A relay is an electromechanical device that is used on a printed circuit board (PCB) to control one electrical circuit by opening and closing contacts in another circuit. Relays have a number of important applications in electronics where they act as electrically operated switches for controlling high power devices safely and reliably using lower power circuitry.
How relays work
A relay contains a wire coil surrounding an iron core and mechanical contact blades or terminals. A small current flowing through the coil magnetizes the core, which pulls the movable contact arm towards the fixed contact. This closes an electrical connection between the normally open (NO) and common (COM) relay contacts.
When coil current is removed, a spring pulls the contacts back to their resting NO position. Some relays also have normally closed (NC) contacts that do the reverse. The coil current needed is a small fraction of the current the relay will switch, allowing low power circuits to drive safely isolated high power loads like motors.
Table: Relay Terminals
| Terminal | Description |
|---|---|
| COM | Common terminal that connects to NO or NC contact |
| NO | Normally open terminal |
| NC | Normally closed terminal |
| Coil pins | Applied voltage magnetizes internal core to activate relay |
Another variation known as a solid state relay uses a thyristor, TRIAC or transistors internally instead of mechanical contacts. But they function logically the same, with a lower power control signal operating a switch for the main circuit.
Why use relays on PCBs?
Relays bring unique capabilities making them indispensable for countless electronic systems and products. Some key benefits of using PCB mount relays are:
1. Switch high voltages and currents
Relays can handle switching anything from a few milliamps up to 800 amps thanks to their mechanical design using metals like silver or platinum. No discrete solid state device comes even remotely close in terms of on-resistance or power handling. Additionally, voltages up to a thousand volts can be managed reliably.
2. Provide electrical isolation
The electromechanical relay contacts provide isolation between the coil control side and the switched load circuit. This makes relays ideal for safely switching mains power or operating sensitive low level logic devices like microcontrollers from high voltage devices, eliminating risk of damage. Opto-couplers provide similar but lower power isolation capability.
3. Reverse polarity protection
Having contacts physically open unless activated provides implicit reverse polarity protection from damaging connected components. This allows flexible wiring without risks of shorts that solid state devices are prone to under incorrect polarity conditions.
4. Fail safe operation
Relays will fail with contacts open once coil voltage is removed. This gives inherently fail-safe operation, critical for guarding motors or actuators from unintended activation. Compare this to transistors and FETs which typically short or fail closed risking uncontrolled high current when damaged.
5. Resistance to environmental noise
Unlike solid state devices, electromechanical relays are generally immune to electromagnetic interference, transients, static discharges or other environmental hazards that cause unintended switching events. This characteristic makes relays suitable to operate reliably in noisy industrial environments. Low susceptibility to inadvertent turn-on/off also suits applications like medical equipment.
6. Low control power needed
Relay coils need just tens to hundreds of milliwatts typically to enable switching kilowatts of power. This low driving power requirement allows use of small and inexpensive logic circuitry and extended operation even from low capacity backup batteries.
7. Long service life
With rated contact pressures and material combinations rated for millions of switching cycles, precision manufactured PCB mount relays outlast comparable solid state devices. Proven reliability over decades of operation complete the value proposition that makes relays a staple in electronics.
Types of PCB mount relays
Multiple relay form factors and terminal arrangements suit different PCB applications and layout requirements:
1. PCB Power Relays
These medium to high capacity PCB relays switch between a few amps to over 200 amps at up to 660VDC/440VAC voltage levels. Multiple contact blocks are common with auxiliary NO/NC connections. They handle motors, heaters, power supplies, battery chargers, etc.
2. Miniature Signal Relays
Low power signal relays operate sensors, logic circuits, small solenoids, RF antenna switches, audio routing, LEDs etc. They switch under 2A and under 60V typically. Miniature size occupies less PCB area with plug-in single-in-line (SIL) or dual-in-line (DIL) sockets preferred for easy replacement.
3. Latching Relays
These relay variants remain engaged or latched even after coil power is removed. No power required to hold state. Momentarily pulsing the set or reset coil contacts toggles between open or closed relay states. Useful for battery powered circuits.
4. Reed Relays
Reed relays feature a slim reed switch hermetically sealed in a glass envelope and are used where high sensitivity and fast switching speeds up to 1ms are needed. Typical for instrumentation, medical equipment, telecom.
5. Solid State Relays (SSRs)
Functionally replacing electromechanical relays, SSRs use back to back thyristors or transistors for totally silent no-contact operation. Fast switching and resistance to vibration suit many applications, despite limited voltage/current ratings.
