As the “intelligent switch” of the electrical control system, relays are widely used in industrial automation, automotive electronics, home appliances and other fields. Their core value lies in controlling high-current circuits through small currents, realizing electrical isolation and safe operation. This article combines authoritative technical manuals, industry standards and manufacturer's information, in-depth analysis of the core components of the relay and its principle of cooperation, to provide a scientific basis for selection and maintenance.
Coil: wound from copper wire, usually wound on the core (ferrite or silicon steel), the number of turns of the coil and the diameter of the wire to determine its resistance and inductance.
Iron core: strong magnetic conductivity materials like electrical pure iron (used to concentrate magnetic lines of force to strengthen magnetic fields).
When the coil is energized, according to Ampere's Loop Law, the current generates a magnetic field and the iron core is magnetized to form a strong electromagnet. For example, when the relay coil voltage is 5VDC, only 40mA current is needed to drive it.
Rated Voltage: Normal working voltage of the coil (e.g. 12VDC, 24VAC).
Suction Voltage: Minimum voltage to actuate the contacts (usually 70%-80% of rated value).
Release Voltage: Maximum voltage to reset the contact (about 10%-30% of rated value).
The armature is mostly made of soft magnetic material with high permeability and low remanent magnetism, and some high frequency relays use PoMo alloy.
The structure is often designed as lever type or reed type to reduce the motion inertia and improve the response speed.
When the coil is energized, the armature is attracted by electromagnetic force and drives the contacts to close or break.
When the coil is de-energized, it relies on spring reset. Experimental data show that the action time of small relays is usually in the range of 5-20ms.
Normally open contact (NO): breaks when coil is de-energized Motor starting, lighting control
Normally closed contact (NC): closes when the coil is de-energized Safety interlocks, alarm circuits
Changeover contact (CO): simultaneous NO/NC circuit switching, redundant design
Silver tin oxide (AgSnO₂): strong arc resistance, suitable for high current (such as automotive relays, can carry more than 50A current).
Gold-nickel alloy: low contact resistance, used for signal level relays (e.g. PCB on-board communication relays).
Tungsten-copper composites: excellent high-temperature resistance, commonly used in high-voltage relays for power systems.
Adhesion: High current leads to contact welding (suitable arc extinguishing device is required).
Oxidation: Silver contacts are susceptible to the formation of a silver sulfide film when the ambient humidity is high (solution: sealed relays or gold plating).
Reset springs: Ensure quick return of the armature after coil de-energization, typical elasticity coefficients range from 5 to 20 N/mm.
Contact Pressure Spring: Maintains contact pressure between contacts (typically 0.5-2N) to reduce contact resistance.
Stainless steel (SUS304): corrosion resistant, suitable for industrial environments.
Phosphor bronze: high elastic stability, mostly used for precision relays.
Engineering plastics: such as PA66 (nylon) + 30% glass fiber reinforced, temperature resistance up to 120 ℃ .
Metal sealed shell: Argon arc welding package, to achieve IP67 protection level.
Transparent window: easy to observe the contact status.
Safety certification marks: UL, VDE, CE and other certifications ensure compliance with international safety standards.
Magnetic blowing arc extinguishing: the use of permanent magnets to guide the arc stretching extinguished (common in contactors).
Arc extinguishing grids: Metal grids split the arc into short arcs to accelerate cooling.
Buffer diode: eliminates the reverse electromotive force when the coil is de-energized.
RC Absorption Circuit: Resistor-capacitor network suppresses contact sparks and extends life.
Built-in LED, red for suction, green for disconnection.
Workflow:
Control circuit applies rated voltage (e.g. 24VDC) to the coil.
The electromagnet generates magnetic force and attracts the armature to move.
The armature drives the moving and static contacts to close (NO type).
The main circuit is energized and the load (e.g. motor) is energized to run.
After the control signal disappears, the spring resets and the contacts are disconnected.
Applications:
New Energy Vehicle: high voltage relay to control the battery pack and drive system on/off, silver plated contacts, 1000VDC resistance.
Smart home: magnetic retention relay zero power consumption to maintain the state, used for smart switch.
Mechanical life: the number of actions without load (usually more than 10^6 times).
Electrical life: number of reliable operations under rated load (e.g. 10^5 times @ 10A).
Leave 1.5 times the contact capacity (e.g. 5A actual load, choose 10A relay).
Prefer dust and moisture-proof models for industrial environments (IP rating ≥ IP40).
From the generation of the magnetic field of the electromagnet to the precise opening and closing of the contacts, each component of a relay is a combination of material science and electromechanical design. Understanding the characteristics and synergies of these components not only optimizes circuit design, but also significantly improves system reliability. While the component design of the traditional relay continues to evolve with the development of new technologies such as Solid State Relays (SSRs), the core principles remain the cornerstone of the electrical control field.
