They are simulated relays and are what enables a PLC to eliminate external relays. INTERNAL UTILITY RELAYS-(contacts) : These do not receive signals from the outside world nor do they physically exist.Typically they are not relays but rather they are transistors. They physically exist and receive signals from switches, sensors, etc. INPUT RELAYS-(contacts) : These are connected to the outside world.We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations.Appropriate circuits to receive input/output data.The primary reason for designing PLC was eliminating the large cost involved in replacing the complicated relay based machine control systems.Simply program the PLC to count its inputs and turn the solenoids on for the specified time.The bigger the process the more is need for a PLC.What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters. But what if the process included 10 switches and solenoids? We would need 10 external timers. We can do this with a simple external timer.Let’s assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for.PLC LADDER Logic Examples PLC Study Material The user enters a program, usually via software, that gives the desired results.Ĭlick below to get plc examples. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. Programmable Logic Controller ) is a device that was invented to replace the necessary sequential relay circuits for machine control.
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