Relay basics: think coils, poles and contacts. There is also applications. Relays contain an electromagnetic coil which opens or closed a set(s) of electrical contacts. The relay coils are either AC or DC energized but not both. You need a constant supply voltage to the coil and there is a minimum coil pull-in current necessary to operate contacts.
Relays are basically automated switches. As a concept, you can consider each switch as a pole. Relays are typically available in 1,2,3 or 4 pole varieties.
The contact set for each pole is typically SPST or DPST. The contact ratings are in volts and amps. Volts are usually not an issue. Amps need to be considered. The rated amps are generally for resistive loads. Engines are inductive loads. Derating for inductive loads are generally 30-40% of the resistive load. Also, if you are running passenger cars with incandescent bulbs you need to consider in-rush current which is generally x10.
Relays are also available with various terminal types: solder, spade, plug-in, terminal blocks.
Stan's relay modules are an excellent value. The contacts are 10A resistive. Just make sure they fit your application.
