A Relay is an electrical switch that opens and closes under the control of another electrical circuit.

Types of relay:

1. Reed Relay - A reed relay has a set of contacts inside a vacuum or inert gas filled glass tube, which protects the contacts against atmospheric corrosion. The contacts are closed by a magnetic field generated when current passes through a coil around the glass tube. Reed relays are capable of faster switching speeds than larger types of relays, but have low switch current and voltage ratings.  

2. Solid state relay - A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability.

3. Latching Relay or keep relay - A latching relay has two relaxed states. When the current is switched off, the relay remains in its last state. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting when power is turned off.

Pole & Throw:

A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways:

Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive.

Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive.

Change-over, or double-throw, contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal.

1. SPST - Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil. A SPST relay has four terminals in total.

2. SPDT - Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil. A SDDT relay has five terminals in total.

3. DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST relays actuated by a single coil. Including two for the coil. A DPST relay has six terminals in total.

4. DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT relays actuated by a single coil. A DPDT relay has eight terminals, including the coil.

Applications:

• to control lighting circuits with a low-voltage signal,

• to isolate the controlling circuit from the controlled circuit when the two are operating different voltages,

• to perform time delay functions.