An integral part of digital output control systems, the relay is an indirectly operated electrical switch. It is useful for implementing remote control of a system and for controlling high current devices with a low current control signal.
They operate on the same principles as a solenoid, except that the core is fixed in place. The relay has an input coil that uses a voltage/current to create a magnetic field. As the coil becomes magnetized it pulls a metal switch (or reed) towards it; thus, making an electrical contact. A contact that closes when the coil is energized is normally open. In contrast, a contact that the reed touches when the coil is not energized is normally closed.
In the figure to the right, the magnetic force pulls the armature toward the coil, and through the mechanical lever, forces the electrical contacts closed to close the controlled circuit. The springiness of the contacts cause them to open with the coil is de-energized. Relays can have normally open contacts (as the figure depicts), normally closed contacts, or various combinations of both.
Relays are used to let one power source close a switch for another. It is often the case that the other switch has a high voltage/current. The relay can accomplish this control while keeping the switches isolated.
Prior to the development of electronic programmable controllers, relay functions were performed by real hardware relays that were physically wired together. When a programmable controller is used, the only real hardware relays required are those for the actual outputs to the machine.
A solid state circuit used to store a sequence state or to combine multiple relay logic paths are referred to in design and programming as "control relays". In comparison to the hardwired relay method, control relays are solid-state memory locations. The digital code contents of these memory locations represent the state of a relay contact. Instead of using a meter to determine whether a real relay contact is set open or closed, the appropriate memory locations need only be examined.
Relay outputs are one of the most common outputs. Relays can be used with both AC and DC loads, as the figure to the left illustrates. A load is a term used to describe whatever is connected to our outputs. It is called a load because the outputs are being loaded with something. If no load is connected to the outputs, and the output is connected directly to the power source, then damage to the outputs would most likely occur. Some common forms of loads are solenoid, lamp, motor, and etc. These loads come in a variety of electrical sizes.