PLC Ladder Logic Fundamental: Contacts and Coils

The electrical control system basic purpose is to determine whether a load should be turned ON or turned OFF, under what circumstances and when it should happen. Just remember the concept of current flow to understand a ladder program – a load is turned ON when the current can flow to it and is turned OFF when the current could not flow to it.

The ladder diagram fundamental element is a “Contact”. A contact has only two states: open or closed. An open contact breaks the current flow whereas a closed contact allows current to flow through it to the next element. The simplest contact is an ON/OFF switch, which requires external force to activate it. Limit switches are those small switches that are placed at certain location so that when a mechanical device moves towards it, the contact will be open.

If a contact is connected to a load and the contact is closed, the load or lamp will be turned ON.  Now if instead of wiring the switch to the lamp directly as suggested in the above diagram, you could connect the switch to the PLC’s input and connect the lamp to the PLC’s output, and then write the above ladder program to perform the same job. Of course it makes little sense to use a PLC if that is all you want to do. We will see how a PLC can simplify wiring shortly.

Mow, let’s say if there are 3 switches that must work together to control the lamp. A master must be ON, and one of the two controls switches ‘controlsw1’ and ‘controlsw2’ must be ON while the other must be OFF in order to turn ON the lamp. We can wire all 3 switches to 3 inputs of the PLC and the lamp to the PLC output.

A contact with a “/” across its body is a Normally-Closed (NC) contact. What it means is that the ladder program is using the inverse of the logic state of the input to interpret the diagram. If “Master” and “controlsw1” are turned ON but “controlsw2” is turned OFF, the lamp will be turned ON since the inverse logic state of an OFF state “controlsw2” is true. An imaginary think current flowing through the “Master” contact, then through the “controlsw1” and at the end through the normally closed “controlsw2” contact to turn ON the lamp.

In other word, if “controlsw1” is OFF but “controlsw2” is ON, the lamp is also turned ON because the current could flow via “Master” and then through the lower parallel branch via N.C. “control sw1” and N.O. “controlsw2”.

Relay Coils

A contact can also be activated by the presence of an electrical current. This makes it possible for a control system to control the turning ON or OFF of a large load by using electrical current to activate a switch that can conduct high current. The simplest form of this contact type is a relay.

A coil of wire is wound around an iron core that turns it into an electromagnet in traditional electromagnetic relay. When current passes through the “coil” the magnet is “energized” and the force is used to either close a contact or open it.

Ladder logic programming language borrows some of those terms used to describe the electromagnetic relay for its own use. You connect a relay coil to the right end of the ladder diagram just like an output. In a PLC, there are hundreds of internal relays which are supposed to behave like the typical electromagnetic relay. Unlike an output which has a physical connection out of the PLC, when an internal relay is turned ON, it is said to be energized but you will not see any changes in the PLC’s physical I/Os.