2 diffrent light settings on 1 switch

Thread Starter

ryan barnes

Joined Apr 22, 2019
9
hi first time posting id like to know if this circuit would work and what rectifier diode i need they are both 12v devices i want just the headlights on when the switch is down and both on when switch is up this is a 3 way switch.
kays new lights.png
 

Tonyr1084

Joined Sep 24, 2015
3,625
Hello Ryan, and welcome to AAC.

The way you have drawn the circuit will do what you're asking. As far as which diode you'd need, you will need one that can handle twice the current the headlights draw. What confuses me is - when you turn your headlights on - they have their own circuit. So why do you need this circuit to control headlights and light bar? I'm not saying you're wrong in doing so, I'm just wondering why.

I'd start by considering the amperage of the fuse that controls the headlights. If it's a 20 amp fuse then you would want a diode rated at 40 amps. If you know anything about Ohms Law, 40 amps at 12 volts is 480 watts. Likely your headlights draw (each) about 55 watts for a total of 110 watts. I wouldn't want to use a diode that couldn't handle 200 watts. Ohms law predicts 480 watts at 40 amps on a 12 volt circuit. But auto electrics can easily see 14.5 volts. I still think a 40 amp diode should be fine. But this is all estimation on my part. Others here with more experience may have more sound advice. Perhaps controlling a relay would be a better solution.

In one position you power just the headlights. In the other position you power a relay that jumps both the headlights and the light bar together. Automotive relays are typically rated for 40 amps. That might be a better way to go. Let us know if you have further questions. For now, the way you've drawn the circuit appears to work. But how long it will work remains to be seen. A relay will probably do better.
 

Yaakov

Joined Jan 27, 2019
1,600
@Tonyr1084 has it right, of course. A relay is almost certainly necessary to avoid frying the switch.

@Tonyr1084: in some places it is illegal to have auxilliary lighting that can be turned on when the headlights are off. If that is the reason for this, I would be inclined to use a relay powered by the ordinary headlight circuit to enable the light bar rather then modify the headlight circuit at all.
 

Tonyr1084

Joined Sep 24, 2015
3,625
@Yaakov has an excellent point. Headlights, and all exterior lights for that fact, are regulated by the DOT (Department of Transportation). It's that way to keep the lights on cars working in ways that other motorists expect. For instance: When you step on the brake pedal, it's expected your brake lights will light up in the same way all other cars do. Same is true of blinkers (turn signals). Headlights and fog lights as well.

I added fog lights to a truck years ago. It had a relay that could only be activated when the headlights were on. Meaning I couldn't just drive with the fog lights only. That was recently, within the past 7 years. However, when I was a kid driving my 72 Nova, I installed fog lights that I could turn on whenever I wanted regardless of whether the headlights were on or not. I got a "Fix-It" ticket and had to modify them so they could not be on unless the headlights were on. It's been that way for many years. There's a possibility you can go for many years and not get stopped for "Unauthorized Use of Lights" on a motor vehicle. A fix-it ticket is a bit of a nuisance, but if you're in an accident caused by "UUL" your insurance might deny your claim and you could find yourself "Out Of Pocket" on that one. It's a remote possibility, but a possibility nonetheless.
 

Wolframore

Joined Jan 21, 2019
1,301
Couple comments - @Yaakov and @Tonyr1084 are right of course... but according to your diagram it looks like the headlights are on in both settings so it may just be disclaimer to let you know the issues with messing with lighting in a car.

If it's off-road like an ATV it's up to you.

A separate circuit with separate fuse would be a good idea. That way if a fuse blows you still have some redundancy.
You can still enable powering headlights but with some sort of isolation like a relay as mentioned above. All the high current circuits should be switched with a properly rated relay.

As an exercise your circuit would work... there are some issues that go with that and the devil is in the details especially in an area so tightly regulated like automotive - Safety First.
 

