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PCB Relay: Types, Design and Applications

Relays are critical switching and protection devices in electronics. As electronic devices become increasingly compact, welding tiny relays onto circuit boards is necessary.

These devices are known as PCB relays. If your electronics project requires this device, here is a close-up look at how to fit it into your circuit board.

Keep in mind there are different types and kinds of PCB relays. Let's take a look!  

What is a PCB Relay?

  A relay is a device used for power management (controlling a high-voltage circuit) using a secondary low-power circuit. Compared to transistor-based switching, this component has the benefit of galvanic isolation.

PCB relays operate like regular relays but are compact enough to fit on a PCB. Usually, they have high voltage ratings and attach to the board via through-hole technology due to their weight and size.   A PCB relay is placed next to a transistor. Compare the sizes.

A PCB relay is placed next to a transistor. Compare the sizes.

Source: Wikimedia Commons  

PCB Relays Function

  Even though they are smaller and have lower voltage ratings, these miniature relays operate like the standard ones.

They have the following functions that usually depend on their type.

  • High voltage switching: These electromechanical devices help switch to high-frequency applications and high-voltage signals. They can work with high currents and voltages.
  • Overcurrent protection: These relays stop high currents from flowing in the wrong direction in a circuit.
  • Thermal protection: Such relays trip if the temperature exceeds a preset threshold.
  • Differential protection: These relays trigger when two circuit parts have different currents or voltages.
  • Frequency protection: Such relays trip when the AC frequency is too high or too low.
  • Time delay: These relays trigger for a limited period.
  • Semiconductor: Such relays switch electronically.

 

Types of PCB Relays Based on Operation Principle

  PCB relay categorization depends on structure, positioning, application, and contacts. Using these factors, we have the following classification of relays.   

Electromagnetic Relays

  Electromagnetic relays contain electrical and mechanical components that move mechanical contact sets when activated.

The primary part of this system is the electromagnet, which creates a magnetic field to open or shut the contact sets.

You can use AC or DC in this relay, and the most common kinds are the induction and attraction types.   An electromagnetic relay

An electromagnetic relay

Source: Wikimedia Commons  

Solid State Relays

  Switching in a solid-state relay uses semiconductor elements, not mechanical components.

Also, this device has a higher power gain than the electromagnetic PCB relay.

This high gain is due to their lower energy control relative to their high power output.   Solid-state relays

Solid-state relays

Source: Wikimedia Commons  

A solid-state relay delivers faster switching capabilities than a conventional PCB power relay.

Additionally, it operates quietly and has a long lifespan due to the absence of contacts.

Some examples include photo-coupled and transformer-coupled relays.  

Hybrid Relays

  These relays include electromagnetic and electronic components operating together but in parallel positioning.

The electrical part of the relay handles rectification, while the electromagnetic component takes care of the output section.

The primary benefit of this setup is lower energy loss compared to solid-state relays, resulting in reduced power consumption.  

Thermal Relays

  Thermal relays switch by reacting to temperature changes and shifting contact positions.

Since temperature changes cause the switching, these relays protect devices from overheating.

These include motors and bimetallic features like temperature sensors.  

Types of PCB Relays Based on Throws and Poles

  Based on their throws and poles, you can differentiate PCB relays into four categories.

These four categories are similar to those that split regular switches: SPST, SPDT, DPST, and DPDT.  

SPST

  Single-pole single-throw relays have four connections, whereby the primary and secondary circuits have two terminals each.

The primary circuit remains constant in all four types, so the throws and poles vary on the load circuit.  

A 5-pin SPDT switch

A car relay (PCB-mounted)  

In this setup, the secondary circuit has a switch normally open (NO) or normally closed (NC).

With the NO type, the coil closes the circuit when energized. But with the NC type, the coil opens the circuit when energized.  

SPDT

  Single-pole double-throw switches have two circuits on the secondary side (double-throw), which allows them to control two devices.

The NC side remains powered by default, but the contact switches to the NO side when you energize the coil.

This action turns off power flow to the NC side, diverting it to the NO side.  

A 5-pin SPDT switch

A 5-pin SPDT switch

 

DPST

  Having double poles, each with a single throw, DPST is equivalent to two SPST circuits.

Therefore, this relay can power two devices concurrently.

Remember, SPDT relays can control two circuits but power only one at a time. But DPST PCB relays can power or turn off both concurrently.  

DPDT

  Double-pole double-throw switches are like two SPDT contacts in one device.

Therefore, these relays can control four circuits, where energizing the coil can power two NO circuits and turn off the other two NC contacts.

When off, the relay reverses the contacts, powering the lines off (NC) and turning off the NO contacts.  

How Does a PCB Mount Relay Operate?

  A PCB relay features a coil in the energizing circuit aligned with the contact points.

As current flows through this circuit, it creates an electromagnetic field that operates through the metal plate connected to the armature.

This magnetic field can shut or open the contacts, which is the switching effect.

You can then use the throw count, pole count, or break count to control the relay's contacts in several ways.  

Relay Pin Terminals

  Since there are different relay types, each has a unique set of pins. Electromagnetic or electromechanical power relays have the following four terminals.   Relay Pin Terminals   Reed relays have a similar design to the electromagnetic type and have positive and negative terminals.

On the other hand, solid-state relays contain input, load, and output terminals, while thermal relays have two terminals connected in series with motors.   A reed relay A reed relay Source: Wikimedia Commons  

Common Applications of PCB Relays

 

  • Household appliances and electronics like video game consoles and amplifiers
  • Mobile/smartphone accessories and telecommunication equipment to control data transmission
  • Surgery and medical instruments like ultrasounds and MRI scanners
  • Military and aerospace systems in avionics, missile control, navigation, and more!
  • Vehicle electronics for illumination switching, to control the powertrain, and to manage engines.

