Functional parts of rp and a devices and conventional symbols used in them

LECTURE 3
FUNCTIONAL PARTS OF RP AND A DEVICES AND CONVENTIONAL SYMBOLS USED IN THEM
Contents:

• 
  Types of relay
  
• 
  Symbols
  
• 
  Characteristics
  

Relays are essential for automation systems and for controlling loads. Also, relays are the best way for galvanic insulation between high and low voltage portions of a circuit. There are hundreds of different relay types. Let’s find out first how a relay operates.

Fig 3.1. Different types of relay

Relay Types

There are so many different types of relays, that it would be literally impossible for me to add them in this article.
Therefore, it will categorize the types of the relays in terms of:
1. Turn ON/OFF operation
2. Coil
3. Contacts
Category 1. Turn ON/OFF operation
Normal relays
There are basically two types of relays in this category. The first type is the normal on/off relay. This relay changes state as long as the electromagnet is actuated, and goes back into relax state when the electromagnet is not actuated any more.

Fig 3.2 This is the most common relay type and is used widely in automation.

Toggle relays

This type of relay operates just like a toggle flip flop. When the coil is once actuated, the relay will change state, and will remain in this state even if the coil in no more actuated.
It will only change state again on the next pulse that will actuate the coil. This is very handy in modern house lighting.
Having this relay instead of a switch, you can turn on and off the lights with one pushbutton. You press the pushbutton once and the lights are turned on. On the next pushbutton press, the lights are turned off.