Relays and their Applications LAB

Do Not Submit This Lab Instructions

Prelab:

Not all relays have identifiable pins. However, they can easily be identified using the simple procedure given below. Two of the pins are coil terminals. Depending on the relay, a measurement of 200-1000 Ω resistance is expected across these two pins. A relay may have two or more sets of contacts. Each set consists of a NO and a NC contact. Each contact has two terminals; however, in each set of contacts, one pin is shared by the two contacts as shown in Figure 1. Again, C1 and C2 are common terminals between the two contacts on each side, and it does not mean that they have to be connected to the common ground (a common misconception!). Figure 1 is a generic diagram and does not necessarily represent the order of the pins on your relay.

Figure 1

A single-pole double-throw relay is shown below Figure 2.

Figure 2. A single-pole double-throw relay consisting of a coil and contacts.

SIMPLE RELAY APPLICATIONS

Emergency Lighting System

The circuit shown below in Figure 4 can be used to simulate an Emergency Lighting System, that is, a scheme to illuminate some lights from a Backup Power source when either the Primary Power source or a strategic Lamp fails.

Figure 3. An Emergency Lighting System.

The Main lamp is connected to the Primary Power through the relay coil. The Emergency lamp will be connected to the Backup Power if either the Primary Power or the Main lamp fails. The circuit is shown in the normal mode of operation, with the Main lamp connected to the Primary Power.

In this scheme, the relay coil and Main lamp are connected in series to the Primary Power, a 12 Volt battery. As long as the Primary Power remains on and the lamp is good, the relay will be energized, opening the normally closed (NC) contact, and the Emergency lamp will not be connected to the Backup Power. If the relay coil becomes de-energized due to either the loss of the PRIMARY POWER or the failure of the Main lamp (an open circuit), the relay contact will return to the normally closed (NC) position, connecting the Emergency lamp to the Backup Power, restoring lighting. In a “real-life” situation, the relay coil and Main lamp would be connected to 120 Volts AC while the Emergency lamp would be powered by a 12 Volt battery. In addition, there would be provisions to keep the 12 Volt battery fully charged while the Primary Power is on.

Backup Power System

The circuit shown in Figure 4 shows a method of providing Backup Power to a load (the lamp in this case) when the Primary Power source fails.

Figure 4. A Backup Power system.

The lamp X1 is normally energized by the 12 Volt Primary Power source. The circuit is shown in the Backup mode due to the loss of Primary Power.

The Primary Power, a 12 Volt battery, is connected across the relay coil. As long as the relay is energized, the lamp will be connected to the Primary Power via the NO relay contact. When the Primary Power is removed, or fails, as shown, the lamp will be connected to the 9 Volt Backup Battery via the NC relay contact.

Lab Report:

Complete Week-1 lab Worksheet available under the Files (Week-1 Lab Folder), go to Modules, and under Lab assignment submit your lab report.

Week 1 Lab Instructions Page 1

Week 1 Lab Instructions Page 3

K

K1

1mH 1 Ω

PrimaryPower

12 V

J1

Key = O

BackupPower

9 V

X1

9 V

NO

NC

COM

K

Relay

1mH 1 Ohm

Coil

Contacts/Terminals

K

K1

1mH 1 Ω

PrimaryPower

12 V

J1

Key = O

BackupPower

9 V

Emergency

9 V

NO

NC

COM

Main

9 V