This is my first exposure to a solid state relay. I tested it first by applying voltage to pins 1 and 2, then I checked for continuity on pins 3 and 4 but got nothing. I finally figured out that I only need to test pins 3 and 4 for voltage. I also did a resistance test between the pins. This worked great for my project. I have an e-bike that provides useless 6V at 500mA for the headlights. I used this relay to connect to my 48v battery.