How does a relay work in a circuit?
A relay uses an electromagnet to open or close a separate circuit, letting a small control signal switch a larger load while keeping the circuits electrically isolated.
In practice, a relay sits between a low-current control circuit and a higher-current or higher-voltage load. When the coil is energized, a magnetic field pulls a movable contact (the armature), flipping the state of one or more contacts. When the coil is de-energized, a spring returns the armature to its original position. This simple action enables automation, safety interlocks, and remote control across many industries. Modern relays can be mechanical or solid-state, and there are many configurations to suit different switching needs.
Core components of a relay
The essential parts of a relay work together to convert an electrical signal into a controlled switch.
- Coil: an electromagnet that, when energized, generates a magnetic field in the core.
- Armature: a movable metal piece that is pulled toward the coil’s magnetic field to drive the switch.
- Contacts: electrical paths that open or close; typically a Common (COM) contact paired with Normally Open (NO) and Normally Closed (NC) contacts.
- Spring or mechanical return: returns the armature to its resting position when the coil is de-energized.
- Core and housing: provide the magnetic circuit and protect internal parts.
- Suppression/Protection: diodes or snubbers to absorb voltage spikes from inductive loads (especially on DC coils).
Together, these parts enable a controlled, isolated switch that can be driven by a small electrical signal in one circuit to control a larger load in another.
How the relay operates in a circuit
A step-by-step look at the typical operation of a standard electromechanical relay helps explain how it fits into a circuit.
Before energizing the coil, the relay’s contacts are in their default state: the COM contact is connected to NC, and NO is open. Energizing the coil causes a magnetic pull that moves the armature, switching the connection to COM from NC to NO, thereby completing or interrupting the load circuit. When the coil is de-energized, the spring pulls the armature back, returning the contacts to their original NC/NO arrangement. The coil side and the load side remain electrically isolated, which protects sensitive control electronics from the load’s voltage and transients. In DC-coil circuits, a flyback diode is often used to suppress the voltage spike when the coil is switched off.
These mechanical actions are fast enough for many control tasks, but they introduce contact wear and bounce at each operate cycle. Designers must account for switch lifetime, arcing at higher currents, and whether the relay is suitable for AC/DC loads and the required switching frequency.
Common types and configurations
Relays come in a variety of configurations designed to handle different numbers of circuits and types of control.
Contact configurations
The number of poles and throws determines how many separate circuits a relay can switch at once and how they’re arranged.
- SPST-NO (Single Pole, Single Throw, Normally Open): closes a circuit when energized.
- SPST-NC (Single Pole, Single Throw, Normally Closed): opens a circuit when energized.
- SPDT (Single Pole, Double Throw): one common contact can connect to NO or NC.
- DPST (Double Pole, Single Throw): two independent circuits switch together, both turning on or off.
- DPDT (Double Pole, Double Throw): two poles, each with NO and NC, allowing more complex switching.
In practice, a designer selects the configuration to match how many separate loads must be controlled and how the control signal should interact with the load circuit.
Coil types and actuation
Relays use different coil designs and actuation methods depending on the application.
- DC coil: powered by a DC control signal; common in electronics and automation.
- AC coil: powered by an AC control signal; often used in power controls and industrial equipment.
- Latching vs non-latching: latching relays hold their state after the drive signal is removed, typically using a second pulse or a magnet, reducing coil power for long-term states.
- Solid-state relays (SSR): use semiconductor switches and opto-isolators; provide fast switching and long life but drop some voltage and may require heat sinking.
Choosing the right coil type depends on drive availability, required switching speed, energy efficiency, and isolation needs.
Practical considerations for designers and users
Real-world implementation requires attention to ratings, protection, and environmental factors that affect reliability.
- Coil voltage and current compatibility with the control electronics; ensure proper driving circuitry (transistors, drivers, or microcontroller interfaces).
- Load current and voltage ratings for the contacts; overspec the relay for inrush and surge handling.
- Electrical isolation between control and load circuits; ensure adequate creepage and clearance distances per applicable standards.
- Flyback suppression and snubbers to protect driving electronics from inductive spikes, especially on DC coils.
- Mechanical wear, bounce, and switching frequency; durability matters in high-cycle applications.
- Thermal considerations; high load or continuous duty can heat the coil or contacts, affecting performance.
By balancing these factors, engineers can select relays that meet reliability, speed, and safety requirements for a given application, whether in consumer electronics, automotive systems, or industrial automation.
Summary
Relays translate a small, safe control signal into a larger, isolated switch. Through a coil-driven magnetic field, a movable armature changes the state of electrical contacts, enabling or cutting power to a load. With choices in contact configuration, coil type, and mounting, relays fit a broad spectrum of control tasks, from simple lighting circuits to complex factory automation, while providing isolation and protection for control electronics.
What triggers a relay to switch?
In one mechanism, two opposing coils with an over-center spring or permanent magnet hold the contacts in position after the coil is de-energized. A pulse to one coil turns the relay on, and a pulse to the opposite coil turns the relay off.
How does a relay work step by step?
The operation of solid state relays or SSRs is similar in principle. But unlike electromechanical relays it has no moving parts the solid state relay uses the electrical.
What is the purpose of a relay in a circuit?
A relay acts as an electrically operated switch, allowing a low-power signal to control a high-power circuit. Its main purpose is to provide electrical isolation and enable a small current to control a much larger one, which prevents damage to sensitive control circuits and allows for safe switching of high-voltage loads. They can also be used to control multiple circuits with a single signal.
Key purposes of a relay
- Switching high-power circuits: Relays allow a low-amperage switch, sensor, or electronic signal to control high-current loads like motors, lights, and other heavy-duty equipment without the risk of overheating the control switch.
- Electrical isolation: The control (input) circuit and the load (output) circuit are physically separated, so the dangerous high voltage or current in the output circuit does not affect the low-voltage control circuit.
- Controlling multiple circuits: One input signal can be used to operate a relay that controls several independent circuits simultaneously.
- Automation: Relays are essential for automation, allowing for the control and sequencing of operations in systems like industrial control panels, home appliances, and telecommunications.
- Safety: Relays are used in safety systems to isolate hazardous voltages from the control equipment and to implement safety functions like emergency stops.
What is the 85 and 86 on a relay?
On a standard automotive relay, pins 85 and 86 are for the control circuit (the coil). Pin 85 is the ground for the coil, and pin 86 receives the positive power from a switch, which energizes the coil to activate the relay.
Pin functions for a standard automotive relay:
- Pin 85: Ground
- Connects to the negative side of the circuit, typically the vehicle's chassis or battery ground.
- Pin 86: Coil Power
- Connects to the positive side of the control circuit, which comes from a switch or another control signal.
- Pin 30: Common Power
- Connects to the positive power source, such as the battery, often through a fuse.
- Pin 87: Normally Open (NO) Output
- Connects to the device being controlled (e.g., headlights, fan). When the coil is energized, this pin connects to pin 30, completing the circuit and sending power to the device.
