What is a relay?

A rough guide for the interested by Brod Bass, of Kuhnke. ©2005

A relay is an electrically operated switch It comprises a coil (of wire) , a plunger, and contacts, all housed to gether within a case or body. Energising the coil creates a magnetic field which moves the plunger. The movement of the plunger causes a moving contact to move toward (or away from) a non moving contact. It is the coming together of these contacts ( making) or separation of them (breaking) that has the effect of switching. Wires or electrical connections internally link the coil to the external connections to which the operating supply will be connected Similarly internal contact connections go to other terminals for connection to the circuit being switched. The connections to a relay are usually in the form of pins or blades in the base, which may plug into a matching socket to which external wiring is connected conventionally. Many smaller relays are soldered directly into circuits

A relay may have supplementary features. Common examples include manual test button which can override the operation of the coil plunger so that a circuit can be tested without having to power the coil. An indicator light can be fitted to show if and when the coil is energised; this is often referred to as a coil LED (Light Emitting Diode) or coil indicator. Electrical protection devices can be integrated into the relay which protect the coil from electrical faults. These features are by no means standard in all industries.

Relay coils are wound for a particular voltage. Most manufacturers produce a range of coils for the most commonly used voltages 24VDC is a popular and safe voltage for control automation, 115VAC is often used in North America 230VAC is found in UK building systems, but there are many more. A coil will activate its plunger as the voltage increases from zero to its normal rated level (nominal voltage ), this is know as the pull in voltage ( often around 80% of nominal). As power is removed from the coil the plunger returns to its rest position as the voltage drops below its drop-out voltage ( often around 20% of nominal) The connections to power a coil are via 2 pins or blades, with particular emphasis on the safe separation of coil operating circuitry from the switching contacts.

The most basic relay has only 2 contacts. You either bring them together by operating the coil (or manual override), known as making or you separate them, known as breaking In this case the relay is defined as single make contact or single break contact These two versions will each have 4 terminals, one for each individual contact and 2 for the coil.


Contacts can be arranged to perform different actions when the coil is energised. There are 3 common contact arrangements Make and break are described above. A changeover or throw arrangement combines a set 3 of contacts. which comprise two fixed , and one moving one. The moving one has a rest position in contact with one of the fixed contacts. When the coil is energised the moving contact breaks away from this first fixed contact and moves to make contact with the second fixed contact. In practice this moving contact has two contact points. This arrangement of 3 contacts is known as a single pole changeover(SPCO) or single pole double throw (SPDT) A single pole changeover relay will have 5 pins for connection. 3 for the contacts, 2 for the coil.

A changeover relay can be the simple single pole version described above, or muti pole. The plunger of the coil can be used to changeover more than one set of 3 contacts. Each set is often known as a pole. So we have 2 pole, 3 pole 4 pole, even 6 pole and 8 pole relays. Each pole has 3 terminals. So a 4 pole relay will have in total 14 terminals or pins: 12 for the contacts, and 2 for the coil.

We can define a particular relay by the coil voltage, the contact arrangement, and the pin out ( the number of connecting pins and their dimensions ) but most importantly we must consider what electrical load is being switched An electrical load is defined in terms of voltage (particulary AC or DC which behave very differently), current and type such as inductive load or resistive load A relay may be used to switch very low currents, say 50mA or much higher ones, say 50A. A DC load can cause an arc to form as contacts break in a way that an AC load will not. An inductive load can create an inrush current that is 10 times higher than the nominal load It is this consideration of load that finally determines the specification of the relay for a given application, and the one that may require special contact materials or arc suppression components .

The different combinations of variables can make selecting or replacing a relay a complex task, particulary when trying to relate the application to a range of standard products. The Relay Company have created a web site that seeks to make this easier for both the occasional relay user and the seasoned professional