Electrical question

[Ranger Mark]

This is a little involved and maybe stupid but, here it goes.

2 scenarios:
#1, You have a relay to let's say a light bar. Across the coil, 85 is wired to ground and power is run to a switch then down to 86.

#2, same relay, same light bar. Wire from source through a fuse to 86. 85 is wired to the switch which is wired to ground.

What are the benefits of either? I see Ford uses option 2 a lot. Is this common for automotive wiring?

 

[Uncle Gump]

Ground side switching....

All the voltage is dropped across the relay coil which means less wear and tear on the switch.

I typically do most of my wiring this way...

 

[adsm08]

Right. Ground side switching is easier on the switch contacts, but otherwise the two are basically identical from a functional standpoint.

 

[Curious Hound]

I usually use power side switching. Either one works.

I'm not scientist enough to argue it. But according to ohms law and kirchoff's law, the same potential is available across the switch contacts either way. With the switch open, you will read 12volts across the contacts. The relay coil isn't dropping any voltage until current flows through the switch. When the switch is closed, current is the same in both cases. Same potential for arcing at the contacts in both scenarios. Either way should be fine.

I don't like having power on my load device with the switch open. That most likely comes from dealing with higher voltage circuits all these years. I feel safer knowing my load is de-energized when I'm working on it.

 

[RonD]

Using the ground to activate a relay is for two good reasons
First is less 12volt wires in the vehicle to possibly short out
Second, you just need ONE wire from engine bay into a switch in the cab, you can get the ground from cab metal

And you can use lighted switches on this same as 12volt switching, i.e. it lights up when ON, and no light when OFF, there is 12volt on that one ground wire to the switch, and power flows when switch is on, so it will light up a light, just FYI
But the 12volts on the wire from the relay can't "short out", i.e. if that wire shorts to ground all that happens is the relay activates, thats all, no melted wires or blown fuses

 

[Curious Hound]

Using the ground to activate a relay is for two good reasons
First is less 12volt wires in the vehicle to possibly short out
Second, you just need ONE wire from engine bay into a switch in the cab, you can get the ground from cab metal

And you can use lighted switches, i.e. it lights up when ON, and no light when OFF, there is 12volt on that one ground wire to the switch, and power flows when switch is on, so it will light up a light, just FYI
But the 12volts on the wire from the relay can't "short out", i.e. if that wire shorts to ground all that happens is the relay activates, thats all, no melted wires or blown fuses

Those reasons make sense. Thanks, Ron