Electrical Relays Explained: How They Work and Where to Use Them

Electrical relay is a device that enables one electrical circuit to be controlled by another. Its main advantage lies in the fact that a relatively small control current can switch on and off equipment that consumes significantly more power. Importantly, the current of the power load does not flow through the control electronics.

For example, a microcontroller cannot directly power a large electric motor or heater. However, it can control a relay whose contacts are rated for the required voltage and current. This principle is widely used in cars, household appliances, industrial automation, and smart home systems.

Let's examine how a relay works, its key components, why an electromagnetic coil causes contacts to switch, and how a small electrical signal can control a powerful load.

What is an electrical relay and why is it needed?

A relay is an electrical or electronic switching device that changes the state of one or more circuits when a control signal is received. In a classic electromagnetic relay, switching occurs due to a magnetic field generated when current flows through the coil.

The main function of a relay is to control an electrical load without the need for the load current to pass directly through a button, sensor, or electronic controller.

To understand the principle, imagine two separate circuits:

• Control circuit - sends an electrical signal to the relay, usually consuming relatively little power.
• Power circuit - supplies power to the connected load: a motor, lamp, heater, or other equipment.

These circuits perform different tasks. The control circuit determines when the equipment should turn on, while the power circuit provides the necessary electrical energy.

For example, an electronic controller operates at 5 V, while the connected motor is powered from a 230 V mains supply. The controller cannot directly switch such a load but can control the relay coil via a suitable driver. When the relay is triggered, its contacts close the motor's power circuit.

As a result, the relay does not amplify current or create extra electrical power. It simply allows a low-power control device to switch a separate circuit that gets energy from its own power source.

Electromagnetic relays also provide galvanic isolation: there is no direct electrical connection between the coil and the power contacts. Control is transmitted via the magnetic field and mechanical movement, which allows circuits with different voltages to be separated-though the safety of this separation depends on the relay's design and specifications.

Despite the widespread use of transistors and electronic switches, electromagnetic relays remain popular for several reasons:

• Versatility: Suitable models can switch both AC and DC.
• Low closed-contact resistance: When contacts are healthy, power losses are minimal.
• Physical circuit separation: When contacts open, an air gap ensures electrical isolation.
• Simplicity: Only the specified coil voltage needs to be applied to trigger the relay.
• Diverse configurations: A single relay can switch multiple contacts or select between different circuits.

However, mechanical relays have a limited contact lifespan, make a characteristic clicking sound, and require a certain switching time-these are inherent to their physical design and operation.

Electromagnetic relay construction

An electromagnetic relay is relatively simple: inside the case are a coil with a metal core, a movable armature, a return spring, and a system of electrical contacts. These elements together convert an electrical signal into mechanical movement.

Unlike a transistor, which controls current through semiconductor properties, an electromagnetic relay physically moves contacts-hence the familiar click when it switches.

Coil, core, armature, and contacts

The main element is the relay coil-a winding of insulated copper wire. When current passes through it, a magnetic field forms around the turns, interacting with the ferromagnetic parts and generating the force needed to switch contacts.

A typical relay includes:

• Relay coil - creates a magnetic field when control current flows. Its characteristics determine the required supply voltage and power consumption.
• Core - concentrates the magnetic flux and enhances the attraction of moving parts.
• Armature - a movable metal part attracted to the core when the magnetic field appears.
• Return spring - returns the armature to its initial position when coil power is removed.
• Electrical contacts - open, close, or switch the load circuit.

All components are housed in a case that protects the mechanism from external influences. Depending on the purpose, the case may be plastic, sealed, or specially designed for harsh environments.

When voltage is applied to the coil, the magnetic field attracts the armature to the core, moving the contact group and changing the electrical connection between relay terminals. When the coil is de-energized, the spring returns the armature and contacts to their original state.

