5 Key Facts About the Electromagnetic Relay Working Principle

If you’re still building your foundation in basic electricity, start with this beginner-friendly overview: 🔹 “Electricity 101: The Complete Beginner’s Guide to How Power Really Works”
After reading it, the concepts in this article will make a lot more sense.

In this guide, we’ll break down the electromagnetic relay working principle using real-world examples from home and industrial control.

▶️ Watch now: How does an electromagnetic relay work? (60-second version)

The basic electromagnetic relay working principle can be explained in under a minute: energize the coil, it turns into an electromagnet and pulls the moving armature, and that movement opens or closes another set of contacts.

In other words, a small control current safely switches a much larger load current. This short video uses simple animation to show you what’s really happening behind that little “click” sound inside the relay.

Introduction | Why you should understand electromagnetic relay basics and applications

If you’ve ever opened an electrical or control panel and seen a row of little rectangular blocks that go “click” when you press a button and a motor, pump, or A/C unit starts up, you were probably looking at electromagnetic relays (also called electromechanical relays) doing their job.

Many people feel that relays are abstract or mysterious, but what they actually do is very simple:

They let you use a small control current to switch a much larger load current, keeping the dangerous high-voltage, high-current side safely separated from the control side.

In this guide, we’ll walk through real-world examples so you can clearly understand:

• What is the working principle of an electromagnetic relay?
• Why do you see them in so many electrical and control panels?
• Where do electromagnetic relays show up in everyday applications like pumps, HVAC, and automation?
• What should you pay attention to when selecting and wiring relays so you don’t create hidden safety risks?

After this, when you open a panel and hear those little blocks “clicking” away, you won’t just think they’re noisy—you’ll know exactly what they’re protecting and why they’re there.

Chapter 1 | What is an electromagnetic relay? (one-sentence version first)

If we had to explain the electromagnetic relay working principle and applications in one sentence, it would be this:

An electromagnetic relay uses an “electromagnet + moving metal armature” to change the state of one or more sets of contacts, so a small control current can safely switch a larger load current; you’ll find it in panels, control cabinets, pump controls, and motor-start circuits.

Inside every relay, there are really two different “worlds” you need to keep separate in your mind:

1. Coil side (control side)
   • Runs on lower-power control voltage, often DC 12 V, DC 24 V, or AC 120 V / 240 V
   • Is driven by switches, pushbuttons, PLC outputs, thermostats, or other control devices with small currents
2. Contact side (load side)
   • Connects to the actual equipment: lighting circuits, receptacle circuits, motors, pumps, heaters, fans, and so on
   • Handles much higher currents, such as 5 A, 10 A, 20 A or more, depending on the relay

You can picture it like this:

The coil is like the person tapping a key card at the door; the contacts are the door mechanism that actually locks and unlocks access to the high-power world behind it.

The card tap is low-risk; what happens on the other side of the door involves much higher energy. The relay stands in between, keeping the control side safe while it switches the high-voltage, high-current side for you.

Chapter 2 | Electromagnetic relay working principle: coil, core, armature, and contacts

Let’s break down the basic structure of an electromagnetic (electromechanical) relay in a simplified way:

• Coil – a length of copper wire wound around an iron core
• Iron core – concentrates the magnetic flux and turns the coil into an electromagnet
• Moving armature – the metal lever that gets pulled in or released by the magnetic field
• Spring – returns the armature to its original position when the coil is de-energized
• Contacts – NO (normally open), NC (normally closed), and COM (common) terminals that actually open or close the circuits

The working process of an electromagnetic relay is really just three steps:

1. Coil energized → core becomes an electromagnet
   When the coil side receives voltage and current, the coil produces a magnetic field, and the iron core becomes an electromagnet.
2. Armature is pulled in → contacts change state
   As the armature is pulled toward the core, it moves the mechanical linkage connected to the contacts.
   That motion either closes contacts that were normally open, or opens contacts that were normally closed.
3. Coil de-energized → spring returns everything to normal
   Once the coil loses power, the magnetic field disappears. The spring pulls the armature back, and all the contacts return to their default states.

Once you understand the electromagnetic relay working principle, the “click” sound you hear in a panel finally makes sense.

