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Old 11-18-2006, 08:50 PM   #1
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Default Q: relay wiring

Is there any advantage to wiring the hot (+12v) lead through a relay, vs. ground? I'd always practiced running the +12v through as few connection/junctions/switches as possible when the ground is an available option. I've got a fan that's too much of a load for the stock fan circuit wires- nearly kills the car with the lights on when it starts up. - rob
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Old 11-19-2006, 12:31 AM   #2
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With something like a fan motor, it makes no difference whatsoever whether a switch (or relay) is placed on the supply side or the return side of a load.

With regard to minimizing connections, you're talking about series resistance losses. Any interconnection, no matter how clean, presents some small resistance. As the number of connections goes up (and the quality / cleanliness goes down) the amount of resistance in the circuit increases, creating a voltage drop in the circuit. The goal, as you are obviously aware, is to keep this drop as small as possible.

The important thing is that it does not matter where in a circuit this resistance is placed relative to the load. Resistance before the load and resistance after the load will reduce the power through the load in the same manner. Put another way, maintaining good, clean grounds is just as important as good, clean supply wiring.
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Old 11-19-2006, 11:28 AM   #3
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makes sense- thanks joe
Just made me wonder because I could find any relay wiring diagrams that switch the ground part of the circuit and couldn't figure out what I was missing. I'd always been told to switch the ground side to reduce the potential for shorts if there's loose connection wire. That might say something about crimping skills though.
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Old 11-20-2006, 12:57 PM   #4
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You're right, safety is about the only determining factor. Putting the switch after the load means that if you accidentally drop a wire and short it to ground, the load merely turns on. Beats blowing fuses and melting wire.
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Old 11-27-2006, 11:16 AM   #5
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Here's how I wired my big 16" spal fan using a relay and the existing/OE radiator fan circuit. Works like a charm, virtually eliminated the idle drop during the fan's startup load.
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