Introduction to Industrial Automation

Table 2.2  Contact States and Current Flow with Regard to Relay State

Relay

Contact

Non-Energized

Energized

Normally closed (NC)

I

NC

I

Normally open (NO)

I

NO

I

Table 2.3  Possible States of Two Simple Changeover Two-Way 

Contacts

Relay

Contact

Non-Energized

Energized

SPDT

I

I

NC

NO

I

I

DPDT

I

1

I

1

NC

NO

I

2

I

2

NC

NO

I

1

I

1

I

2

I

2

---

Hardware Components for Automation and Process Control 

◾

 

35

The issue of selecting the suitable relay for an application depends on many parameters that 

concern the kind of construction; operation principle; size; and electrical characteristics, such as 

coil nominal voltage, the contact’s rated current, the number of contacts, and many others. The 

manufacturers of relays offer detailed catalogs containing tables with all mechanical and electrical 

characteristics of the relays to customers, as shown in Figure 2.17, which summarizes the required 

information for general purpose relays of plug-in socket, or PCB mounting type. For power 

relays, the same technical characteristics are available, but their selection is mainly based on the 

rated current of the main contacts. Furthermore, the power relays are also characterized by the 

power being able to feed to the electric motors.

We will close the section on relays with some general conclusions:

 

◾

Relays are used either as power switches or as auxiliary logic components.

 

◾

Relays permit a low-voltage circuit to control another high one or in general different volt-

age one.

 

◾

Relays can be used as current “amplifiers”.

 

◾

Relays provide complete electric isolation between the control signal and the power signal, 

that is between the auxiliary control circuit and the power circuit.

 

◾

By using relays with multiple poles of contacts, it is possible with one low-voltage signal to 

control the operation of many loads, each one with a different voltage.