So the electromagnetic relay working principle boils down to this:

You use “coil current → magnetic force” to drive a mechanical movement in the armature and contacts, and that mechanical movement performs the electrical switching.

That familiar “click” or “clack” sound you hear in a panel? That’s literally the armature slamming into or releasing from the core and dragging the contacts along with it.

Chapter 3 | How to read relay specs so you don’t pick the wrong one

Once you start applying electromagnetic relays in real projects, you immediately run into the selection problem. In the field, the most common questions are things like:

• “What coil voltage should I choose for this control system?”
• “Is the contact rating enough for this motor or heater?”
• “How many poles and what combination of NO/NC contacts do I actually need?”
• “Should I use a plug-in relay with a socket, a DIN-rail module, or a PCB relay?”

3-1 Coil voltage: match your control circuit

Common relay coil voltages include:

• DC coils – DC 12 V and DC 24 V are very common in control panels and PLC systems
• AC coils – AC 120 V and AC 240 V (typical for North American systems), sometimes 208 V or 277 V depending on the application

The key rule is simple:

Whatever voltage your control side actually provides, choose a relay with the same rated coil voltage. Don’t mix and match “almost” values.

For example:

• If your PLC output provides 24 V DC, choose a relay with a 24 V DC coil.
• If you’re directly using utility AC to drive the coil, and the control voltage is 120 V AC, choose a relay with a 120 V AC coil.

3-2 Contact rating: how heavy is your load?

Relay contact ratings usually include information like:

• Current, voltage, and load type (for example AC-15, AC-3, resistive vs inductive load)
• Typical markings look like: 10 A 250 V AC (resistive), 7 A 250 V AC (inductive / motor), etc.

A simple rule of thumb:

If you’re switching motors, pumps, or other loads with high inrush current, give yourself a safety margin. Don’t choose a relay whose rating is just barely equal to the running current on paper.

Inductive loads (motors, solenoid valves, transformers) hit the contacts much harder than pure resistive loads (heaters, incandescent lamps), especially at startup.

3-3 Poles and contact types: how many NO / NC contacts do you need?

You’ll see marking such as:

• 1P1T, 2P2T, 4P2T – indicating how many poles and throws (how many independent contact sets)
• Each set of contacts is usually labeled as NO (normally open) or NC (normally closed).

Practical examples:

• If you want one relay to both start a motor and turn on a pilot light, you’ll need at least two contact sets, one for the motor circuit and one for the indicator.
• If you’re doing interlocks or more complex logic (for example, one machine can only start if another is stopped), you’ll often use multiple NO/NC contact combinations to build “AND” and “OR” conditions in hardware.

3-4 Mounting style: plug-in socket, DIN-rail, or PCB relay

In modern control panels and industrial electrical cabinets, the most common style is:

• DIN-rail relays with plug-in bases
  They’re easy to maintain and replace—you can pull out a failed relay and plug in a new one without rewiring the whole panel.

PCB (printed circuit board) relays, on the other hand, are more common in:

• Appliance control boards, chargers, power strips, HVAC control boards, and other electronic equipment
  They’re soldered directly to the board by electronic technicians, and end users usually never see them.

Chapter 4 | Where electromagnetic relays show up in real life

Now for the fun part: where do electromagnetic relays actually show up in real installations?

4-1 Water tank, well, or sump pumps: float switch + relay

In many buildings, wells, or basements, pump control is often built around a combination like this:

• A float switch or level sensor – detects the water level in a tank, well, or sump
• An electromagnetic relay or contactor – switches the pump’s power on and off

The reason is straightforward:
The float switch itself usually can’t safely handle the full pump current, so it sends only a small control signal. The relay then uses that small signal to switch the pump’s much higher load current on or off.

4-2 Cranes, pumps, and fans: start/stop and safety interlocks

In industrial sites, plants, and mechanical rooms, you’ll often see combinations like:

• Start / Stop pushbutton stations → drive the relay or contactor coil
• Electromagnetic relays or motor contactors → actually switch the motor or fan power
• Overload relays or other protective devices → trip if the motor overheats or draws too much current

In these circuits, the electromagnetic relay often plays a “logic” role, for example:

• Only when condition A is met and condition B is safe do you energize the main contactor.
• If any protection device trips—overload, high temperature, low level, emergency stop—the relay coil drops out and the whole system shuts down immediately.

