PCB Relay- A Guide to Working Principle and Design



PCB relays, also known as Printed Circuit Board relays, are crucial in electronic circuits. They are electronic switches designed to be mounted on a printed circuit board. PCB relays are compact and have a low profile, making They are responsible for controlling high-power devices, signals, or currents using a low-power signal. These little guys are everywhere, from household gadgets to heavy-duty industrial machinery. This article will help you learn about PCB relays’ working principles and the primary considerations for designing them.

PCB Relay on Circuit Board
PCB Relay on Circuit Board


What is a PCB Relay?

When you think of an athletics relay activity, what comes to mind? Similarly, when it comes to electrical circuits, envision a similar scenario where the control circuit hands off the metaphorical “baton” to a relay, which in turn transfers it to the load. The controlled switching action is essential for the device or system to work correctly.

PCB relays function as electrically operated switches crucial in ensuring the effective operation of devices and PCBs. PCB relays, also known as printed circuit board relays, function in a similar manner to other relays. PCB relays are power management tools utilizing a secondary low-power circuit to govern high-voltage circuits. However, they have the advantage of being more compact and having lower voltage ratings.

 120VAC PCB Relay
120VAC PCB Relay


Given their sizes and weight, these relays can be integrated into printed circuit boards as through-hole components. Whenever the requirement arises to manage a high-voltage circuit with a secondary low-power circuit, a relay is employed. An added benefit of relays is galvanic isolation, a feature that isn’t attainable through transistor-based switching methods.

PCB relays are handy when it comes to protecting your circuits. These nifty devices have the ability to trip and cut off the power when they detect a surge of current flowing through the armature coil. If there’s an excessive current passing through the PCB relay, it can trip and cause an open circuit. This, in turn, interrupts the flow of current in the device. But, you know what? Relays aren’t the only option for protecting high-power circuits from ESD.


What Features does a PCB Relay have?

PCB relays perform similarly to their conventional relays, demonstrating comparable functionality, yet they do exhibit a tendency to be more compact, accompanied by a preference for lower voltage ratings. These elements are available in various conventional configurations that are observable in larger relays. Different PCB relays offer different features, some of which are given below:

High-Voltage Potential: These PCB relays are purposefully engineered to operate with high voltage levels and prevailing currents.

Time delay: As this feature suggests, these relays are primed to initiate action for a restricted duration.

Overcurrent: This particular relay type is designed to unseal, effectively halting the surge of excessive currents from meandering along an incorrect trajectory within a circuit.

Thermal: In the case of thermal relays, their activation occurs when the temperature surpasses a predefined threshold.

Semiconductor: Among other relays, solid-state relays facilitate electronic switching.

Differential protection: This particular PCB relay iteration can activate when disparate portions of a circuit harbor distinctive voltage or current readings.

Protection against Fluctuation: Solid-state relays are programmed to trigger when the alternating current’s frequency escalates beyond or descends below permissible parameters.


How Does PCB Relay Work?

A coil, armature, and some contacts are all needed to make a PCB Relay work. Since relays perform the same job as switches, they share the same jargon. The PCB Relay’s moveable armature is its common terminal, linking the device to its surrounding circuit.

A 3D view of Relay Internal Components
A 3D view of Relay Internal Components


The Relay usually operates when electricity is transmitted via its coil, creating an electromagnetic field. The armature of the Relay is activated, and either a static contact is broken or made. When the connections are closed, and the Relay is de-energized, the connection is broken because the movement opens the contacts. When the roles are reversed, the converse is true.

The armature can revert to its unstrained resting state by cutting power to the coil. In industrial motor starters, gravity is often used in addition to the spring to generate the pulling power that releases the armature.

Demonstration of Relay’s terminals
Demonstration of Relay’s terminals


Most relays are built with a quick response time keeping in mind from the factory. It lessens noise in low-voltage uses and restricts arcing in high-voltage ones. But it’s important to remember that depending on their function, different relays might include a different number of contact sets.

Diagonal diodes on the coil are critical whenever the coil is being supplied with current. Upon deactivation, it will dissipate the energy released by the collapsing magnetic field, which otherwise would cause a surge in voltage that might damage the semiconductor’s components.