A key point is that the coil and contacts are electrically separated: the coil gets control power, while the contacts operate a separate load circuit. For example, a relay with a 12 V coil can switch a 230 V circuit, if its construction and ratings allow.

The coil voltage and allowable contact voltage are different: the "12 V" marking usually refers to the coil's nominal voltage, not the maximum load voltage.

Normally open and normally closed contacts

Relay contacts can be in different positions depending on whether the coil is powered. The states are denoted by the international abbreviations NO, NC, and COM.

The normal state means the relay coil is de-energized:

• NO (Normally Open): The contact is open without power; it closes when the relay is triggered.
• NC (Normally Closed): The contact is closed without power; it opens when the relay is triggered.
• COM (Common): The shared contact connected to either NO or NC depending on the switching mechanism's position.

For example, a lamp wired through an NO contact will only turn on after the relay is triggered; when power to the coil is lost, the contact opens and the lamp turns off. With an NC contact, the lamp stays on until the relay is triggered, then turns off.

In a changeover relay, COM connects to NC when the coil is de-energized. When power is applied, the armature moves, COM disconnects from NC and connects to NO. Not all relays have all three terminals; some only have NO or NC contacts, and some have multiple independent contact groups for switching different circuits simultaneously.

The contact configuration is chosen to suit the required control logic: NO for activating a load on command, NC for keeping a connection until the relay operates.

How a relay works: controlling high power with a small signal

A relay operates by converting electrical energy into mechanical motion. When current flows through the coil, a magnetic field forms, attracting the armature and switching the contacts. This allows a small control circuit to turn powerful equipment on or off.

The crucial point is the separation of the control signal and load power. The relay does not transfer energy from the low-power controller to the high-power device; it only opens or closes the current path from a separate power source.

How a small current switches high power

Consider a relay with a 12 V coil controlling a 1000 W heater connected to a 230 V supply. There are two independent circuits: one powers the coil with 12 V DC, the other supplies the heater with 230 V AC.

1. Control voltage applied: 12 V is supplied to the coil, causing current to flow through the winding.
2. Magnetic field created: The coil's current generates a magnetic field, attracting the armature to the core.
3. Contacts move: The armature mechanically closes the NO contact.
4. Load switched on: Current flows from the 230 V supply through the closed contacts, turning on the heater.

A 1000 W heater draws about 4.35 A, while the relay coil may consume just a few dozen milliamps-depending on the model.

This demonstrates how a relatively small control current can switch a much larger load. However, the energy for the heater comes directly from the mains, not the relay coil. The relay is not a power amplifier-it acts as an electrically controlled switch, letting one circuit control another.

Unlike a manual switch, where a person directly opens or closes contacts, a relay does this automatically via a magnetic field. Note: even a small relay coil may require more current than a microcontroller output can supply, so a transistor driver is often used to provide the necessary coil current.

What happens when the relay is switched off

When the control voltage disappears, current through the coil decreases, the magnetic field weakens, and the armature is no longer held in place. The return spring moves the armature back, and the contacts return to their original state. If the load is wired via NO, its circuit opens and power is cut off.

The mechanical movement of the armature and contacts produces the characteristic relay click, both when switching on and off, with the volume depending on the relay's design and size.

But de-energizing the coil also produces self-induction. The coil stores energy in its magnetic field; when the control circuit opens and current drops rapidly, the changing magnetic flux induces a voltage spike in the coil. This voltage tries to maintain the current. Without protection, a high-voltage pulse may appear across the coil, potentially damaging the driver transistor or other electronics.

Protection components used include:

• Flyback diode - mounted across a DC coil to provide a path for the self-induction current after power is removed.
• TVS diode - clamps dangerous voltage spikes and can switch off the relay faster than a regular diode.
• RC snubber - suppresses transients and overvoltages in circuits, especially when switching inductive loads.

A standard diode is suitable for DC coils, but not for AC coils, which require different suppression circuits.

