Introduction to Industrial Automation

2.1.6   General Purpose Relays

General purpose relays are usually miniature relays, used either as auxiliary components for the 

implementation of automation circuits, or as switches for supplying very small electric loads, such 

as electric valves, micromotors, small fans, alarm sirens, etc. Their use as auxiliary relays or as aux-

iliary contacts of a power relay means they are necessary components for the implementation of 

the Boolean logic functions described in the automation circuit. This issue will be further analyzed 

in Chapter 4, where the logical design of the automation circuits is presented.

In general, there is a huge variety of general purpose relays regarding their size, shape, number 

of contacts, coil voltage, and mounting methods. All of the relays of this type have the same opera-

tion principle as that of power relays and an example of their typical form is shown in Figure 2.9. 

These relays are mounted either on a specific base for accepting electric wires (easy replacement 

of a relay), or directly on a PCB board as a component of an electronic circuit. Figure 2.10 shows 

the two side views of a general purpose relay in its physical dimensions, where the basic parts of 

this relay are evident as are the coil, the movable armature and contacts, the double fixed contacts, 

and the return spring. General purpose relays are protected by a plastic case to protect the mecha-

nisms from dusty and corrosive environments. The movable contact arm, called common terminal 

(C), has two contact tips and it is located between two fixed contacts, which forms a normally 

(a)

(b)