Relays are electromagnetic switches that are used in a wide variety of applications to control electronic circuits. When current passes through the coil in the relay, it creates a magnetic field which causes the relay’s switch (called the armature) to move and make or break an electrical connection. Relays allow a low power circuit to control a higher power circuit safely and reliably.
One of the most common relay voltages is 5V. There are several reasons why 5V relays are so ubiquitous:
Compatibility with logic circuits
Most logic circuits and microcontrollers operate at 5V. Using a 5V relay allows for straightforward integration between logic circuits and higher power loads. A 5V relay can be driven directly from a microcontroller I/O pin without additional components.
Wide availability
5V relays are mass produced in huge quantities for industrial, commercial and consumer electronics. This makes them inexpensive, readily available as off-the-shelf components from many manufacturers.
Lower power consumption
Relays with lower coil voltages like 5V require less power to switch on the coil. This makes them well suited for battery operated, portable and low power applications.
Safer and easier to use
Compared to relays with higher coil voltages, 5V relays are safer for users and engineers to work with. There is less risk of accidental electric shock or sparks/arcing when switching 5V circuits. The lower voltage also reduces insulation requirements and creepage/clearance considerations.
Standardization
The ubiquity of 5V logic and power rails in circuits has led to standardization on 5V relays across many industries. Standardization leads to interchangeability, lower costs and improved reliability.
Characteristics of 5V Relays
While there are variations between relay models and manufacturers, some general characteristics are common among 5V relays:
Coil Resistance
To operate from 5V, relay coils typically have a resistance in the range of 60-400 ohms. This results in a coil current draw of 10-80 mA. Relays with lower coil resistance have higher current draw but more driving power.
Coil Power Rating
Most 5V relay coils have power ratings from 50mW to 500mW. The coil power determines how much current can be provided by the driving circuit. Exceeding the rated coil power risks overheating and damaging the relay.
Switch Contact Ratings
5V relays are available with a wide range of contact configurations and current/voltage ratings. Most common are 1-pole single-throw (SPST) and 1-pole double-throw (DPDT) types rated for currents up to 10A at 250VAC/30VDC. Contact materials are typically silver alloy or gold plated.
Switching Speed
The operating time for 5V relays to close or open contacts is typically in the 1-20ms range. Faster switching speeds around 1-5ms are preferable for higher speed control applications.
Coil Resistance
To maximize mechanical and electrical life, 5V relays specs will list maximum switching cycles from tens of thousands to millions of operations. High reliability industrial relays may be rated for 100 million cycles.
Size
Most 5V relays follow standard packages or form factors. Common sizes are “mini” relays (10-20mm dimensions), “power” relays (30-40mm) and PCB mount relays (12-18mm). Miniature SMD relays are also available.
Advantages of Using 5V Relays
Given their widespread use, 5V relays offer a number of advantages:
Simplicity of driving circuits
5V relays can be driven directly from common logic chips like 74HC595 shift registers, ULN2803A drivers and microcontroller I/O pins without additional circuitry. The low voltage is unlikely to damage electronics if connected improperly.
Safety
Since the control circuit runs at only 5V, there is minimal shock hazard to users. The relay isolates and switches the higher power load. Troubleshooting circuits with 5V control logic is safer compared to higher voltages.
Low power operation
Battery powered and portable electronics can take advantage of lower power consumption from 5V relays. Even driving multiple relays from a microcontroller or batteries is feasible.
Compact size
PCB mount and miniature 5V relays allow fitting relays into dense PCBs and small enclosures. Mini relays take up less space compared to larger power relays.
Noise immunity
With a 5V coil, electromagnetic noise pickup is reduced compared to higher voltage coils. This improves reliability in noisy industrial environments. Twisted pair wiring for coils helps further.
Affordability
Economies of scale make 5V relays very inexpensive, often costing less than a dollar even for industrial grade devices. They are produced in vast quantities by many manufacturers.
Availability
Popular 5V relays are stocked by major electronic distributors and suppliers in small quantities for prototyping, and large volumes for production. Fast worldwide shipping is common.
Example Applications of 5V Relays
Some typical applications taking advantage of 5V relays include:
Home Automation and IoT
5V relays are ubiquitous in home automation systems and internet of things (IoT) devices. They allow microcontrollers to control lights, fans, motors, valves and other AC mains powered devices. WiFi or Bluetooth connected modules equipped with relays can switch high current appliances.
Automotive/Transportation
Cars, trucks and other vehicles use 5V relays for engine control, headlights, power windows, power locks, wiper motors, solenoid valves and various other functions. 5V coils are driven from engine control modules and body control modules.
Test and Measurement Equipment
Automatic test systems, lab instruments and measurement electronics often need to switch signals or loads during tests. Small 5V control relays perform this switching cleanly and reliably.
Vending Machines
Switches for motors, lights, coin counters and other mechanisms in vending machines often rely on 5V relays that are controlled by the machine’s computer board. The low voltage minimizes electrical hazards.
HVAC Systems
5V relays switch the higher voltages and currents needed for compressors, fans, pumps and valves in heating, ventilation and air conditioning (HVAC) systems. The relays interface to digital thermostats and controllers.
Other Types of 5V Relays
Beyond electromechanical types, other 5V relay technologies include:
Solid State Relays
These contain a semiconductor switching device like a TRIAC instead of mechanical contacts. Often used for quieter, faster AC switching.
Reed Relays
Miniature relays enclosed in a glass capsule using magnetic reeds to create switch contacts. Ideal for PCB usage with high density.
Hybrid Relays
Combine electromechanical contacts for low current switching and solid state outputs for higher power loads. Provides flexibility in a single device.
Latching Relays
Bistable relay designs where the armature “latches” into position magnetically without holding coil current. Allows low power intermittent operation.
Selecting the Right 5V Relay
With such a broad array of 5V relays available, here are some tips for selecting the optimal relay for a given application:
Check contact ratings
Make sure the relay’s contact amperage, voltage and power capacity exceeds the load requirement with a healthy safety margin. Also factor in whether the load is resistive, inductive or capacitive.
Consider switching speed needs
Evaluate if the relay’s operating time is fast enough. Faster is better for higher speed control applications.
Estimate expected lifespan
For products requiring high longevity and reliability, choose a relay rated for extended lifetime cycles. Industrial quality versions outlast consumer grade models.
Check approvals and certifications
For commercial products, look for relays with regulatory safety approvals like UL or CE. Transportation and high reliability applications may need additional qualifications.
Evaluate size and mounting
Make sure the relay’s form factor fits within the desired enclosure and PCB layout constraints. Through-hole and surface mount options are available.
Research manufacturers
Stick with established relay brands known for quality and performance. Ask distributors for recommendations if unsure.
Leave a margin on coil power
When driving relays from a power supply, regulator or batteries make sure there is adequate overhead beyond just meeting the minimum coil voltage and current.
Conclusion
In summary, 5V relays offer an easy interface for lower voltage digital logic circuits to control higher voltage/current loads reliably and safely. The widespread availability, low cost, and simple operation of 5V relays makes them the first choice for many automation, industrial and everyday consumer applications. By selecting the proper relay type for their design needs and constraints, engineers can take advantage of the versatility of 5V relays for controlling just about any system or process.