This is a direct result of inductance physics: a coil resists sudden changes in current because altering the magnetic field induces a voltage (EMF) in the opposite direction. For a deeper explanation, see Inductance and Self-Induction: A Simple Explanation for Beginners.

The behavior of the power contacts when switching off an inductive load is also important. If the relay breaks the circuit of a motor, electromagnet, or other inductive device, the stored energy can cause an electrical arc between separating contacts. Such arcing accelerates contact wear and, if the relay is not properly rated, can damage or weld the contacts. The allowable load depends not only on maximum current but also on the type of equipment being switched.

Thus, a relay rated for a certain current with a resistive load may not safely switch a motor with the same nominal current. For inductive loads, inrush currents, voltage spikes, and application category must be considered.

Types of electrical relays and their differences

Relays vary by switching method, construction, and operating conditions. While their main role-controlling a circuit via an external signal-remains the same, different types solve this in different ways.

The most common are electromagnetic relays and solid-state relays (SSR). The former use mechanical movement of contacts; the latter switch the load with semiconductor components. This affects speed, durability, power consumption, and environmental suitability.

Electromagnetic vs solid-state relays

An electromagnetic relay uses a coil and moving contacts. Applying control voltage generates a magnetic field, moving the armature and changing contact positions. The main benefit is physical disconnection of the circuit-when contacts open, an air gap isolates the load; when closed, low resistance ensures good connection.

However, mechanical parts wear out over time. Each switch cycle causes contacts to touch and separate, and electrical arcing can damage the surfaces. So, relay lifespan depends on both the number of operations and the nature of the load.

A solid-state relay (SSR) has no moving contacts. Instead, it uses semiconductor elements such as triacs, thyristors, or transistors. When the control input receives a signal, the circuit turns on the power device, allowing current to flow through the load-without moving parts.

SSR advantages include silent operation, suitability for frequent switching, and no mechanical wear. However, even when on, semiconductors cause power losses (heat), so high-current SSRs may need heatsinks. Also, many SSRs pass small leakage currents even when off, so the load might not be fully disconnected.

However, SSRs are not a universal replacement for electromagnetic relays. Some SSRs are only for AC, others for DC; their power elements and control circuits differ, so the device must match the specific load.

For occasional switching of lights, heaters, or motors, a standard electromagnetic relay is usually sufficient. For silent, high-frequency switching, SSRs may be preferable.

Many electronic switches are based on field-effect transistors (FETs), which control current flow via an electric field. Learn more in How Field-Effect Transistors Work: MOSFETs Explained, Structure and Applications.

Relay vs contactor: What's the difference?

Besides relays, contactors are widely used in electrical engineering. They operate similarly: a control coil creates a magnetic field that moves mechanical contacts.

The main difference is in their purpose and construction.

Standard electromagnetic relays are often used in electronics, control, and automation systems, switching signal lines, small motors, lighting, and other loads within their ratings. Contactors are mainly for switching power circuits, especially for controlling large motors, heaters, or industrial equipment.

Contactors for high current may have large contacts, arc-suppression systems, and robust features for heavy-duty operation. For example, a small controller can operate a contactor coil that switches a three-phase industrial motor, without passing motor current through the controller itself.

However, the difference isn't just current rating: there are power relays for high loads and compact contactors for moderate currents. The choice should be based on application, construction, electrical characteristics, and category of use.

The type of load is especially important. Motors may draw much higher starting currents than their rated value, so contactors for motors are selected according to starting and stopping conditions, not just working current. Industry uses application categories, e.g. AC-1 for non-inductive loads and AC-3 for certain asynchronous motor controls.

Thus, a contactor is a specialized electromagnetic switch designed for power circuits, while standard relays are more common where compact size, varied contact configurations, and control of individual signals are needed.

Where relays are used and how to choose the right device

Relays are found wherever automatic switching of electrical circuits is needed: cars, appliances, industrial systems, security, and smart home devices. Despite similar control principles, requirements vary widely. Some relays handle small signals; others control motors or heaters of several kilowatts.