Three manners of operation exist through which the Relay on the PCB operates.

Normally Open Mode: In this mode, the Relay’s contacts are open when the coil is not energized. When the control signal activates the Relay’s coil, the contacts close, allowing current to flow through the connected circuit. The NO mode is suitable for applications where you want the Relay to close the circuit when activated, such as turning on a light or a motor.

NO Mode of PCB Relay
NO Mode of PCB Relay


Normally Closed Mode: In this mode, the Relay’s contacts are closed when the coil is not energized. When the control signal triggers the Relay’s coil, the contacts open, interrupting the current flow in the connected circuit. The NC mode is useful when you want the Relay to open the circuit when activated, such as for safety shut-off mechanisms.

NC Mode of PCB Relay
NC Mode of PCB Relay


Changeover (CO) Mode (Single-Pole Double-Throw – SPDT): Also known as the SPDT mode, the changeover mode provides both NO and NC contacts within a single relay. When the Relay’s coil is not energized, the NC contact is closed, and the NO contact is open. Upon coil activation, the NC contact opens, and the NO contact closes. This model is versatile and used when switching between two circuits, like choosing between heating and cooling systems in a thermostat.

Most relays found on printed circuit boards are built with quick response in mind. It reduces noise at low voltages and prevents arcing at high voltages. In computer circuits, they are utilized for arithmetic and other mathematical processes. PCB relays have widespread applications in various fields, including motor control, robotics, medicine, multiplexing, home appliances, data collection, and more.


Types of PCB Relays Based on Operating Principles

PCB relays can be broken down into several categories according to their construction, operating principles, and contacts. Using these factors, we can classify relays in the ways described below.

Electromechanical PCB Relay

In these relays, mechanical contact sets are actuated by electromagnetic relays, which have both electrical and mechanical components. The electromagnet’s primary function is to produce a magnetic field that may be used to open and shut the contact sets. Both alternating current (AC) and direct current (DC) with attraction and induction can be used in this Relay.

Solid State Relays

Instead of employing mechanical parts, semiconductors activate a solid-state relay. This technology provides a more significant power increase than the electromagnetic PCB relay. Their high gain results from inadequate energy control relative to their high power output.

A solid-state relay can switch much more quickly than a PCB power relay. There are no moving parts, so it operates quietly and lasts long. There are several types of relays, such as photo-coupled and transformer-coupled types.

Thermal Relays

Temperature-sensitive contact adjustments allow thermal relays to perform switching functions. Relays that switch as the temperature rises or falls are used to keep machinery from overheating. Bimetallic parts, such as motors and temperature sensors, fall within this category.

Hybrid Relays

These relays incorporate electrical and electromagnetic components that work in tandem. The electromagnetic part of the Relay controls the output, while the electrical part manages rectification. This setup has a lower energy loss than solid-state relays, reducing power consumption.

Hybrid relay
Hybrid relay


Electromechanical PCB Relay Solid State Relay Thermal Relay Hybrid Relay
Advantages Versatile, can handle

varying loads

Quick switching,

silent operation

Temperature sensing,

machinery protection,


Efficiency, reduced

power loss, suitable

for energy-sensitive


Disadvantages Mechanical wear, audible

clicks, moderate switching

Limited power handling,

heat generation

Limited applications,

slower switching

Complexity, higher


Typical Use Cases Household appliances,

automotive systems

Industrial automation,

robotics, electronics,

medical devices

Motors, HVAC systems,

thermal protection


applications, control


Examples Relay modules in

appliances, automotive

control systems

Solid state relays in

automation systems

Thermostats, Thermal

overload relays,

motor starters

Hybrid automotive

relays, power

distribution systems


Types of Relay PCB based on Throws and Poles

SPST: It has two interchangeable terminals, representing a “single pole, single throw.” If you consider two for the coil, there are four connections on the Relay. If you come across the terms SPNC or SPNO, know that their use is meant to clear up any confusion caused by the SPDT’s pole height (closed or open).

One pole double throw is what this term refers to. It’s wired so that one end can connect to the other two. If you count the two for the coil, the Relay has five terminals.

