Hi everyone and thank you for your time, I am having a problem where on my car my position lights work only during night time if left on automatic and in my country by law you need to have at least position on in daytime so I wanted to make them work when the engine is running during the day but still leave them work with automatic lights as intended... The way I plan on doing this is by adding another positive wire that will connect directly to the led but I need a circuit or a component that I am unfamiliar with(but in my head I am sure something like this exists) that will let 12v pass from the second wire(+12v(2)) but when there is voltage present on the fist wire(+12v(1)) to cut off the second wire and use that source instead. I've been looking online for a while but I just can't describe to google exactly what I need so I wasn't able to find any good solution that would be simple and reliable... the power consumption should be around 20w btw... Anyways any help is appreciated and thanks in advance

 

In most countries, when a law is passed, things already in existence are grandfathered in. So if your car was manufactured before the law requiring daytime lights went into effect, there is a good chance that your car is exempt. So if the only reason you are wanting to do this is because you think the law is requiring you to, you need to check the specifics of the laws in your country. If older vehicles aren't exempt, then it's also possible that there is some provision for you to get the needed modifications done at little to no expense. It would be worth taking some time to look into it.

 

In at least two areas where I have lived or passed thru, when the local government passes a rule, there is no escape from "The Man".

BUT for the application that the TS has a simple solution will be diodes, which will allow two different poser sources to operate a light and not interfere with each other. The diodes must have adequate steady state current ratings for the lights used, and voltage ratings above the system supply voltage, which probably the lowest voltage rating of 50 volts will be totally reasonable.

 

I am really new to anything component related but this is what I came up with so I don’t even know if it will work but I guess I’ll try anyway. What do you think?

 

diode will have forward voltage drop. perhaps something like this would do

 

Certainly the diode will have a forward voltage drop. typically less than a volt. That is not at all likely to be any problem at all. In addition the diode will not consume any power aside from what is wasted in that very small diode voltage drop. A realy installation where there are five connections that must be made correctly,and where some wrong connections may cause immediate damage does not seem like a best choice for somebody with rather limited experience. I quote:" but I need a circuit or a component that I am unfamiliar with(but in my head I am sure something like this exists) ", while adding a series diode will be immediately obvious if it is reversed and cannot do any damage if it is reversed. Likewise, adding a second diode will be obvious if it is reversed. Also, the diode installation will be simpler and cost quite a bit less.

 

Certainly the diode will have a forward voltage drop. typically less than a volt. That is not at all likely to be any problem at all. In addition the diode will not consume any power aside from what is wasted in that very small diode voltage drop. A realy installation where there are five connections that must be made correctly,and where some wrong connections may cause immediate damage does not seem like a best choice for somebody with rather limited experience. I quote:" but I need a circuit or a component that I am unfamiliar with(but in my head I am sure something like this exists) ", while adding a series diode will be immediately obvious if it is reversed and cannot do any damage if it is reversed. Likewise, adding a second diode will be obvious if it is reversed. Also, the diode installation will be simpler and cost quite a bit less.

I have enough experience to get everything wired correctly, I just don’t have much knowledge about electronic components and what they do like transistors and how I could use a capacitor or a resistor in circuit… About voltage drop I don’t know how big difference it will make in the brightness of the led hopefully not much, as you say one volt isn’t really much… and about two diodes do you think something like in the picture? Would this work without the relay or would there be a problem having to 12v inputs to the led?

 

I have enough experience to get everything wired correctly, I just don’t have much knowledge about electronic components and what they do like transistors and how I could use a capacitor or a resistor in circuit… About voltage drop I don’t know how big difference it will make in the brightness of the led hopefully not much, as you say one volt isn’t really much… and about two diodes do you think something like in the picture? Would this work without the relay or would there be a problem having to 12v inputs to the led?

NO, since in this case both 12 volt feeds originate at the same source, presuming that the vehicle has only one charging system and only one battery. AND, certainly the LED device will only draw the current it requires. That is how it alwaays works.

Two diodes will work, with the benefits I described. The diodes will be wired with the "arrow" pointing toward the "position light", or lights. The function is similar to a one wy check valve in a fluid system.
What a diode does is to conduct electricity in one direction quite well, and in the opposite direction very poorly. So in this application it will allow either power source to pass thru to the position indicator light. In this application the diodes will prevent either source from sending power to the other source.