How To Choose Relay

Post By: Tom Rowse On: 14-12-2023 Read Time: 5 minutes - Guides

Choosing a relay that’s suitable for your application requires a thorough understanding of the relay’s characteristics. Once you’ve found an appropriate relay, you can confirm its suitability and prove it in the actual operating environment where you’ll be using it. 

There are several principles that you can apply to guide your selection, according to the separate parts of the relay, namely:

  • Contact

  • Coil

  • Performance

  • Use environment

  • Shape and installation

  • Other 

For each part of this analysis, there are several principles to apply. These include the primary parameters you need to determine for each part, plus any influencing factors.


  • Contact load – AC, DC, size, type (inductive or resistive)

  • Contact form – NO, NC or conversion, several groups of contact pairs

  • Electrical durability – frequency of use, number of expected actions

  • Contact material – what material

  • Contact resistance – size, test conditions

You should also take into consideration influencing factors such as ambient temperature, whether the contact and contact material match and whether the action in AC loads is synchronised with the load. The relay mustn’t exceed the specified maximum current value and should be tested and confirmed in operation. Many contacts have a gold-clad layer, which performs better on medium and small loads.

When determining whether the contact load is adequate for your use application, you must confirm the size of the load and its type, as different loads operate under different steady-state values. Unless stated otherwise, the specified load is usually resistive. Different contact materials are used for different load types or ranges, even for the same type of relay. 

Unless stated otherwise, electrical durability is usually shown as a nominal value, measured under operating frequency, certain temperature, rated load and load ratio. This value will differ for other load types and switching frequencies. The electrical durability of loads above 2A is usually greater for flux- and dust-proof relays than those encapsulated in plastic. If it’s possible in your environment, you can improve the relay’s service life by using dust- and solder-proof circuit breakers.


  • Rated voltage – size, direction, AC or DC

  • Action voltage – influence of size and power fluctuation

  • Release voltage – influence of size and power fluctuation

  • Maximum allowable voltage – size, time

  • Coil resistance – size, input power consumption

  • Coil temperature rise – what is the quantity, red insulation grade

Influencing factors are ambient temperature, power fluctuations and drops in electric voltage when driven by a semiconductor. You must confirm that the rated voltage can be supplied by the working circuit to the relay coil for the relay to work reliably. If the relay is driven by a transistor, there will be an associated voltage drop. This means the voltage reaching the relay coil will be below that specified for the drive circuit. 

It’s sometimes possible to apply the maximum allowable voltage to the coil for a short time. This can reduce the relay action time, but you must ensure the relay doesn’t overheat or get damaged. If the relay is polarised, you’ll need to confirm the coil voltage’s polarity. You must also select the appropriate coil resistance for the relay to work reliably so that the working circuit can supply the relay with the appropriate nominal coil power consumption.


  • Packaging method – open, dust cover, plastic package, flux-proof

  • Dielectric strength – size, location

  • Insulation resistance – size, location

  • Impact resistance – size, stability or strength

  • Anti-vibration performance – size, stability or strength

Additional influences include the relay’s environmental conditions and security requirements. The various packaging methods have different requirements for the relay’s post-processing, to ensure its reliability. Choose the correct insulation resistance and dielectric strength for your application, to prevent the line from suffering short circuits, breakdown, etc. The same applies to the impact resistance and anti-vibration performance parameters, so the relay doesn’t fail during use.

Use Environment

  • Ambient temperature – high or low, time

  • Atmosphere – temperature, presence of harmful gases

The packaging method, insulation class and service life may also influence these factors. The relay usually works normally when the ambient temperature is within the range specified by the manufacturer. If the use environment is dusty, with high humidity or condensation, you should choose plastic-encapsulated relays. Dust contamination will soon cause relay contacts to fail, while high humidity and condensation accelerate corrosion in the relay’s structure.

Use sealed relays in environments containing silicone, because silicone accelerates relay contact failure. You should test and prove in use that plastic packaged relays work effectively in any environment containing harmful gases like H2S, SO2, NO2 etc. The dust cover and flux-proof type can’t be used in this kind of environment, but in a normal ambient atmosphere the dust- or flux-proof relays work better. This is because they offer greater electrical durability than plastic-packaged relays.

Shape And Installation

  • Shape – shape size, shape height

  • Outlet form – PCB, plug-in, QC, screw fixed

  • Welding method – manual, reflow, wave soldering, etc, and whether to clean

  • Installation clearance – gap, close

Influencing factors include the installation method and size requirements. The external dimensions of relays are created with specific tolerances, so you should design the circuit and installation clearance according to the maximum specified size. Ensure that the relays you choose can be reflow soldered if that’s what your application demands. Normal relay terminals are designed to be lead-free, with a recommended soldering time of two to five seconds at 240 to 260 Degrees. Choose the outlet’s shape and installation method to match the actual configuration of your relay’s application.


  • Safety certificate – TUV, UL, VDE, CQC, depending on area

  • Special requirements and circumstances – customer request

As a general rule, VDE and TUV certification apply in Europe, while UL/CUL certification applies to North America. But most countries recognise all these certifications, owing to their international authority.

Applying The Principles To Relay Selection

Knowing how to choose a relay will save you time and money in breakdowns and failures. Follow the various principles of selection, then match the appropriate relay parameters and functions to the requirements of your application. It’s wise to bench-test your relays to confirm that you made the right choice and to prove them in operation to be sure they’re working optimally.