6. Automotive Relays
Hardened relays allow direct PCB mounting onto engine compartment controllers and interior applications in cars/trucks for reliable performance despite vibrations, temperature swings, and voltage transients from the vehicle electrical systems.
This snapshot shows the incredible versatility relays deliver for all kinds of PCB implementations. Next let’s cover best practices for integrating relays during the schematic capture and PCB layout process.
PCB Design guidelines for relays
While deceptively simple devices, proper integration of relays deserves attention to harness their full capabilities while minimizing susceptibility to electrical or mechanical issues.
1. Schematic considerations
- Specify relay contact ratings for required load currents and voltages
- Include protection diodes for driving inductive DC loads
- Add clamping circuits if driving long wires or cables
- Consider redundancy for critical functions
- Review timing diagrams for intended logic functionality
2. Placement tips
- Keep relay placement close to associated components
- Ensure contacts are de-rated for required operational life
- Orient relay terminals for easy PCB routing
- Allow clearance for relay actuators to operate freely
3. PCB Layout provisions
- Use larger pad sizes for contacts carrying higher currents
- Include generous clearance between high voltage traces
- Route controlled impedance traces to weakly driven control pins
- Bypass coils adjacent to terminals with capacitors
- Add text labels indicating relay locations and functions
4. Enclosure and testing advice
- Prevent flux residue buildup on relay contacts during assembly
- Safeguard against metal filings shorting exposed contacts
- Perform contact resistance or continuity testing to validate assembly
- Conduct burn-in tests and environmental stress screening where warranted
Relay Form Factors
Relays are produced in a variety of mechanical packages to suit integration into different PCB assemblies. Common configurations include:
1. Through-Hole DIP
The predominant relay package, dual in-line (DIP) through-hole pins allow direct insertion into breadboards or PCB sockets for easy replacement later. 2 to 12 pins are typical in popular widths between 0.3″ to 0.6″.
2. SMT Relays
Suitable for automated SMT assembly, these surface mount relays sit flat to the PCB without protruding. They take less space but may compromise serviceability or inspection access.
3. PCB Sockets & Bases
For frequently switched relays, sockets allowing insertion/removal of relays simplify maintenance. Screw terminals are also common. Bases provide added mounting stability.
4. Open Frame Power Relays
No enclosure allows bolt or screw mounting of larger power relays to chassis or heat sinks if needed. They are often potted after installation for environmental sealing.
This glimpse of relay packages demonstrates the versatility offered to designers across form factors, current ratings, sizes, and termination schemes.
Now that fundamental relay workings, use cases, types, design needs, and form factors covered, let’s address some common questions around applying PCB relays.
Frequently Asked Questions
Q1. How do you determine the right contact rating for a power relay?
Consider requirements both in terms of current load and operational longevity. Rule of thumb is select a relay where rated current falls between 50% to 80% of intended load current to avoid overheating contacts. Also ensure contact material and switch ratings exceed the target device operating cycles by at least 4X for margin and wear effects.
Q2. When should you use a solid state relay instead of an electromechanical relay?
For applications needing fast, no-bounce quiet switching, extended mechanical lifespan exceeding 50 million operations, operation in high shock/vibration environments, or very small PCB footprint, solid state relays are preferable despite higher on-state resistance and limited overload capacity. Electromechanical types are favorable where high current loads or fail-safe operation is must.
Q3. Can you drive a 12V relay coil directly from a microcontroller?
While possible, it is not recommended, as back EMF voltage spikes from the collapsing magnetic field in the relay coil on switch-off can easily damage GPIO pins. Always use a protection diode across the relay coil. For 24V or higher coils, a buffer transistor stage is required for reliable operation.
Q4. How do you test a relay on a PCB?
Basic relay testing validates coil actuation and contact closure electrically. Use a DMM to check coil resistance & energization voltage/current. Monitor relay contact closure timing relative to coil power, and validate open/closed contact resistance values fall in expected ranges. Also inspect for any stuck or abnormally bouncing contacts visually.
Q5. What special considerations apply for selecting automotive grade PCB mount relays?
Key considerations for auto relays include extended temperature rating -40°C to +125°C, protection against voltage transients typical in vehicle systems, higher current contacts to manage surge loads, low emission housings compliant with headlamp switching devices, and additional testing/qualification covering humidity, vibration, and mechanical shock. Hermetic sealing may also be required.
Conclusion
As elaborated above, relays form critical bridges between electrical domains due their unique advantages. They continue serving numerous essential applications thanks to proven operation over decades. With sound design practices relays integrate seamlessly into PCB implementations ranging from high volume consumer goods to niche industrial equipment. When next faced with challenging switching needs, remember to consider the versatile relay as an indispensable solution!