4-3 The tiny relays hiding inside appliances and chargers

If you’ve ever taken apart a switched power strip, an oven, a heat pump, or an EV charger, you may have noticed small black rectangular blocks labeled 10 A, 16 A, or similar. Those are usually PCB-mounted power relays.

They’re commonly used to:

• Switch compressors, heating elements, or fan motors on and off
• Add delays and sequencing so equipment doesn’t hard-start repeatedly and damage itself

You rarely see these relays directly in everyday life, but they’re quietly working inside many of the devices you use every day.

Chapter 5 | Common misconceptions and failure signs

5-1 Common misconception: “A relay just amplifies current,” right?

A lot of people think the main job of an electromagnetic relay is simply “use a small current to amplify or drive a bigger current.” That’s part of the story, but in practice relays also help you achieve:

• Electrical isolation – the control side and load side are separated, improving safety and reducing noise coupling
• Hardwired logic control – using multiple NO/NC contacts to build AND, OR, and interlock logic without software
• Protection coordination – combining relays with overloads, under-voltage protection, leakage/ground-fault protection, and other devices

So an electromagnetic relay is not just a “bigger switch.” It’s a key building block in the overall control and protection scheme.

5-2 Failure signs: weird noises, bad contacts, burn marks

Some common signs that a relay is failing include:

• Excessive or random clicking / buzzing – the relay seems to pull in and drop out repeatedly
• When the coil is energized, the contacts sometimes conduct and sometimes don’t (you tap the relay and it suddenly “works” again)
• After opening the relay or inspecting it closely, you find blackened or welded contacts, discolored plastic, or a burnt smell

At that point, don’t try to “nurse it along” for a few more months. Use a multimeter or relay tester to confirm the condition, and if you see any doubt at all, replace the relay. It’s not worth risking damage to the panel, the equipment, or the building because of a $10–$30 part.

Conclusion | Once you understand relays, every panel looks different

Let’s recap the key ideas:

• Electromagnetic relay working principle: energize the coil → create magnetic force → pull in the armature → open or close the contacts.
• Electromagnetic relay applications: from water-level and pump control, to industrial motor control and appliance control boards, relays show up almost everywhere electricity needs to be switched safely.
• Selection essentials: coil voltage, contact rating, number of poles/NO/NC contacts, and mounting style all matter—none of them are “random details” you can ignore.
• Practical mindset: relays exist to let you safely control large currents with smaller ones, isolate control circuits from power circuits, implement hardwired logic, and coordinate with protective devices.

Next time you open a panel and see a whole row of electromagnetic relays clicking away, pause for a moment and think about how much current they’re quietly handling for you—and how much protection and automation logic they make possible in that small piece of hardware.

If you plan to dive deeper into motor control, protective relays, or PLC-based automation, mastering the electromagnetic relay working principle early will save you a lot of headaches later in design, troubleshooting, and system upgrades.

📌 Further reading on this site:

• “What Is an Electric Motor? From Electricity to Motion”
  First understand how motors work, then see how electromagnetic relays can start, stop, and protect them from a distance.
• “How Current Affects Battery Life: Why Your Batteries Wear Out Faster Than You Think”
  Another angle on the relationship between current and lifetime, this time focusing on portable devices and battery protection.
• “What Is a Variable Frequency Drive (VFD) and How Does It Control Motors?”
  Compare simple on/off control with relays versus continuous speed control with VFDs—and see where each one makes more sense.

📌 Further reading (external, in English):

• How a Relay Works and How to Use It in Circuits
  A clear, diagram-based explanation of relay structure and basic wiring—great if you like hands-on examples.
• Basic Electromechanical Relay Guide
  A step-by-step tutorial on using an electromagnetic relay to drive loads in practical DIY projects.
• Basics of Solid-State Relays (Texas Instruments)
  A deeper dive into how solid-state relays differ from mechanical relays in terms of lifetime, noise, and switching behavior.

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