DPST: It indicates a single toss with two poles. It is equivalent to two SPST relays or switches activated by a single coil and has two sets of terminals. This device has six terminals if you count the coil terminals as two.

DPDT: It contains two rows at the end that shift over, signifying a double pole double throw. It is the same as activating two SPDT relays or switches with a single coil. The coil is one of its eight terminals.


PCB Relay Design Considerations

Several important considerations must be remembered when designing a printed circuit board (PCB) relay. These considerations can significantly impact the overall performance and reliability of the Relay, so it’s crucial to keep the following points in mind.

  1. Prioritize Simplicity: The guiding principle in PCB relay design should center around achieving straightforwardness.
  2. Integration in Circuit Design: When devising a circuit, it’s vital to factor in including the PCB relay. Forge an optimized connection to ensure consistent and dependable performance.
  3. Minimal Contacts for Efficiency: Pursue the PCB relay design with the minor requisite contacts to achieve circuit efficiency and cost-effectiveness. A meticulous evaluation of contact needs is instrumental in securing peak electrical and mechanical reliability.
  4. Voltage and Current Match: The inclusion of applied voltage and current ratings in the design of a printed circuit board relay is indispensable. Thoughtful deliberation of these parameters guarantees the intended operation of the assembled circuit.
  5. Temperature Range Optimization: The desired operational temperature range must be a cornerstone in designing a printed circuit board relay. Tailor the design to align with optimal functionality within the specified temperature boundaries.

While smaller relay options are available nowadays, it’s important to remember that the process still requires careful attention to detail. Every step, from soldering to heat application and cleaning, is crucial in ensuring a successful relay installation.

To ensure optimal performance and reliability, it is advised to go for a PCB thickness of 1.6 millimeters. This thickness provides the necessary stability and durability required for mounting relays effectively. By adhering to this recommendation, you can rest assured that your Relay will be securely attached to the PCB, minimizing the risk of any potential issues down the line.

The mechanical blueprint of your product should additionally account for safeguarding relays against vibrations and shocks. So as you craft your PCB, strategically position relays at a distance from transformers, semiconductors, or components that radiate heat. Mount the Relay to render any impact or vibration perpendicular to the operational direction of the Relay’s armature.


PCB Relay Design Requirements

Creating an efficient and reliable PCB relay design requires careful attention to several essential requirements. By following these steps, you can ensure that your PCB relay functions optimally within your electronic system.

  1. Load Specifications: Determine the load voltage and current ratings the Relay will control. This information is crucial for selecting a relay with appropriate contact ratings.
  2. Contact Configuration: Choose between ordinarily open (NO) or normally closed (NC) contacts based on your application’s requirements. Consider how the Relay’s initial state will affect your system’s functionality.
  3. Coil Voltage: Select a relay with a coil voltage that matches your control circuit’s voltage. A mismatch could lead to unreliable operation or potential damage.
  4. Contact Form: Decide on the number of contact sets you need (single-pole, double-pole, etc.) based on the complexity of your circuit and the number of loads you’re controlling.
  5. Response Time: Evaluate the Relay’s response time—how quickly it switches after receiving the control signal. This can be crucial for applications with tight timing constraints.
  6. Coil Power Consumption: Calculate the power consumed by the relay coil. This is important to ensure that your control circuit can provide sufficient power and won’t be overloaded.
  7. Ambient Conditions: Factor in the environment where the Relay will be used. Consider temperature, humidity, and any potential exposure to dust, vibration, or other elements that could impact its performance.
  8. PCB Mounting:  While designing, choose a relay that can be easily mounted on your PCB. Check the Relay’s physical dimensions, pin spacing, and mounting options.
  9. Schematic and Layout: Create a clear and accurate schematic diagram and PCB layout. Ensure proper separation between high-voltage and low-voltage components, adhering to best practices for signal integrity.
  10. Testing and Validation: Thoroughly test the relay circuit under different conditions to ensure reliability. Check for proper switching, coil activation, and contact stability.
  11. Documentation: Maintain comprehensive documentation, including the relay datasheet, circuit diagram, PCB layout files, and design considerations. This ensures easy troubleshooting and future reference.

