Resistor needed for relay

Hi,

I have this relay: Solid State Relay - 40A (3-32V DC Input) - COM-13015 - SparkFun Electronics

Datasheet: cdn.sparkfun.com/datasheets/Components/General/SSR40DA.pdf

As I find various answers, I’ll try here to be sure: Do I need a resistor between the Arduino and the relay?

No you don't need a resistor,

You can connect the one of the relay inputs directly to the Arduino output pin (and to the other input to arduino gnd).

Great - thank's for the quick reply :)

As I find various answers, I'll try here to be sure: Do I need a resistor between the Arduino and the relay?

Some solid state relays simply have an LED (like an opto-isolator) and those require a resistor. Usually, if a resistor is required they will specify an operating current range, and a "forward voltage" for the LED of around 2V.

If you see a 3-30V range or something like that, the relay is "ready to go". ...And most of those heavy-duty AC solid state relays with screw terminals don't require a resistor.

Note the voltage drop in page 7. these will tend to heat (1.6W per amp) much more then an electromechanical 'contactor'