An electronic relay is an electromagnetic switch operated by an electrical signal from an outside source, then opens or closes a circuit. We use very often relays in security systems.
In this article we will show you
- how relays work
- Types of relays
- Relays in security systems
The invention of a relay
Joseph Henry invented relays in 1835. He applied them in telegraphy. Relays were also applied to switch calls in telephone exchanges.
Transistors can do the similar job to relays. They are tiny components; working as amplifiers and switches further, they switch very faster. Therefore, transistors replaced relays in electronics but they are still in use. As we do in security systems.
How a relay works?
The diagram shows a lamp controlled by a relay. The lamp is powered by an outside electrical source. As shown on a relay diagram. When a coil is energized through a control switch, it generates the magnet field. Thus, the open contact will start to be attracted. Then it will close. As a result, a power will flow to enlighten the lamp.
On the other hand, as soon as the current is off, the contact will go back to it’s initial position thus the lamp will turn off.
Types of relays
- NO ( Normally open)
This type of relay closes the circuit when it is activated and opens it when it is inactive.
- NC ( Normally closed )
In this kind of relays, the contact is closed. Therefore, the current flows by default. Moreover, it stops flowing when a relay is activated.
- Double throw contact
This type of relays controls NO and NC contact with a common terminal
Relays in practice
A relay circuit is used to realize logic functions.
With Relays, we can control high voltage circuits with the help of low voltage signals.
Relays in security systems
All detection devices are based on relays, thus the detection triggers the relay then it switches its position from close to open or vice versa. Therefore, the alarm will be sent. The diagram The diagram below shows the detection process . In this case its not the power that triggers the relay however it triggers because of the detection
Anti burglary panels are built with programmable outputs. They can supply voltage at specific events. Thus, the output triggers a relay to activate any other circuit.
The diagram is a CR624 relay, wired to a programmable output on a DSC alarm panel. If an output supplies current to the relay. It changes it’s position then closes the lamps circuit, and then the light will flare.
The alarm panel and relay diagram can be applied with any detection event. For example, An air conditioning goes on if a certain degree of temperature is reached. a garage door opens if we press a button.etc.
Access control and relays
Access control panels are equipped with relays. Actually, opening a door in general is based on a relay mechanism. Let’s see it through a following diagram diagram how a relay plays a role in opening doors: :
An access granted will make the relay switch from the open position NO to a close NC. As a result, the current will flow from the power source to the door strike then the door opens.
As we see, relays play a very important role in security systems. Thanks to them, we can build so many logical circuits. We can activate or deactivate so many actions automatically following certain events
