7.3. Relays
Relays, their types
A relay is a device designed for remote controlling another device. The word relay is defined as "to pass on". That is what the electromagnetic device does. A relay uses low voltage and low current to cause the switching off/on of high voltage or high current, usually at a distant or remote location.
The relay operation principle is based on the solenoid and its ability to generate an attracting magnetic force. One or more sets of contacts are associated with the moving armature of a relay. These serve as electrical contacts and make or break when the relay current is switched on and off. The relay contacts can be arranged for a variety of functions, such as SPST (single-pole single-throw), SPDT (single-pole double throw), DPDT (double-pole double-throw), or other desired combinations. The advantage of the relay is the substantial pulling power that can be developed with a small coil current. The contacts themselves can be made quite large and can handle and switch high values of electrical power. An extremely small amount of control power can be used to switch much higher voltages and currents in a safe manner.
For example, the control relay is electromagnetically operated and held. Energising of the magnet coil causes the normally open contacts to close and the normally closed contacts to open. De-energising of the coil causes the contacts to switch back to their original state. The latching relay, the control relay, and the timing relay work together to form a logic system for making automated plants possible. The control relay is designed to switch inductive and resistive loads in both ac and dc circuits. By far the greatest number of applications involve the switching of inductive loads in ac circuits.
Electromechanical relays have a limited number of open-close cycles due to physical wear of the contacts. When control circuit relays are operated at maximum rated load, the life of the contacts is usually less than that of the remainder of the device. If the application requires a large number of operations during the life of the contacts, the contacts must be applied at values less than their maximum make and break ratings.
Solid-state relays. Solid state refers to relays made with silicon or germanium materials that operate on the same basic principles as transistors and diodes. In most instances the relay is nothing more than a transistor, either PNP or NPN type*. In other instances the solid-state relay is a silicon-controlled rectifier (SCR). Of course, the circuit arrangements are such as to allow the switching needed for the relay action. Other features, not readily available in electromechanical relays, are also available in this type of relay. As with everything, there are advantages and some limitations or disadvantages. One of the first differences noted between electromechanical and solid-state relays is the absence of a coil and no contacts. The solid-state relay needs very low voltage and current to do its job of switching. The transistor or SCR do the actual switching and the change in control voltage causes the semiconductor device to conduct or not conduct according to the control voltage applied to its elements.
*transistors can have different junctions: PNP (positive-negative-positive), when the transistor is forward biased and NPN (negative-positive-negative), when a transistor is reverse biased.
Reference:
Miller, R. et al. (2013) Industrial Electricity and Motor Controls, McGraw-Hill Education.