Which side of a circuit do you switch, pos or neg?

[DaveInDenver]

Don't the electrons flow from negative to positive, rather than positive to negative. I thought that was the whole concept of the positive ground, popular in vehicles in the late 50's early 60's.

If that is the case wouldn't it be better to fuse the negative side??

Of course the theory may have no practical application.

I dunno why they grounded positive on old cars, but the phone company does it to reduce electrolytic corrosion. With the (+) grounded and the (-) floating, you never have a positive potential to ground. The (+) has 0V and the (-) side has -48V. Maybe that was the thinking, but since the car frame is isolated from true ground by the tires then you always have a potential no matter how you assign polarity. If you dragged grounding chains, then having a positive ground might make a difference.

But you are right that conventional current flow, positive to negative, is opposite of electron flow. It's just a convenient convention that everyone follows. But in terms of fuses and switches, it doesn't matter. If the circuit is interrupted, there is no flow of electrons anywhere in the circuit. They don't flow to the stop point and wait. If there's no where to go, then they don't even start. So with the switch open in the negative side, there's no current coming down the wire anywhere. The only reason it might matter is if the device is designed to bleed current off into ground and by switching the negative side it's possible that you leave things charged that aren't supposed to. Can't think of anything that would really care, but I don't have a fridge, computer, GPS or anything. I did put my radio relays on the (+) side of their power with their coils being negative switch from the ignition key, but I doubt it really mattered.

 

[adrenaline503]

Why not? Nissan did on your truck. Look up how your dome light works. Its how all your gages work, Its how all the sensors on the motor work. In fact Nissan does a lot of negative switches.

Most electronics work on negative switching with diodes.

It doesn't matter as long as you fuse the hot. Negative trigger/switch in some instances means less wiring. If you don't need a indicator light then its a single wire switched to ground.

I in fact prefer Negative switches especially when I am using a relay. I can run them on 24 gage wire and keep all the high current wires short and close to the source and device.

Huh, I guess never looked into how my truck was wired, I guess thats a good thing. I was always taught that hot was always fused, then switched then loaded. Maybe the military just wants everything wired the same way every time. I know that electronics used a lot of negative/ground switching.

 

[madizell]

I know how the relay works, but how do you do a double switch on them? Just wire two powered switches to the one post on the relay?

Yes. I have one circuit that is wired this way and it has worked just fine for years. Rear facing lights can be triggered by the transmission switch for back-up lights, or by an overhead switch for camp lights. Each provides a 12V signal to the same post on one relay. Even if both are turned on simultaneously, it still works since the relay does not care and the signal is still 12V. Nor do the independent switches care. If you provide 12V to both sides of a switch (back feed), the result is still 12V through the switch.

 

[ntsqd]

Huh, I guess never looked into how my truck was wired, I guess thats a good thing. I was always taught that hot was always fused, then switched then loaded. Maybe the military just wants everything wired the same way every time. I know that electronics used a lot of negative/ground switching.

I'll guess that is EXACTLY why you were taught to do it that way. So that everything is the same. In an organization like the military, standardization and uniformity are a requirement. Imagine the headaches if everyone did it differently.

 

[DaveM]

Yes. I have one circuit that is wired this way and it has worked just fine for years. Rear facing lights can be triggered by the transmission switch for back-up lights, or by an overhead switch for camp lights. Each provides a 12V signal to the same post on one relay. Even if both are turned on simultaneously, it still works since the relay does not care and the signal is still 12V. Nor do the independent switches care. If you provide 12V to both sides of a switch (back feed), the result is still 12V through the switch.

Thats what I wanted to hear. I'll try it that way first. Thanks.

 

[madizell]

Here's another option for wiring LED's and such in the back: Run one power wire to the lights, then run alternative ground wires to switches wherever you want them, then ground each switch. This way the power runs are limited, and the circuit will function much like the dome lights, i.e., more than one switch will work one light, which is probably why domes are wired that way in the first place. No reason to use expensive relays on low power circuits used in LED lighting.

 

[DaveM]

Here's another option for wiring LED's and such in the back: Run one power wire to the lights, then run alternative ground wires to switches wherever you want them, then ground each switch. This way the power runs are limited, and the circuit will function much like the dome lights, i.e., more than one switch will work one light, which is probably why domes are wired that way in the first place. No reason to use expensive relays on low power circuits used in LED lighting.

Well that had been my initial idea, but I got convinced that the power side should be switched not the neg. Now that I've read all the posts, I guess it's ok to switch either. I had never considered the fact that the dome lights are actually ground switched.

 

[kjp1969]

I always prefer to switch ground so that a short to ground on the switch wire will simply complete the circuit, not blow the fuse. Blowing a fuse isn't the end of the world, but can be a nuisance.
Kevin

 

[Grim Reaper]

Huh, I guess never looked into how my truck was wired, I guess thats a good thing. I was always taught that hot was always fused, then switched then loaded. Maybe the military just wants everything wired the same way every time. I know that electronics used a lot of negative/ground switching.

AC power like in your home it is generally considered best to switch the hot. That may be where it came from. Low volt DC its not really an issue either way.

 

[michaelgroves]

Here's another option for wiring LED's and such in the back: Run one power wire to the lights, then run alternative ground wires to switches wherever you want them, then ground each switch. This way the power runs are limited, and the circuit will function much like the dome lights, i.e., more than one switch will work one light, which is probably why domes are wired that way in the first place. No reason to use expensive relays on low power circuits used in LED lighting.

This works well. And if you want a relay, you can do the same thing - run one power to the relay, and then alternative ground wires to switches wherever you want them, then ground each switch. The output from the relay is then conventionally wired to the lights.

 

[madizell]

This works well. And if you want a relay, you can do the same thing - run one power to the relay, and then alternative ground wires to switches wherever you want them, then ground each switch. The output from the relay is then conventionally wired to the lights.

At the time I wired my HID's for alternate switches, I never thought to use a ground side switch for the purpose, and instead provided two power sources. Now that I think about it, it would have been just as easy to do it the other way around.

 

[ntsqd]

I think "switch on the hot side" has been ingrained in us all. I know it's taken me a while to realize that it isn't always the best way.

 

[DaveInDenver]

I think "switch on the hot side" has been ingrained in us all. I know it's taken me a while to realize that it isn't always the best way.

That's definitely the case and there are reasons for doing things various ways. First and foremost, there's the NEC and that is very specific about things. But when doing low voltage stuff, there are advantages to switching the ground side. But there's nothing wrong with sticking the switch in the positive side, either. Whatever works. In any case, always put a fuse as near to the positive battery terminal as possible, if your truck is negative ground.