What Are Relays?

Relays have a long lifespan and have the ability to handle high levels of current and power. They are normally used in situations where a large current would otherwise exceed the capacity of switches and wiring.

A relay contains an iron coil that is surrounded by a moveable spring loaded part called the armature. When the coil is powered it generates a magnetic field that closes an electrical contact.


The function of a relay is to allow one input to activate many outputs and perform functions that are impossible or very difficult to do with other components. Relays can also perform advanced Boolean logic functions and are used in a wide range of electrical and electronic systems.

A relay’s coil can be powered by low-voltage DC like 5, gate driver 12, or 24 volts, or AC such as 24 volts or 120 or 230 volts. The coil wire is wrapped around a core made from an iron or other material. When the coil is energized, current flows through it and creates a magnetic field that can move contacts to open or close the circuit.

The number of poles on a relay determines how many circuits it can control at once. A single-pole contact can only break an electrical circuit in one place, while a double-pole switch breaks it in two places. Relays with a higher pole count have more switching terminals. A DPDT relay, for instance, has 12 switching terminals and can handle more current than a SPST switch.


Typically the relay has a heavy-duty frame that houses and supports the moving parts. A coil of wire is wound around the frame and causes an electromagnetic field which, when energized, pulls down on the armature. This completes the ‘high current circuit’ between the terminals and the relay is said to be ‘on’. When the coil is de-energized a spring pulls the armature back to its ‘at rest’ position.

The relays contacts come in different configurations, referred to as poles, breaks and throws. A common configuration is Single-Pole, Single-Throw (SPST) which has two terminals that can be connected or disconnected. Normally-open (NO) contacts connect the circuit when the relay is activated; they disconnect when the relay is inactive. They are also called Form A contacts or early-make contacts.

If the relay is exposed to long-term hot and humid conditions, or it is stored in bare packaging, its insulation can degrade. This results in electrochemical corrosion which can cause the metal parts of the relay to oxidise and break down. The result is arcing which can produce a short circuit between the relay’s contact terminals.


Relays are very reliable in benign environments, however they are subject to wear and tear and will have a limited number of switching operations at full rated load. This limitation is called mechanical life.

Routine maintenance consists of checking the relay for proper operation by operating the controlled device (circuit breaker, auxiliary relay, or alarm). It also includes verifying that all internal logic functions used in the protection scheme are functional. In addition, for communication assisted protection schemes, the end-to-end testing of both receive and transmit communication circuits should be performed.

Never touch the terminal section (charged section) of a relay with your hand while the power is supplied. This could result in insulation failure between the contacts, contact welding or burning of the relay itself. If possible, enclose the relay in an enclosure to keep it from being exposed to the elements. Also, make sure pigtail Quadrant Analog Multiplier wires and screw terminals are properly attached so that they do not short circuit. It is also a good practice to wrap each wire in the coil with electrical tape so that no one can inadvertently touch it while the relay is powered.


Relays can fail in a number of ways, but they are usually easy to repair. Most of the problems are caused by contamination, electrical arcing, and physical wear on the contact plates. These are all things that can be fixed easily with a multimeter and a little patience.

The problem with contamination is that it can degrade the life of a relay. If the contacts become contaminated with pitting or corrosion, the current will have to pass through those areas instead of through the coil windings. This can cause arcing on the contacts that causes them to “weld” together and produce a short circuit in the system.

This can also happen when the coil is energized for too long. The arcing can produce high voltage spikes that can damage components in the circuit. This is why it is important to test all relays after repairing them. If the problem is not resolved, it may be necessary to replace the relay. Fortunately, O’Reilly Auto Parts has a great selection of relays to help you get your vehicle running again quickly.


Relays come in a variety of sizes and power ratings to suit a range of applications. They are basically electric switches that open and close circuits by moving parts.

The main components of a relay are the frame, coil and armature. The coil of wire causes a magnetic field to be created and the armature opens and closes the contacts. When the current is switched off a spring returns the armature to its original position.

Different types of relays have different configurations of these parts, and some have multiple contacts. For example, a relay can have a single-break (SB) or double-break (DB) contact. The type of contact that is used determines which part of the electrical circuit it opens or closes.

When a relay coil is powered and the switch is closed, it generates a large reverse EMF that can damage semiconductor devices such as transistors. To prevent this, a flyback diode is always connected across the coil to neutralize this reverse back EMF. This is often a simple 1N4007 diode.

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