By meticulously addressing these PCB relay design requirements, you’ll be well on your way to developing a functional and dependable relay circuit that seamlessly integrates into your electronic system.

PCB Relay Mounting Considerations

It’s essential to take care while installing PCB relays to get the most out of them and keep them in working order.

It would help if you cleaned the PCB thoroughly. Putting the Relay on a clean, flat surface is essential to prevent short circuits and guarantee adequate contact between the PCB and the Relay.

When attaching PCB relays, static electricity must also be taken into account. Before running the Relay, it is recommended that you discharge any static energy from your body.

Ensure you follow the relay manufacturer’s instructions for mounting hardware and torque settings. A malfunction or short circuit might be caused by improper relay installation, which could also harm the PCB.

To guarantee appropriate installation and functioning, the Relay should be soldered onto the PCB in accordance with the manufacturer’s instructions. Avoiding damage or errors using the right equipment and methods while soldering or attaching the Relay to the PCB.

Common Applications of PCB Relays

PCB (Printed Circuit Board) relays are often employed in electronic and electrical systems to switch electrical signals or power. You can find some typical uses for PCB relays below.

Machinery and equipment control

PCB relays switch high-power loads like motors, heaters, and solenoids in industrial control systems. Automation systems use them to monitor and control processes.

Mobile phone accessories

PCB relays switch signals and regulates data transmission and reception in telecommunications equipment.

Vehicle electronics

Engine management, powertrain control, and illumination use PCB relays in automotive electronics. Airbag control modules and other safety systems use them.

Household electronics

PCB relays are used in power amplifiers, DVD players, and video game consoles. Signal switching, power control, and overload protection use them.

Instruments for surgery

MRI scanners, infusion pumps, and ultrasound equipment need PCB relays. They switch power and signals and control equipment.

Military and Aerospace Technology

Avionics, navigation, and missile control systems use PCB relays in aerospace and defense. Vital system signal switching and management use them.

DIY PCB relay

Creating a DIY PCB relay involves designing a simple circuit, creating the PCB layout, fabricating the PCB, assembling the components, and testing the circuit. Here’s a step-by-step guide with a circuit diagram explanation:

Step 1: Circuit Design

Design a primary relay circuit using a microcontroller to control the relay coil. This example uses an Arduino as the microcontroller.

An NPN Transistor PCB Relay Circuit Diagram
An NPN Transistor PCB Relay Circuit Diagram
  • An NPN transistor acts as a switch to control the relay coil.
  • The microcontroller’s digital pin provides a control signal to the transistor’s base.
  • The transistor turns on when the microcontroller’s pin goes high, allowing current to flow through the relay coil.
  • This energizes the relay coil, closing the Relay’s mechanical contacts.

Step 2: PCB Layout Design

Use PCB design software (such as KiCad or Eagle) to create the PCB layout—place components, including the Relay, transistor, and connector pins.

Step 3: PCB Fabrication

Print the PCB design onto a copper-clad board using a laser printer or plotter. Then, etch the board to remove excess copper, leaving the traces and pads.

Step 4: Component Assembly

Solder the components onto the PCB according to the design. Ensure proper placement and orientation. Attach connector pins for external connections.

Step 5: Testing

Connect the DIY PCB relay to the microcontroller (Arduino) and a power supply. Upload a test code to the microcontroller that toggles the digital pin connected to the transistor’s base.

Step 6: Code

Here’s a simple Arduino code example to control the Relay:

Arduino Code for PCB Relay
Arduino Code for PCB Relay


Step 7: Final Testing

Apply power and run the test code. The Relay should click on and off, indicating successful circuit operation. You can verify the Relay’s contacts using a multimeter.

Note: Always follow proper safety precautions when working with electronics, especially high voltage or current levels.

Remember that this is a basic DIY PCB relay example. More complex relay circuits involve additional components like protection diodes, opt couplers, and power sources. Always double-check your design and connections before applying power to the circuit.


When working with electronics, PCB relays are a must-have component. It’s a tool for establishing contact between two electrical circuits. Every designer must know how these relays work since they are a crucial part of printed circuit boards.

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