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

[DaveM]

I installed a Blue Sea 6 circuit fuse block in the rear cab of 2003 DC Tacoma. I will be running my RTT LED dome light off of this and need to wire in the circuit w/ switch. It will be a low draw LED light so I won't use a relay. Do I put the cab mounted switch in the pos line or the neg line?

thanks,
dave

 

[ntsqd]

With no relay or computer (or both!) involved, the common practice is to switch on the plus side. Though that's not to say that you can't do it otherwise.

 

[adrenaline503]

Always switch on hot. Do not switch on ground.

 

[DaveInDenver]

There isn't much difference electrically between switched positive and switched ground. A lot of Toyota circuits are switched ground (like headlights). But the difference is that on the positive side you keep the switch between the fuse and load, so less wire is exposed to potential shorting with the switch open. It only really matters because the truck body and frame are negative and it's safer.

 

[DaveM]

There isn't much difference electrically between switched positive and switched ground. A lot of Toyota circuits are switched ground (like headlights). But the difference is that on the positive side you keep the switch between the fuse and load, so less wire is exposed to potential shorting with the switch open. It only really matters because the truck body and frame are negative and it's safer.

Makes sense to me. Thanks.

New, but related question:

If I have a circuit in the bed of the truck that I want to be able to switch from either the cab or the bed, how do you wire that up?

I imagine having a single line hot line from the fuse block that is switched in the cab and a line from the fuse block that is switched in the bed. They both join in a Y connection after the bed switch so that either one will close the circuit to the light. Will the work or will that cause a problem is both switches get closed?

 

[pete.wilson]

Hey

Keep in mind, if your switching the hot lead (positive wire) without a relay, use a switch that is at leat 1.5 times the amount of current of your lights (most are about 15amps, same goes for fuses or circuit breakers). You should not have any problems since the LED's are fairly low in current draw to begin with. I just don't like to see companies who sell aftermarket lights with 55W bulbs and not using a relay but use only some cheap switch. Thats a problem waiting to happen IMHO.

Pete Wilson

 

[adrenaline503]

Makes sense to me. Thanks.

New, but related question:

If I have a circuit in the bed of the truck that I want to be able to switch from either the cab or the bed, how do you wire that up?

I imagine having a single line hot line from the fuse block that is switched in the cab and a line from the fuse block that is switched in the bed. They both join in a Y connection after the bed switch so that either one will close the circuit to the light. Will the work or will that cause a problem is both switches get closed?

The best way to do this is to have the two switches go to a relay. A relay is just a big switch that a little switch activates. This way, either small switch will flip the big switch. Plus, you dont have much of a load on your switches, the relay takes it instead. If this doesnt make sense I can explain further.

 

[DaveM]

The best way to do this is to have the two switches go to a relay. A relay is just a big switch that a little switch activates. This way, either small switch will flip the big switch. Plus, you dont have much of a load on your switches, the relay takes it instead. If this doesnt make sense I can explain further.

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?

 

[Robthebrit]

Yes you can double wire a relay with two swithes but be careful. The type of switch you use is important because as you power the relay via switch#1, switch#2 is back powered due to them being linked at the relay. Also consider what happens if both switches are on.

Imagine you have illuminated switches, when switch#1 is active, switch#2 will also light up. If you use multi pole switches then what happens when you back power gets much more complicated depending on what else is connected. A very serious gotcha for our type of vehicles is when the two switches are powered by different batteries, switch#1 comes from the truck battery and switch #2 comes from the aux battery. If either one is switched on then no problem but if you switch them both on you have linked the batteries and they will attempt to equalize which may involve hundreds of amps going through a circuit made to switch a relay coil.

In theory you can use a diode to stop this but a little off the self signal diode may as well not be there when the batteries get linked together, the best you can hope for is the diode simply acts like a fuse and melts.

If you are using simple toggle switches, its all from the same batter and you won't forget what you did then no problem. The safest way to have two switches is to simply use two relays and wire them in parallel.

Rob

 

[DaveM]

Are there relays that accept input from more than one switch? Seems like that would be the most elegant solution.

I have only one batt so no chance of draining from equalizing. The in cab switch will be a LED illuminated toggle switch (in/out/ground), the in bed switch is a simple water proof on/off (in/out) push button switch. I kinda like the idea that the cab LED would come one when the bed switch is on. The light I'm powering is a multi LED Aux light, no idea of the rating hough.

 

[grahamfitter]

Makes sense to me. Thanks.

New, but related question:

If I have a circuit in the bed of the truck that I want to be able to switch from either the cab or the bed, how do you wire that up?

I imagine having a single line hot line from the fuse block that is switched in the cab and a line from the fuse block that is switched in the bed. They both join in a Y connection after the bed switch so that either one will close the circuit to the light. Will the work or will that cause a problem is both switches get closed?

House wiring would use two three-way switches wired back to back. I don't see why that wouldn't work in a vehicle. Ultimately there are three wires between the switches - two between the switched contacts and one for the return. Sorry its been a long, long, time since I drew a circuit diagram. Hopefully the colorful napkin makes up for that.

I think you can get a 12V 3 way switch that illuminates when under load but having never used one I don't know if they work wired back-to-back like this.

View attachment 17209

(Oh and feel free to correct me if this is complete nonsense!)

Cheers,
Graham

 

[Martyn]

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.

 

[adrenaline503]

Robthebrit is right, two relays in parallel is better. I have seen double switch relays, but not in 12v. Its a little tricky for me because I am used to using milspec switches, so I rarely have to worry about back feeds or looping burning a switch. I know on a ship we always switch on hot, since we dont have a true earth ground. I imagine a vehicle would be the same.

 

[Robthebrit]

House wiring would use two three-way switches wired back to back. I don't see why that wouldn't work in a vehicle. Ultimately there are three wires between the switches - two between the switched contacts and one for the return. Sorry its been a long, long, time since I drew a circuit diagram. Hopefully the colorful napkin makes up for that.

I think you can get a 12V 3 way switch that illuminates when under load but having never used one I don't know if they work wired back-to-back like this.

View attachment 17209

(Oh and feel free to correct me if this is complete nonsense!)

Cheers,
Graham

That would certainly work and would be perfectly safe but in this case an internally illuminated switch would work too as both switches would light up only when the rely was on, either switch would switch off the relay and the lights. The only potential issue it finding the double pole switches and running some additional cable.

Rob

 

[Grim Reaper]

Always switch on hot. Do not switch on ground.

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.