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voltage variations and spikes.

Switched outputs - A voltage is supplied to the PLC card and the card switches it to different outputs using solid state circuitry (transistors, triacs, etc.) Triacs are well suited to AC devices requiring less than an amp. They are sensitive to power spikes and might inadvertently turn on when there are transient voltage spikes. A resistor may need to be put in parallel with a load to ensure enough current is drawn to turn on the triac. The resistor size can be determined by

Transistor outputs use NPN or PNP transistors up to 1A typically. Their response time is well under 1ms.

7.24 THE PLC ENVIRONMENT

7.24.1 Electrical Wiring Diagrams

•PLC’s are generally used to control the supply of power to a system. As a result, a brief examination of electrical supply wiring diagrams is worthwhile.

•Generally electrical diagrams contain very basic circuit elements, such as relays, transformers, motors, fuses, lights, etc.

•Within these systems there is often a mix of AC and DC power. 3 phase AC power is what is delivered universally by electric utilities, so the wiring diagrams focus on AC circuits.

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an insulated cabinet. In the figure below, each box could be a purchased module(s).

A physical layout for the control cabinet

• After the Layout for the cabinet is determined, the wire paths must be determined. The figure below lays out the wire paths and modules to be used.