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Old 08-10-2014, 12:36 PM   #1
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Existing lights new switches 12v advice.

So I have kept my existing 12v lights in my bus, I am using them for fully lighting the whole bus up at once. They are wired after the ignition to a switch in the cab at the drivers seat. I want to wire an additional switch at the middle of the bus but I want it to be before the ignition. This is all off of the existing starting batteries btw. So my question is, will it cause some sort of problem if both switches are powered on at the same time in the different location.
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Old 08-10-2014, 03:08 PM   #2
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Re: Existing lights new switches 12v advice.

My assumption is that you want to be able to turn on/off all your roof lights either from one end of the bus or the center somewhere. If that is the case, wire the lights to a fused terminal that is energized at all times and hook it up like you would a hallway switch in your house. All it takes is a pair of on/off/on toggle switches and you are good to go. Google hallway switch wiring, go to images and the first image you get will have a very clear wiring diagram at the top center of the screen.

Your initial wiring proposal would back feed the circuits the ignition thinks are shut off if both switches are on at the same time even if the ignition is off--not good. I have 3 sets of these hallway switches in my tiny Skoolie and find them to be very handy. Jack
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Old 08-10-2014, 03:46 PM   #3
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Re: Existing lights new switches 12v advice.

Yea, I thought it would back feed. The issue is I want to do 3 way switches like you suggested, but I want to have 3 switches Total running it. I think I'm just going to disconnect the switch in the cab. Or. Make sure not to have them both on at the same time.
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Old 08-10-2014, 04:30 PM   #4
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Re: Existing lights new switches 12v advice.

Yup, just disconnect the original switch. The same Google page has wiring diagrams for three switches. Jack
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Old 08-11-2014, 03:32 PM   #5
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Re: Existing lights new switches 12v advice.

If you use standard two-way switches (ON/OFF) in the circuit you'll be able to turn off all the lights with any switch but you'll need all switches ON to turn them back on. Basically, any switch can break the circuit but all need to be ON to get light anywhere.

A pair of three-way switches (ON/OFF here & ON/OFF over there) will let you switch them from either of two switches but be aware that you wire them differently depending on where the power is coming from.

Now, if you want THREE switches controlling the same circuit you'll need 2 three-way switches and 1 four-way switch. Circuits containing four-way switches are a major cause of insanity in beginning electrical workers.

Here's a link to some wiring diagrams:

http://users.wfu.edu/matthews/courses/p ... tions.html
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Old 08-11-2014, 04:41 PM   #6
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Re: Existing lights new switches 12v advice.

Here's a crude drawing of the old & new circuits:



I'm assuming that the "present" circuit is fused somewhere already so if you bypass the dash switch you'd not need to add a fuse. If you later powered your lights from a separate house battery a new fuse would be needed.

You can see how the three-way switches provide a second path for current.Three-ways have three connector terminals while two-ways have only two terminals. No worries about back feeding, by the way. The battery only has 12 volts of potential so if you were to run two wires back to the battery, put switches on both wires and ran both wires to a single light you'd only get 12 volts with both switches ON. The light would see the two wires as one conductor.
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Old 08-11-2014, 06:06 PM   #7
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Re: Existing lights new switches 12v advice.

Hey Roach. You are right about the two batteries and the lights thing but he could still back feed the full circuit which is meant to be disconnected by the ignition assuming more is connected to the same output terminal than just the lights because the back feed has the same effect as turning the ignition on. True, the original light switch would have to be turned on as well to complete the circuit--but that would be very easy to forget about over time. Jack
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Old 08-11-2014, 06:45 PM   #8
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Re: Existing lights new switches 12v advice.

I see what you're getting at - if battery power was to come from a second battery bank (or even come directly from the starting bank) AND the original wiring back to the ignition switch was left intact then there would be battery voltage on the ON side of the key switch even when the switch was OFF. Took me a while to wrap my head around this one.
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Old 08-11-2014, 09:59 PM   #9
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Re: Existing lights new switches 12v advice.

I've seen the backfeed problem, where you could not turn off a vehicle engine if an improperly designed accessory was turned on using ignition bypass battery power while the vehicle was running. For the moment, I will be ignoring the backfeed issue from inserting another power source downstream in the drawing, and concentrate on the switches.

You can have many more than two switches on the light circuit. I had a work van that I wired so that I could turn the dome lights on or off from the dash, the sliding side door, or the back doors. Of course, this was after I disconnected the door switches, so I didn't kill the battery leaving the doors open all day to get my tools in and out.

For the diagram Roach posted, the two SPDT (single pole, double throw) switches shown should be the ON-ON variety, and not ON-OFF-ON with a center OFF position like Jack posted. If someone leaves an ON-OFF-ON switch in the center OFF position, the lights cannot be turned back on from any other switch regardless of which traveler wire is selected.

With ON-ON switches, one traveler wire is always hot, and one is always connected to the lights. If the traveler wires chosen are a match, the lights are on. If the switches select opposite wires, the lights are off.

What are called 4-way switches for household wiring, we would call DPDT (double pole, double throw) when buying toggle switches. Again, we are looking for ON-ON DPDT switches. (I used push-push latching switches, so there was never any confusion caused by the position of any toggle handle. Just push the nearest button to turn the lights on or off.)

All the 4-way switches desired are placed in series to interrupt the "traveler wires" in the drawing, between the source SPDT and the "destination" SPDT. Cut the two traveler wires wherever a 4-way switch is to be installed. Wire all four cut ends to the switch so that in one position, the two traveler wires continue straight across as they are currently drawn. If you flip the 4-way switch, the switch crosses the two wires (picture an "X") so that the path from the upper wire continues on the lower one, and the path from the lower wire continues on the upper one.

Regardless of the positions of all the other switches, the 4-way switch in your hand can create or undo a traveler wire cross-over to power or extinguish the lights. You can put as many 4-way switches in series between the two 3-way switches as you like. Any switch in the series can turn the lights on or off by causing the hot path through the traveler wires to either match or not match the switch position selected at the far end.
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Old 08-12-2014, 12:57 AM   #10
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Re: Existing lights new switches 12v advice.

Make sure you are using switches rated for DC. While you can use an AC switch, they will "wear out" fairly quickly. With today's internet sales, you should have no problem finding a decently priced 12vDC switch (either rocker or toggle style).

Avoid super cheap as they may not be well made. I say that because I bought cheaply priced 12vDC flourescent lights. After a few burned out within a short time frame, we pulled them and found scorched areas under the still operating lights. This is why we put 110vAC lights in the bus and power them off of an inverter. I am not saying for you to use AC lighting, just not really cheap switches. Sometimes cheap is okay, othertimes cheap is not a good idea.
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