PCB Relay | Power Relay Types, Design and Applications
Updated: Sep 9
1. Introduction
PCB relays play a significant role in switching and protecting electronic circuits from higher voltages or currents. Power relays generally operate motors, lighting, and other loads without direct electrical coupling and are mounted on PCBs. These relays play a crucial role in electronic systems by providing safety and isolation. This article provides an overview of PCB power relays, their types and designs, and their best uses in day-to-day life.
2. PCB Power Relay Fundamentals
PCB power relays are designed to switch higher loads for electrical components, such as motors, heaters, and other high-load devices. The most common categories of power relays are
Electromechanical relays (EMRs) use a coil to move contacts, opening and closing power circuits.
Solid-state relays (SSRs): SSRs use semiconductor devices such as triacs, MOSFETs, and IGBTs to open and close power circuits for electronics. SSRs have no moving parts like EMRs.
Both power relays are soldered to the PCB and control the high-voltage devices. The key features of the PCB power relays are given below:
Power relays have defined contact ratings for voltage, current, and load.
It has a specified coil or input rating.
It has proper isolation between the low-voltage and high-voltage load paths.
It has standardized footprints for automated assembly and replacement.
3. Types of PCB Power Relays
PCB power relays are categorized by their applications. The most common types of PCB power relays are given below:
General-purpose PCB power relays
These kinds of power relays are used to turn power on and off for lamps, heaters, and small motors. The power relay rating ranges from 5 A to 30 A for current and up to 250 VAC or equivalent for DC rating for the PCBs. These general-purpose relays are available in SPST, SPDT, and similar configurations, which are the default choices for several home appliances and industrial control boards.
Latching PCB power relays
These power relays switch the power circuit with a short pulse. These relays help reduce power consumption by using a battery for short pulses and are most often used in smart meters, automation, and power distribution modules.
High-Inrush/ Motor Load PCB Relays
The high-Inrush power relays are designed for high-current systems such as HVAC, Compressors, Motors, and Pumps. In this type of relay, specific contacts and geometry minimize welding and erosion.
Solid State PCB Power Relays
Solid-state PCB power relays use an LED or optocoupler on the input side and triacs, MOSFETs, or IGBTs on the output side to switch the power system. This kind of relay provides noise-free, fast switching during system operation. However, solid-state power relays generate heat during switching. It also experiences a small drop in voltage and current. These PCB power relays are mostly used for frequent switching and instrumentation.
4. Internal Structure and Operation
The electromechanical and solid-state PCB relays have distinct internal structures and operation mechanisms, which are given below:
Electromechanical PCB power relays
Coil: It is energized by the low-voltage signal that creates the magnetic field.
Core and armature: These magnetic components move when the coil is energized by a low-voltage signal.
Spring: Returns the unit to its default state when the coil power is removed or no low voltage is applied.
Contacts: The conductive elements that open or close the power circuit, essentially acting as switches.
Housing: The enclosure that protects the overall switching system and provides safety.
Solid State PCB power relays
The control signal and semiconductor devices control the input and output of the solid-state PCB power relays, respectively. There is no mechanical movement involved in this relay system. Dedicated isolation structures ensure the isolation and safety of the solid-state power relays.
5. Design Considerations for Power Relays
The following are the major design considerations for the PCB power relays that must be accounted for to ensure efficient and reliable PCB power relays:
Electrical rating
Contact voltage and current: The electrical rating specifies the maximum voltage and current for relays that must exceed the maximum operating voltages by a margin.
Load types: Define load types, such as resistive, inductive, and capacitive, during power relay design.
Coil voltage and power: Match these to the PCB power budget for safety.
PCB layout and isolation
Maintain the clearance and creepage distance between the coil and contact pins. Use traces and appropriate copper thicknesses for high-current paths to avoid overheating. Increase isolation by accounting for the required number of slots and cutouts on the PCBs. The high-voltage traces should be away from the RF circuit and other sensitive devices to protect them.
Coil or Input drive design
Select the transistor or MOSFET, or the driver IC, properly to control switching. Adding a flyback diode across the DC coil helps control voltage spikes. Limit the current through the LED in SSRs to protect the system.
Thermal and mechanical management
Consider losses in the coil traces to minimize heat generation in the PCBs. Avoid relays in confined areas to minimize heat generation, or use a proper thermal dissipation system. Identify vibration and shock areas, and design relays to withstand harsh environments.
6. Applications of PCB Power Relays
PCB power relays are used in various applications, including home appliances, HVAC, industrial process control, and energy systems. The most common applications of PCB power relays are as follows:
Home appliances: To control the heaters, pumps, and fans in washing machines, refrigerators, and electric ovens.
HVAC systems: To turn on and off the blower motors, dampers, and valves, and control the lighting and power outlets inside the building.
Industrial process control: To switch the valves, pumps, and actuators using PLC drivers and to detect smoke and fire for the alarm systems.
Energy system and power supply: In the battery protection system, input selection for the power supply system, and limiting the current in energy systems
Automotive industries: To control the power distribution in the electronics units, seat heater, and window lift for the vehicles.
7. Common Design Challenges and Best Practices
Various challenges arise when designing PCB power relays. The most common are given below:
Contact life and reliability
The fast switching under heavy loads accelerates contact wear in electromechanical relays. Rapid switching also causes voltage spikes, which can damage the system. SSRs with zero cross-factors and appropriate snubber networks can solve this challenge by limiting voltage spikes in PCB power relays.
Noise and electromagnetic interference
Electromechanical relays generate internal noise, and EMI can also affect sensitive devices. Separating power traces from sensitive devices with proper grounding can solve these problems.
Compliance and Approvals
Improper or non-standardized relays can cause operational issues. Therefore, high-voltage systems must use relays that meet safety standards. Clearly defined safety standards must be documented to certify the system, making it more reliable and trustworthy for consumers.
8. Summary: EMR vs SSR on PCBs
The overall summary of the electromagnetic power relays and solid-state power relays on PCBs is given in the table below:
9. Conclusions
PCB power relays bridge low-power control electronics and high-current, high-voltage loads. These relays switch various electronic devices. Selecting the right relay ensures safety and reliability in applications such as HVAC, industrial, and home appliances. Proper PCB layout, isolation, and protection systems can make PCB power relays more efficient and durable.
10. FAQs
1. What are the types of PCB power relays?
Electromechanical and solid-state PCB power relays are the most common power relays used to switch circuits in various applications.
2 How can I choose electromechanical and Solid-state PCB power relays?
Choose electromechanical PCB power relays when you need lower cost and low on-state losses. Otherwise, if you need a longer lifespan, fast switching, and sufficient power, go for solid-state PCB power relays.
3. Are PCB power relays safer for switching main voltages?
Yes, the rated and safety-approved PCB power relays are safe for switching higher voltages.
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