Before discussing the main issues, it helps us understand what it is used to relay. Relays are simple devices that use magnetic fields to control the switch, as shown below.
Figure 01. Relay
When the voltage provided at the input coil, the currents produce magnetic fields are created. This field that will attract the metal switch to him and would touch the other switch. As a result of this mechanism is a series of previously open circuit to closed circuit. Nature relay like this (to be closed after a given circuit voltage) is called normally open. Thus, the normally closed relay is a relay that will become an open circuit after a given voltage. Generally, the relay depicted by schematic diagrams using a circle that describes the input coil. Contact output is given by two parallel lines. Normally open contact is described with two lines and will be open (non-conducting) if not given energy. Is normally closed to the contrary and is described by two lines with a diagonal line intersecting the second line. When not given energy, the state relay is closed (conducting). Relay is used to state a single source (either open or closed) energy state can regulate energy sources (open or closed) others usually have a greater flow and the two energy sources are mutually insulated from each other (not connected directly). Relay is the main component of the use of simple PLC.Contoh relay indicated by the following figure.
Figure 02. Analogies Ladder Diagram
In this system, the first relay on the left image is normally closed and will continue until there was a current of the voltage applied to the relay (input A). The second relay is normally open and no current will flow until the voltage applied to this relay (input B). If current flows in the second relay first, then the currents will also flow in the third relay and will close the switch at the output C. Circuit as depicted in this schematic diagram Ladder to the drawing. Logically, this diagram can be read as follows: C will be "on" when a "off" and B "on". In Boolean logic is formulated as C = A '. B
Figure 03. Digital Gate Equivalent schematic
The following figure represents a more complex example of the application in the PLC, with two pieces of key inputs.
Figure 04. Example Applications in PLC