Common Types of Relay

relays You can choose from a wide range of relays but do you know what type to use in your project? Well, sometimes is hard to choose, especially if you’re not sure which model you need or how a specific one works. In this article we will learn about different types of relay and what to use in you circuit to get the desired result.

1 ≡ Electromechanical Relays

These type of relays are made of a coil that induces a magnetic field when it get energized and moves the armature, which closes of opens the contacts.

electromechanical relay

normal closed (NC) electromechanical relay (click on image to see the animation)

Usually there a 2 types of relays:

a) Latching – it can have one or two coils and it can remain in the last position when the current is turned off. These type of relays are useful in applications where power consumption must be low because they require no current to maintain their position.

b) Non-Latching – it has a spring or magnet that maintains its initial state of NC (normal closed) when no current is flowing through the coil. When current flows through the coil then the contact is opened as you can see in the animation above.

The electromechanical relays can be classified by switching type:

  • Single Throw (ST):
    • Normally Open (NO) – the circuit is connected when the relay is active and disconnected when inactive.
    • Normally Closed (NC) – the circuit is connected when the relay is inactive and disconnected when active.
    • Double Pole Single Throw (DPST) – it has 2 terminals and 2 contacts.
  • Double Throw (DT):
    • Single Pole Double Throw (SPDT) – it has one common terminal and 2 contacts
    • Double Pole Double Throw (DPDT) – the contacts can control 4 circuits. It has 2 terminals and 4 contacts.
2 ≡ Reed Relay

reed relay

The reed relay consists of a switch with magnetic contacts that move under the influence of an external magnetic or the induced field from its solenoid. They have faster switching speed compared to the electromechanical ones but their switching current and voltage is lower mainly because of its contacts thickness.
Here you can read more about it.

3 ≡ Solid-State Relay (SSR)

The SSR is basically a circuit with various electronic components that has the same function as an electromechanical relay. The advantage of using SSR is that it doesn’t have moving parts so it lasts longer and doesn’t make noises.

diy solid state relay

There are many other types of relays but their use is limited, are expensive and are not widely available therefore we will not discuss about them in this article.