Applications of electrical relays

• Automotive: Switching high-current consumers like cooling fans, horns, auxiliary lights. The control signal comes from a button or control unit, while load power is supplied separately, reducing stress on switches and electronics.
• Home appliances: Switching heating elements, motors, compressors. For example, a washing machine's control board might use a relay to turn on the heater or control a fridge compressor.
• Smart home: Automating lights, fans, pumps, etc. The signal may come from a sensor, timer, microcontroller, or central automation system.
• Industry: Used in control cabinets, process automation, and for interfacing controllers with actuators.
• Protective relays: Monitor electrical parameters and act upon detecting deviations (e.g. voltage monitoring relays disconnect equipment if voltage goes out of range).

How to choose a relay by voltage, current, and load type

When selecting a relay, consider both the control and power circuit characteristics. Devices that look identical may have totally different electrical ratings.

Key parameters:

• Coil voltage: The voltage needed to trigger the relay. Common values are 5, 12, and 24 V DC, and AC coil models exist.
• Contact current rating: The maximum load current the contacts can switch under specified conditions.
• Switching voltage: The relay must be rated for the circuit voltage. Allowable AC and DC values may differ.
• Contact type: Determines if the equipment turns on with a signal, turns off, or switches between two circuits.
• Load nature: Heaters, motors, and power supplies impose different switching conditions.
• Electrical life: The expected number of operations under specified load and operating conditions.

It's vital to distinguish between resistive and inductive loads. A heater is mostly resistive; a motor has coils and creates switching transients. Thus, a relay marked "10 A" may not be suitable for all 10 A loads-the rating must be matched to the load, voltage, and conditions given by the manufacturer.

Loads with high inrush currents (e.g. some LED power supplies, motors) can briefly draw much more than their working current at startup, putting extra stress on contacts and accelerating wear.

How to wire a relay for load control

Relay wiring always involves two separate circuits: the control circuit (coil) and the power circuit (load contacts).

In the simplest setup, power is applied to the coil terminals, and the power circuit uses the COM and NO contacts. While the coil is de-energized, these contacts are open and the load is off. Applying control voltage closes COM and NO, powering the load. Removing control voltage opens the contacts and cuts power.

If the opposite logic is required, use the NC contact: the circuit remains closed when the coil is de-energized, and opens after activation.

Important considerations:

• Don't mix up coil and power contacts. Applying the wrong voltage can damage the relay.
• DC coils may require a transistor driver and protective element to suppress self-induction spikes.
• Contacts must be rated for the working voltage, current, and load type.
• Switching inductive loads may require extra suppression components.
• The power circuit should have adequate overload and short-circuit protection.

Warning: Wiring a relay to 230 V mains must comply with electrical safety standards. Never use breadboards, exposed contacts, or relay modules lacking certified insulation and load ratings. Mains work should be performed by qualified professionals.

Also remember, a relay does not replace a circuit breaker, fuse, or specialized protection device-its main task is switching, not preventing short-circuit consequences.

A properly chosen relay provides load control without passing working current through the controller. However, system reliability depends on the relay's characteristics and the design, protection, and operating environment of the entire setup.

Conclusion

An electrical relay enables high-power loads to be controlled by relatively small control currents. In electromagnetic models, this is achieved with a coil that creates a magnetic field and moving contacts that open or close a separate circuit.

The main benefit of a relay is the separation of control and power circuits. This allows a controller, sensor, or button to operate motors, lighting, or heaters without handling the load current directly.

Electromagnetic relays are suitable for most applications with moderate switching frequency. For silent, frequent switching, consider solid-state models. For heavy-duty industrial equipment, specialized contactors may be needed.

When choosing a relay, always consider the coil voltage, contact current rating, load type, inrush currents, and operating conditions. Proper matching of these factors is crucial for reliable and safe operation in any electrical system.