automotive interior lighting

N

NeXuS

Newly Enlightened
Joined
Nov 24, 2013
Messages
4
Hi there,

I want to build some interior lighting for my car, which means i will need something that can handle the 6 to 30v range that is normally found in cars.

I will be using 1w COB bead type LEDs, these are very common form factor for 1w LEDs.

I will be using a single LED for the interior light, but also I will have single LEDs in the foot wells that come on when the doors are opened, and single LEDs under the doors to work as "puddle lights"

for this to work i will need to be able to control each individual LED so that the interior light can be used when the doors are closed, and the footwells are not illuminated.

as far as i am aware, the only way to do this is to have a driver for each LED, there is no simple way of having a pre regulated and current limited supply that a pool of LEDs can use independently.

I understand that i need a driver, because a resistor gives a fixed resistance, and won't maintain a constant current when the voltage fluctuates as much as it does in a car (is this right?)

so i settled on the PT4115 constant current LED driver. I noticed that ebay sells a lot of these LED drivers for 1w LEDs and it uses this chip as its centre. Reading the datasheet, it says that this chip has a wide input range of about 4v to 30v which sounds perfect for my automotive environment.

so i built a simple driver, only to find that what voltage i put in, i get out the other end. so for example i wire it up to a 12v supply and the output to the LED sees 12v as well. i haven't risked wiring an LED up because i don't want to burn it out. The LED is rated for 2.8v

but then i had a thought. if you wire up an LED to a car battery with a resistor the LED will work and not burn out, but it must mean that the resistor is not only limiting the current, but it must be dropping the voltage too.

so if i wire up the LED, will the PT4115 sense the current draw and automatically adjust the voltage too? and the only reason i am seeing 12v at the moment is because there is no LED wired up?

if this is not the case, does it mean i will also need a regulator for each LED? something that takes 6-30v and drops it down to 2.8v and then pass it through the PT4115 to regulate the current? because that is going to be very complex and take a lot of space.



also i have bought an LED driver from ebay, also based around the PT4115, which (according to the ebay listing) is rated at 12v and will drive a 10w LED. i assumed that i could string 9 of my 1w LEDs together and drive them with this driver. i wish to have a cluster of 9 LEDs in the boot to illuminate my audio install.

but when i wired it all up it didn't work. on the other side of the PT4115 was 12v, which i calculated was no where near enough voltage to drive 9 LEDs (in series) and these 10w LEDs are rated to 30v, so i have no idea how this regulator was ever going to drive a 10w LED with 12v.

Am I doing something wrong, or is my maths sound and i've just been conned by cheap chinese crap again?

any tips or help towards my quest would be greatly appreciated!

Thanks!




(ps, this may appear very similar to a previous post i made, but i am genuinely looking for guidance on automotive LED drivers for interior lighting only)
 
I

inetdog

Enlightened
Joined
Mar 4, 2013
Messages
442
If the voltage varies that widely, you really need to use a driver with active circuitry that can maintain constant current.
Once you have limited the current, you can avoid having a seperate driver for each LED by putting several in series or in parallel with a small resistor in series with each to enforce equal current sharing.


Tapatalk...
 
N

NeXuS

Newly Enlightened
Joined
Nov 24, 2013
Messages
4
thanks very much for the reply!

the PT4115 has active current monitoring so that is no problem, its the voltage i am more concerned about.

but i am not sure i understand you, if i want to switch on individual LEDs then they cannot be in series, and if they are in parallel then the current limit would need to change with the number of LEDs lit at any one time. if each LED requires a limit of 300ma, and there are 4 LEDs then to run all 4 would require the current limiter to be set to 1.2a, but if only one of the LEDs is lit, it would be exposed to far too much current.

or do i have that wrong?
 

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