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Understanding relays. Seems to be some confusion about how a relay works and what they are used for. I am going to try to explain it without pictures.
A relay is simply a remotely controlled switch. First a little understanding of how a switch works. A switch is a device that allows a circuit to be opened or closed depending on the position of the switch. A switch, when closed, completes an electrical circuit. If I put a switch between a battery and a light bulb, I can turn the light bulb on and off using the switch. The switch can be on the NEGATIVE side of the circuit, or it can be on the POSITIVE side of the circuit. It makes no difference because either way, you are breaking or completing the circuit using the switch.
A relay is a switch that can turn itself on and off by the use of an electromagnet. A relay is just a switch. The switch is turned on (circuit completed) by turning on an electromagnet. The electromagnet must have a complete circuit to be “on”. The electromagnetic circuit can be turned on and off by the use of another switch. This other switch can interrupt the POSITIVE or the NEGATIVE side of the electromagnet circuit in the relay (although the polarity of the electromagnet circuit does not matter at all). What I am describing is a simple, 4-wire relay. The two big wires that are used as the primary switched circuit, I call the “big wires”. These two big wires are normally open (meaning the circuit is NOT a complete circuit when the electromagnet is turned off). The two small wires that are the electromagnetic circuit, I call the “signal wires”.
As an example, I will describe my relay circuit that I use for my electric supercharger. I use a heavy-duty relay to activate the electric motor that drives the fan. What I did was wire one side of the electric motor directly to ground. The other side gets its power from the relay. So there is simply a switch between the electric motor and the positive battery terminal. This switch is the “big wires” of the relay. The electromagnetic circuit in the relay determines whether or not the fan is on or off. The electromagnetic circuit has its positive side running to the fuse panel to a “key-on” power source. That means that if my key is in the ignition and turned on, the electromagnet has a complete circuit to the POSITIVE side of the battery. It still needs to have a complete circuit to the negative side of the battery to activate the “big wires” so the fan will come on. The NEGATIVE side of the electromagnetic circuit is completed by a micro-switch mounted on my throttle plate. It becomes a complete circuit when the throttle is wide open. So here is the summary for my electric supercharger circuit: I turn on my ignition to supply positive 12-volts to one side of the electromagnetic circuit in the relay. Stepping on the throttle completes the negative side of the electromagnetic circuit. At wide-open throttle the relay’s electromagnetic circuit is completed so that means that the “big wires” are also a complete circuit. That means the electric motor on my supercharger has power (through the big wires on the relay) and ground (directly bolted to the frame).
That describes a simple 4-wire normally open relay. Most relays actually have 5 wires. The fifth wire is a normally closed circuit that is common with one of the “big wires”. There are many different configurations for relays but now you should have an understanding of the most common relay used in automotive systems. You may ask: Why even use a relay? Why not just use a heavy-duty switch? The reason is: To use a circuit that has very small current to activate a circuit that uses a lot of current.
 

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2001 Subaru Impreza RS
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I thought everyone knew that! It's all about the ones and the zeroes.

Nick C.
 
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