Wiring 2 electrical inputs to one source

[nismos14 5,000+ posts]

I have a question here.... what I'm trying to do is safely run a cold cathodes, or weries of cold cathodes from two seperate sources. Basically, my door lights currently control the cathodes you see here: http://www.nissanclub.com/forums/3454073-post7.html .

What I want to be able to do is also use an outboard switch like the switch from my fog lights to turn them on and off whenever I want, not just when the doors are open, but I do want them to turn on with the doors.

Sow how can I have the door trigger the lights, as well as the fog light switch without fuses blowing if I happen to open the door while the fog light switch is set on?

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[helotaxi 5,000+ posts]

Sounds like a job for a pair of relays. Wire the relays in parallel, have the switch trigger one and the door trigger the other. With them in parallel if either or both are on the lights are on if both are off the lights are off. The circuits remain effectively independent of each other so no chance of blowing fuses.

 

[wu501 10+ year member]

^^agreed. relays are your friend //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[nismos14 5,000+ posts]

Sounds like a job for a pair of relays. Wire the relays in parallel, have the switch trigger one and the door trigger the other. With them in parallel if either or both are on the lights are on if both are off the lights are off. The circuits remain effectively independent of each other so no chance of blowing fuses.

Could it overload anything current wise?

 

[Stel 10+ year member]

Just run another switched positive wire to the positive side of the light (assuming it's a 12v light). If your concerned about the possibility of something overloading, fuse the new positive feed to a little above the current demand of the light.

 

[helotaxi 5,000+ posts]

Could it overload anything current wise?

Relays draw almost zero current from the switching source. That's why they're used. Allows a low current trigger to switch a higher current device.

Just run another switched positive wire to the positive side of the light (assuming it's a 12v light). If your concerned about the possibility of something overloading, fuse the new positive feed to a little above the current demand of the light.

Doing that effectively shorts the two sources to each other and will blow all kinds of fuses and potentially mess up some of the car's electronics. Relays will isolate everything and prevent shorts.

 

[Stel 10+ year member]

Doing that effectively shorts the two sources to each other and will blow all kinds of fuses and potentially mess up some of the car's electronics. Relays will isolate everything and prevent shorts.

No. If the sources both have the same 12v potential (both switches on), the potential difference between the two is zero, current will not flow between the sources. If only one switch is open, again current will not flow between the sources. Current will only flow to ground, which the only path will be through the device. Yes a relay would be helpful if this were a high current device, but I highly doubt it is. OP, what is the voltage and current draw of the light?

 

[nismos14 5,000+ posts]

2.1 amps, 12 v.

 

[Stel 10+ year member]

That should be fine for any standard accessory switch. Just make sure the switch is rate for the total load it will supply.

If both lights are independently controlled (if one door is open only that light goes on) and you want to keep it that way, get a 2 pole switch or use two separate switches and wire each switched 12v + feed to a light (or you could use one single pole switch and put two diodes rated at least 2.1a, 12v but thats probably more involved). btw there is only 2 lights I assume? If you have more than 2 and you want only one to turn on with each door opening maybe the diode method becomes easier.

 

[nismos14 5,000+ posts]

Here's the layout:

There will be up to possibly 4 lights wired in parallel on each side of the car so a maximum of 8, with a final load of 5.6 amps. Being on the fog light switch and being on the door switch. I want to be able to safely have the door be open with the fog light switch (oem) on and not have to worry about any electrical circuits shorting out.

 

[Stel 10+ year member]

and btw make sure the circuit you tap for your new switch can handle the additional load: light current draw times number of lights. Was the 2.1amps figure per light?

 

[nismos14 5,000+ posts]

.7 per light.

 

[Stel 10+ year member]

Here's the layout:
There will be up to possibly 4 lights wired in parallel on each side of the car so a maximum of 8, with a final load of 5.6 amps. Being on the fog light switch and being on the door switch. I want to be able to safely have the door be open with the fog light switch (oem) on and not have to worry about any electrical circuits shorting out.

ah ok, it won't short out and the fog light circuit should be able to handle 6amps easily, but like i said, if these are all wired to one switch, now when one door opens, all 8 lights will go on. The question here is if the old door circuit can handle 8 lights, if it can't that fuse will blow or that switch in the door will fry. I'm guessing that circuit can handle it though, there isn't a lot of current draw here. But keep in mind you have the door switch potentially supplying a load it might not have been designed for (ie if only one door is open then current for all 8 lights will be passing through that door switch).

So the alternative would be to put a diode rated for at least 12v and the current it supplies (1.4 amps for 2 lights) in series with each 12v + feed to each door from the fog switch.

Diodes allow current to flow in only one direction, so the one fog switch will still turn on all 8 lights, but if one door is open , only the lights for that door will come one (assuming the fog switch was off), but yes, the fog switch can be on while the door is open without stuff shorting out.

So one more question, currently if one door is open, only that light comes on? and if so do you want to keep it that way?

(I'm off to work)

 

[helotaxi 5,000+ posts]

Or you could wire a 12v relay and not worry about current. Relays are cheap. You could use the diode array but I would still couple it all with relays.

Depending on the vehicle, the door lights are activated by the computer not a direct switch. Apply 12v where it wasn't meant to be and the computer gets a bit upset. A diode could keep it from being a problem but so could a relay and the relay will take care of any current issues as well.

 

[mokedaddy 5,000+ posts]

Or depending on the car you could probably tap directly into the domelight switch which would allow them to turn on with the door open or whenever you turn the domelight on with the switch. But I would definitely use a relay as opposed to running 8 leds directly off a door trigger.