Problem:

I have a old light fitting on my boat that I would like to convert from CFL to LED. The boat electrical system is 12v DC - but in reality can ben 12.5v - 14.8v depending on charging etc. The manufacturer apparently has made a conversion kit using:

Cree CXA1304 x 2 (Vf = 9v)
LM2940-T 10

There is a switch for a High/Low setting.

The circuit they made to do this conversion I have been told (3rd hand) looks like this, and I have constructed this:



I have tried this circuit and with the resistor values given the circuit draws 1.6A @ 12v and the LM2940 gets very hot (140) very fast. (
On low it uses about 0.28A.).

The LED's get similarly hot very fast. Adding a cheap heat sink from china to both reg and led has helped: (it has fins on the bottom)



But eventually it gets to thermal equilibrium and gets almost as hot as it was before.

I have also tried to add a resistor to the input of the LM2940 which reduced the total current draw of the circuit down from 1.6A to 0.8A - this helped a lot. After 30 minutes the LM2940 was at 50 deg C and the Crees heat sink at about 80 deg C - but the resistor I added to the LM2940 input is now the heat problem.

I am wondering if a Meanwell driver might be better at least for not having to deal with the heat in the driver side of the circuit - something like https://www.meanwell.com/Upload/PDF/LDD-H/LDD-H-SPEC.PDF

Unless anyone can think of a way to tweak the linear design?

 

Thoseresistor values are much to low, that is the problem. The first thing that you need is information about the intended current for the LEDs, and the anticipated forward voltage at that current. With that information it will be possible to calculate the correct resistance values for the two resistors.
In addition, that voltage regulator IC, LM2940T-10, does need to have additional 0.10 MFD capacitors across the source and load terminals to avoid possible oscillations that will certainly cause undesired operation modes.