Thread Starter

ryan barnes

Joined Apr 22, 2019
9
in total honesty guys its not for an actual car its for my sons ride on vehicle but that was the only way i could explain it the headlights are just a few leds and i wanted to keep the stock switch instead of making holes in the plastic ill show you what i meanUntitled.png
 

Thread Starter

ryan barnes

Joined Apr 22, 2019
9
i just dont know what diode to buy if anyone could link them for me off ebay maybe theres about 10 leds in total and the light bar im not sure of how to work out amps etc but i know the lightbar is 12v 36w and the leds are 320ma and run at 2.1-3v on resistors
 

Wolframore

Joined Jan 21, 2019
1,301
Fun... as long as you're doing the math and making sure you're not drawing more current than it's designed for it should be fine. I need to hook up a horn for my granddaughters pedal car... she loves hitting the middle of the steering wheel. I'm also working on a slammed rat rod radio flyer for her to take to car shows but that's another project.
 

djsfantasi

Joined Apr 11, 2010
5,680
So, you need a diode that’s ratef for the total current draw.

The LEDs/headlights are 0.32A

You have the voltage and wattage of the light bar. 12V and 36W. You want to calculate the amps. The formula for Watts is
W = V*I​
Or written to calculate current
I = W / I​
So, you need a diode that can handle 3.32A.

But! The diode will only pass current for the LEDs. So, you’ll only pick a diode that can pass 0.32A. Anything higher than that would work. 1A diodes are fairly common.
 

Wolframore

Joined Jan 21, 2019
1,301
You're pulling about 3 Amps on the light bar with 36watts@ 12V. 36/12 = 3 AMPS.

The diode needs to support the power going to your other lights... If you're running 10 LED's at 320 mA each that's 3.2A. Or if it's 10 LED's at 320mA total it's only 0.32 Amps...

Either way the components get bigger for higher current but I would get something rated for 10 A / 30-50V - higher current through diodes can cause resistance to go up so bigger the better.... within reason.
 

djsfantasi

Joined Apr 11, 2010
5,680
The diode needs to support the power going to your other lights... If you're running 10 LED's at 320 mA each that's 3.2A. Or if it's 10 LED's at 320mA total it's only 0.32 Amps...
I think the reasonability test favors 320mA TOTAL. The LEDs run on 2.1-3V. And 32mA apiece is also reasonable assumption. 320mA per LED just doesn’t sound right for 3V.
 

Thread Starter

ryan barnes

Joined Apr 22, 2019
9
yes sorry there are 8 5mm leds that run at 25ma each then 4 8mm leds that run at 30ma each so the total for all lights is 320ma then + 3a light bar so call it 3.4A can someone find a cheap diode for me please and put the link as im finding it difficult to find them
 

Wolframore

Joined Jan 21, 2019
1,301
https://www.amazon.com/1N4004-Axial...ds=diodes&qid=1555954909&s=automotive&sr=1-27

This is rated for 1 A, which is fine... see above explanations. Your diode will pass current ONLY for the 320mA of 5mm LED's.

One last comment - please use at least 20GA or thicker wires for the light bar and put a 5A fuse on it.

https://www.amazon.com/First-Source-In-line-Fuse-Holder/dp/B0002KR8EE
or
https://www.amazon.com/Waterproof-fuse-holder-Blade-Holder/dp/B01DLUQ1BW/ref=sr_1_18?keywords=in-line-fuse-holder+5a&qid=1555955727&s=automotive&sr=1-18
 
Last edited:

djsfantasi

Joined Apr 11, 2010
5,680

Thread Starter

ryan barnes

Joined Apr 22, 2019
9
this is what im about to order all good? and ive already got fuses and wire i wouldn't risk it with something my son has to sit in but thanks for all your guys help very appreciated just gotta wait on china nowUntitled.png
 

Thread Starter

ryan barnes

Joined Apr 22, 2019
9
okay quick question ive just noticed that the negative is on the swtich can i still use the diode if i do it on the negative?
 

djsfantasi

Joined Apr 11, 2010
5,680
Trace the current flow in your circuit. I expect that all you need to do is flip the diode around. The end of the diode with the band should point to the most negative connection. In the original, the most negative connections were on the other side of the LEDs. The converse is also true. The end of the diode without the band should be connected to the most positive connection.
 

Thread Starter

ryan barnes

Joined Apr 22, 2019
9
i completely understand but i wasnt sure if it had to be positive if not its and easy fix anyway i can just chop the positive for the switch and solder the negatives together and heatshrink them
 
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