A relay uses a small current in a coil to create a magnetic field that closes a switch controlling a larger circuit.
Parts
- Coil (electromagnet).
- Armature.
- Spring.
- Contacts (normally open or normally closed).
How it works
- A small current flows through the coil.
- The coil creates a magnetic field.
- The field pulls the armature, which moves the contacts.
- This closes or opens a separate, higher power circuit.
Uses
Cars, appliances, industrial controls, and electric grids.
Types
Electromechanical, solid state and reed relays.
Basic parts
- Coil (electromagnet).
- Armature, a movable iron lever.
- Spring.
- Contacts, either normally open (NO) or normally closed (NC).
Principle
A small current through the coil creates a magnetic field. The field attracts the armature, which moves the contacts to open or close another circuit. When the current stops, the spring returns the armature.
Why use relays
A relay lets a low power signal control a high power circuit, and keeps circuits electrically isolated. This is why car horns, headlights and starter motors are switched through relays rather than directly from the dashboard switch.
Types
- Electromechanical relays, with moving parts.
- Solid state relays (SSR), which use semiconductors and have no moving parts, switching quickly and silently.
- Reed relays, with contacts sealed in glass.
- Time delay relays.
- Latching relays that keep their state without constant power.
Ratings
A typical automotive relay is rated for 30 to 40 amps at 12 volts. Check coil voltage, contact current and voltage ratings.
Flyback diode
When the coil is turned off, it produces a voltage spike. A diode across the coil protects the driver circuit.
Applications
Industrial control, home appliances, telephone exchanges (historically), cars, security systems and HVAC.
History
The relay was invented by Joseph Henry in the 1830s and was used in telegraph repeaters to extend signal range.
Typical failure
Contact wear, welding of contacts from high current and coil failure.
Operation and ratings
A relay is an electrically operated switch. A small current in a coil creates a magnetic field that pulls an armature, closing or opening contacts that control a separate, often higher power circuit. Joseph Henry demonstrated an early relay in 1835, and relays were key to telegraph systems. Common types are single pole single throw (SPST), single pole double throw (SPDT) and double pole double throw (DPDT). Coil voltages are often 5, 12 or 24 volts, while contact ratings may be 10 amps at 250 volts AC. Mechanical life is typically 100,000 to 10 million operations.
Protection and uses
When the coil is turned off, it produces a voltage spike, so a flyback diode across the coil protects the driving transistor or circuit. Solid state relays have no moving parts, switch faster and last longer, but can cost more and produce heat. Relays are used in cars for headlights, horns and starter circuits, in appliances, industrial controls and power protection. Failures are usually due to contact pitting from arcing, coil burnout or sticking. A clicking sound is a common sign that a relay is working.
Based on data from engineering reference texts and the IEEE
