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How to select the right solid state relay ?

solid state relays

When selecting a SSR (solid state relay), it should be based on the application conditions as well as on SSR technical performances. This is important to take into consideration the overcurrent and overvoltage conditions in the circuit versus the SSR ratings.

We have listed all the key questions for choosing the right SSR. It will help you not to forget any key parameters.

-For AC Solid State Relays

-For DC Solid State Relays

 These questionnaires are devided into 5 parts :

1-What is the control voltage or input signal to turn on the SSR?

You will need to determine the control voltage value, means the control voltage of the SSR which is required to energize your load.

Unlike EMR (ElectroMechanical Relays) which are typically controlled by a fixed voltage, Solid State Relays have wide input control signal ranges, either Vdc, Vac, or dual Vac/Vdc.

celduc relais is offering DC or AC or AC/DC control voltage SSR .

2-How many poles do you need to switch ?

When choosing a Solid State Relay, you will need to know how many phases are to be switched to the load.  For a single-phase AC load, you will need a single phase AC switching SSR. For three-phase AC loads, you will need to decide if you want to switch all three lines to the load (in that case you will use a three-phase AC switching SSR), or if you can get by with only switching two lines (in that case you can use our 2 leg three-phase SSRs : two of the three phases are switched by the SSR, the third being directly connected).

For DC load you will need to choose a DC switching Solid State Relay.

3-What type of load do you have?

There is no problem for SSR to switch on/off the normal loads, but for some special loads this is necessary to follow some rules to avoid damages to the device caused by excessive current and overvoltage.

Most of celduc’s SSR (except SO9, SU9, SA9, SMT9, ….which are specially designed for heating control) are designed to a state-of-the-art technology making them suitable for control of inductive loads (solenoid valves, electromagnets, motors,…).

In the use, the switching current flowing through SSR output should not exceed the rated output current under relevant temperature as stipulated in the product data-sheet (you must refer to the tables and thermal curves). The possible inrush current cannot exceed the overload capacity of the relay.

Typical applications solid state relays

The current rating of the SSR is selected according to different load types, depending on whether there is or not inrush current :

SSr current rating according to load type

The voltage of load power supply can’t exceed rated output voltage of solid state relay, and can’t be lower than stipulated minimum output voltage. The maximum value of peak voltage for solid state relay should be lower than the value of the transient voltage. When switching OFF the AC inductive load, single-phase and 3-phase motors, or capacitive loads, an over-voltage, which could be twice the peak voltage of the power supply, may occur.

4-How will you mount / install your SSR ?

After you have defined the technical characteristics needed for your Solid State Relay, you need to choose which SSR will fit your application in terms of housing, connection type, ….

Celduc offers a wide range of Solid State Relays available in different mounting configurations and various wiring possibilities : PCB or Din-Rail mounting, screws connection or removable spring terminals, …

5-What is the ambient temperature where the SSR will be installed ?

Operating ambient temperature is also very important to know, in order to choose the right heatsink. How choosing the right heatsink ?

SSR on heatsink

celduc relais can help you define the right Solid State Relay suitable for your application.

We invite you to download our selection guide catalogue which will give you an overview of celduc's range of solid state relays.