How to properly select Solid State Relays ?

Follow our technical tips for properly selecting Solid State Relays

Solid State Relays (SSR) are essential components in modern automation and control systems, offering silent, fast, and reliable switching for a wide variety of loads. Selecting the right SSR for your application requires careful consideration of several technical factors, including load characteristics, control signals, switching requirements, and environmental conditions.

In this article, we will first provide practical technical tips for selecting the right Solid State Relay (SSR) for your machinery or control system. From determining load voltage and current to considering mounting styles and ambient temperature, we give you the essential criteria to ensure safe, efficient, and reliable operation.

1-Determine your load voltage and current

During a solid state relay selection process, designers must first consider the main specifications and operating ranges required for their specific application, including input, output, load, and installation requirements. You will need to determine the maximum AC or DC voltage and current for your load in order to choose the proper solid state relay.
In many cases, the load will drive the SSR package. For instance, load currents above 8 Amps generally require a heatsink, regardless of the SSR current rating. For example, a 10 Amp SSR may have a free-air rating of 6 Amps at 40 °C ambient temperature, whereas a 40 Amp SSR in the same package may only reach 8 Amps under the same conditions. This difference arises because both SSRs are thermally similar and have the same case-to-air interface. Full SSR capabilities can only be reached with an adequate heatsink, which maximizes heat removal from the power elements inside the SSR

2-Determine the required control voltage or input signal to turn on the SSR?

Unlike EMRs (Electromechanical Relays), which are typically controlled by a fixed voltage, Solid State Relays have a wide range of input control signals, either Vdc, Vac, or dual Vac/Vdc. celduc® relais offers a large range of regulated AC and DC input voltages.
After selecting a suitable SSR package and terminal configuration, the next parameters to consider are isolation, input drive, and output voltage and current. Isolation value and input parameters are normally straightforward to identify. On the other hand, output parameters require a more careful selection based on the application. Most SSR manufacturers have similar operating voltage and current ranges; however, transient overvoltage or blocking ratings may vary by as much as 200 V. In some installations, this difference can mean the difference between a reliable installation with an adequate overvoltage safety margin and one vulnerable to severe malfunction due to voltage spikes. If transient overvoltage capacity is too low, it may even preclude the use of an external transient suppressor.
If you wish to proportionally control your load, additional specifications are needed to choose the proper SSR. celduc® relais offers proportional controllers for this purpose.

3-Define how many poles you wish to switch

When you select a solid state relay you need to know how many poles are to be sw itched to the load. celduc® offers single-phase, two-phase and three-phase solid state relays. For a single-phase AC load, you’ll need a one-pole AC SSR (single phase). For three-phase AC loads, you’ll need to decide if you want to switch all three phases to the load, or if you want to switch two of the three-phases, the third is then directly connected.

4-Take into consideration what type of load you have

Each load type (resistive, inductive, or capacitive) will function better with certain types of SSRs.
Examples: Resistive loads are best controlled with zero-cross Solid State Relays; random SSRs are ideal for inductive loads; for DC loads, DC Solid State Relays will be required.
Furthermore, for abnormal loads, specific instructions must be followed to prevent excessive current and overvoltage from damaging the device. SSR load current selection must also consider installation requirements and the influence of the load on performance, including physical space for pairing the SSR with an adequate heatsink. Minimum and maximum steady-state load currents, as well as non-repetitive and repetitive surge current capabilities, are important factors. When in use, the switching current flowing through the SSR output should not exceed the rated output current under the relevant temperature, as stipulated in the product datasheet.

5-Determine your mounting style : panel or Din-Rail mount

You need to choose which SSR will fit your application in terms of housing, connection type, etc.
Our offers are available in different mounting configurations with various wiring options: PCB or DIN rail mounted, with screw connections or removable spring terminals, etc

6-Measure the ambient temperature

The maximum SSR current rating depends on the ambient temperature where it will be installed. High temperatures can reduce the SSR’s current rating. We recommend mounting the SSR on a heatsink to optimize its performance and reach nominal capabilities. It is essential to know the operating ambient temperature usually inside the electrical cabinet, as this will determine the heatsink selection.

Need help for selecting an appropriate heatsink in your application ? Please download our technical note.

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