to invert output logic.
    • 6.3: Permissive and Interlock Circuits
      This page covers the practical applications of switch and relay logic in control systems, with an emphasis on burner and reversible motor control. It explains permissive contacts for safety, ensuring multiple conditions must be satisfied for activation. The page also discusses interlocking mechanisms to prevent incompatible actions, such as simultaneous energizing of forward and reverse motor contactors, and highlights safety measures like electrical and mechanical interlocks.
    • 6.4: Motor Control Circuits
      This page explains modifications to motor control circuits for continuous operation following momentary activation. It highlights the use of auxiliary "seal-in" contacts for maintaining motor operation until stopped and discusses time-delay relays to manage mechanical load inertia and prevent immediate motor reversal. These relays simplify circuit design by removing redundant interlocking contacts, thereby improving operational efficiency.
    • 6.5: Fail-safe Design
      This page addresses fail-safe design in logic circuits, focusing on safety in control systems through an example of fire alarms that trigger during failures. It explains that open circuit failures necessitate system designs that revert to a safe state. Additionally, it compares challenges in ensuring safety across various applications, such as power generation and distribution.
    • 6.6: Programmable Logic Controllers (PLC)
      This page outlines the transition from electromechanical relays to Programmable Logic Controllers (PLCs) in industrial applications, emphasizing their flexibility and ease of modification through ladder logic programming. It highlights the critical nature of fail-safe design in PLC systems, particularly in motor control, and introduces concepts like internal relays for logic inversion.

    This page titled 6: Ladder Logic is shared under a GNU Free Documentation License 1.3 license and was authored, remixed, and/or curated by Tony R. Kuphaldt (All About Circuits) via source content that was edited to the style and standards of the LibreTexts platform.