 

A relay mounted on a PCB for car function switching

A relay mounted on a PCB for car function switching

  • Industrial machinery and equipment to control heaters and motors
  • Automated systems to control processes

 

PCB Relay Design Considerations

  Consider these design guidelines to ensure the PCB relay remains reliable throughout its lifespan and functions optimally.

  • Make the relay’s design simple and integrate it into the PCB’s design using an optimized connection that ensures dependable and consistent switching performance.
  • Evaluate the required contacts for the circuit to reduce the relay’s pins as much as possible, which will increase efficiency. While these devices are reliable, those with more contacts have more points that can fail. And they are costlier, as well.
  • PCB relays have operating current and voltage ratings, which should match the ratings on the board.
  • Relays also have a maximum operating temperature range; you should design them to operate below what the PCB can handle. Also, consider positioning the device in a neighborhood that doesn’t have hot components. Alternatively, you can design the device to handle hot temperatures without failure if it will sit in a hot area.

 

PCB Design Requirements for Electromagnetic Relays

  An electromagnetic PCB relay will not work if its internal mechanisms get distorted.

Therefore, you must pay attention to its soldering, washing, and heat application during the mounting process.

Most manufacturers provide recommendations for mounting relays, and they include the following.

The relays should also have the standard conductor thickness (35 and 70µm).

However, thermal and electromagnetic relays perform differently when exposed to heat and magnetic fields.

Therefore, you should remove them from semiconductors, transformers, and other heat-generating components during the printed circuit board design process.

Also, mount the relay to have vibrations or shock hit perpendicularly to the relay armature operating direction.   

Types of Relay PCBs

 

  • Single-channel relay board PCB
  • 2-channel relay PCB
  • 4-channel relay PCB
  • 6-way relay PCB
  • 8-relay board PCB
  • 16-channel relay PCB (12V)

Attach the external devices like the load, power source, and Arduino board to the PCB, then test the circuit.

You can expand the code to add more control functions or design a circuit with a DPDT to control more devices.  

DIY PCB Relay

If you are a hands-on person interested in electronics, you can build a PCB relay using these simple steps.

You’ll need a simple 12V relay (SPST or SPDT), an NPN transistor to control power flow to the relay’s coil, and an Arduino board to control the transistor’s switching.  

Step 1: Circuit Design

In this circuit, you should directly connect a 12V power source to the primary coil on one side.

But on the negative side, have an NPN transistor’s collector terminal connected to the relay, with the emitter returning to the power source.

We’ll use an Arduino UNO microcontroller to power the transistor’s base terminal to control power flow through the relay’s coil, which will regulate the secondary circuit.

PCB relay circuit diagram

When the Arduino’s pin switches to high, it will output around 5V, allowing current to flow through the transistor.

This flow will energize the relay coil, contacting the NO terminal to power the load using the connected AC or DC voltage.

Switching back to low (0V) will deactivate the transistor, cutting power flow through the coil.  

Step 2: Coding and Testing

Write this simple code in your Arduino IDE, connect the board, and upload the sketch.

If you’ve made the connections accurately, the relay should connect to the NO circuit (load), turn off, and back again in one-second intervals. You should hear a clicking sound in the relay each second.  

Step 3: Create the PCB Layout

Use the CAD software of your choice to design the PCB layout for the relay circuit. Remember not to include the Arduino board in the design.

You will connect this component to the PCB for external control, so leave a pin for its connection.  

Step 4: Circuit Board Fabrication

Print the PCB layout using a plotter or laser printer, then place it above a copper-clad laminate. Etch the board to leave copper on the areas covered by the layout printout.  

Step 5: Assemble the Board

Assemble the relay and transistor to the PCB, then mount the connector pins.  

Step 6: Connect the External Devices

Attach the external devices like the load, power source, and Arduino board to the PCB,  

How to Solder a Relay on a Circuit Board

  Soldering comes after mounting, flux application, and preheating. You can either do this automatically or by hand. The following steps should guide you through the automatic soldering process.

  • Adjust the solder level to avoid overflowing onto the PC board top using the flow soldering process.

  A wave (flow) soldering machine

A wave (flow) soldering machine

Source: Wikimedia Commons  

  • Solder at 500°F±41°F/ 260°C±5°C for about six seconds

Use these conditions unless specified otherwise, depending on the relay type. However, the relay's performance can degrade on multi-layer boards due to their high thermal capacity.

Therefore, take caution when using such panels. For hand soldering,

  A soldering iron

A soldering iron

 

  • Maintain the tip temperature at 662°F or 350°C
  • Solder for about three seconds

After that, cool, clean, then coat the board.  

How to Test a Circuit Board Relay

  If you recall, NO contacts remain open with an unenergized relay, while NC contacts remain closed with an unpowered relay. You will need a multimeter for this test and use the following steps.

  • First, set the multimeter to continuity check mode
  • Test for continuity between the pole and NC contacts
  • Check for discontinuity between the pole and NO contacts
  • Energize the relay, which should engage and produce a clicking sound. Remember to use the rated voltage
  • Then, check for continuity between the pole and NO contacts
  • Next, test for discontinuity between the pole and NC contacts
  • Lastly, measure the relay coil's resistance using the multimeter. Check whether this value matches the one labeled by the manufacturer.

  Testing a Circuit Board Testing a Circuit Board  

Summary

  To summarize, PCB relays are essential safety devices for circuit boards. Even though they are tiny, they function like standard-sized ones.

However, they require special care when mounting onto a circuit board.

Also, consider the PCB design requirements described above to ensure the relay functions effectively.

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