Definition: [I]"SPDT Relay : (Single Pole Double Throw Relay) an electromagnetic switch, consist of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one normally open terminal (87). When the coil of the relay is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have continuity. When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity. The diagram below shows the relay at rest, with the coil not energized.
Why do I want to use a relay and do I really need to?
Anytime you want to switch a device which draws more current than is provided by an output of a switch or component you'll need to use a relay. The coil of an SPDT relay that we most commonly use draws very little current (less than 200 milliamps) and the amount of current that you can pass through a relay's common, normally closed, and normally open contacts will handle up to 30 or 40 amps. This allows you to switch devices such as headlights, parking lights, horns, etc., with low amperage outputs such as those found on keyless entry and alarm systems, and other components...." Hey, wait a minute, that's us!
Why not to use a SPDT relay to safely switch our power-hungry motors thus protecting our precious contacts? I believe that's the whole purpose of Mosfet
relays. Btw, I know there are more advanced Mosfet applications out there (and i'm pretty sure these can be done with relays too).
So why to use delicate, relatively expensive electronic setup, if you can do it simpler and cheaper? We don't see automotive industry using Mosfets, it's all good old relays. The only drawback IMO of using a relay is it's size, which is usually bigger than a Mosfet assembly.
So what do you think?
BTW, 12 30A auto relay successfully installed and operational in my friend's AUG, more to come.
I'm attaching a simple diagram for connecting a relay. It is in spanish(?) illustrating how to connect a car headlamp, but it is exactly the same with AEG.
