A relay is one of the most used components in industrial automation and control, in this article, I am going to explain what is a relay, the type of relay, and how to correctly use a relay.
What is a relay?
A Relay is an electronically controlled switch. It consists of a set of input terminals for single or multiple control signals and a set of operating contact terminals.
Let’s say you want to turn on a fan when the level of carbon monoxide reaches a certain level, instead of having a carbon monoxide detector and someone to start the fan when it reaches a certain level, most fixed gas detectors will come with a relay and the relay will turn on or off the fan when the co levels reach a certain level.
How do relays work?
A relay consists of two parties, the first part is the magnetic coil, a magnetic coil is used to activate the switching action depending if there is electric power or not.
The second part of the relay is the contacts, contacts as their name suggests are used for power connections with the external devices. Usually are normally open, normally closed, and common contacts.
The electromagnet starts energizing when the current flows through the magnetic coil and then intensifies the magnetic field.
The electromagnet becomes connected to the power source through the contacts to the load and a control switch.
The upper contact arm becomes attracted to the lower fixed arm and then closes the contacts resulting in a short circuit.
The contact then moves in the opposite direction and creates an open circuit once the relay has been de-energized.
The movable armature will return to its initial position when the coil current is off. The force that causes its movement will be almost the same as the half-strength of the magnetic force. Spring and gravity provide this force.
Types of relays
There are four types of relays, and each type of relay will require to be wired differently, if you do not know what type of relay you have, you can check the connection diagram of the particular relay and figure out which type you have.
SPST – Single Pole, Single Throw
An SPST Relay or Single Pole Single Throw Relay is the simplest type of relay out there, it consists of one magnetic coil and one set of contacts. It only connects or disconnects only one contact when it is operated.
SPDT – Single Pole, Double Throw
A Single Pole Double Throw (SPDT) relay is a relay that only has a single magnetic coil and can connect to and switch between 2 contacts.
It is the most used relay type in the industry. It consists of one Coil and 2 sets of Contacts (1 Normally Open & 1 Normally Closed) separated by a Common Contact.
DPST – Double Pole, Single Throw
A DPST or Double Pole Single Throw relay is a type of relay that has two magnetic coils and two Contacts.
Each magnetic coil has one corresponding contact. But both coils are isolated from each other, the same as both contacts are isolated from each other.
It works with two different circuits. It only provides the switching function either on or off. Both the input-output pairs work simultaneously.
DPDT – Double Pole, Double Throw
A Double Pole Double Throw (DPDT) relay is a relay that has 2 magnetic coils and 4 contacts, each magnetic coil has 2 corresponding contacts that it can connect to.
The DPDT Relay represents two sets of SPDT Contacts with the +V connections tied together
Designed for when a single relay is needed to activate/deactivate two external devices, such as a Horn and a Strobe light.
Conclusion
That is it, in this article, we defined what is a relay, and the types of different relays that you can find in the industry, thank you for reading.
