wsr,
We might have to think differently here. I see that you want to design a tail light for the Ghia using Lumiled's 3W Red LEDs. I looked up the car to see what the taillights look like. It is a nice little car. Funny that I believe I seen that type of car in a parking lot a few months ago.
What type of heatsink do you plan for the three LEDs? Do you plan on using the base from an old light bulb and building a heatsink into that (just insert into existing 1157 wiring harness), or do you plan on making a fully custom taillight enclosure? I am asking this because I am very concerned about heat right if you try to push LEDs to that level of brightness that you want.
You might have to reconsider the power going to these LEDs. The more power, the brighter they get, but the hotter they get, too. If you look at the specifications on Lumiled's data sheet, you will find that the Red colored 3W LEDs will drop in brightness quickly as it heats up. For example, at 140 degrees F (60 degrees C), a normally safe temperature for LEDs, the LED intensity drops to about 66%. That can hurt the overall brightness and affect the efficiency of the LED.
What if we brought down the current that goes to these LEDs. I have an old 1W red/orange Luxeon Star fitted as my 3rd brake light. Normally it runs at 350mA and produces 55 lumens. I had to tone down this thing a bit because it seemed to bee too bright compared to my two tail lights.
The reason I say this is because 400 lumens in red, times 2 (one for each tail light), is a whole lot of red light. Way too much, in fact! The 400 lumens that you mentioned that is from the light bulb is really measured in white light. 2/3 or more of that light is lost in the red colored tail light filter. So you are actually be seeing only 133 lumens instead of the original 400 lumens. This particular LED only emits pure red light, and in an efficient way as designed, so all of that red light (or most) will pass through the red filter. With LEDs, a little bit of red light will go a long way!
I am not saying that everything you are thinking is wrong and you should just throw those ideas away. They are good ideas! Well keep them, but we can make it work for you if we make some minor adjustments.
You are on the ball with three 3W LEDs in series. It is true that LEDs get more efficient as they are powered at lower levels of current. The higher the current, the higher the forward voltage of the LED. If you can power the LEDs at a lower current, the overall power (in watts) is lower. Lets compare brightness vs. total power. With three red 3W LEDs running at 450mA each, you will get 170 lumens. Total power is aprroximately 2.97W. With one red 3W LED running at 1400mA, you will get the same brightness. Total power is 4.13W. See the difference? Three LEDs running at 450mA each will be rather cool, allowing the LED to run brighter. Dissipating 3W by means of a small solid piece of aluminum would be easy.
Also, the higher forward voltage (Vf) and lower currents will make your driver circuit run cooler and more efficient. 450mA at 6.9V isn't too bad on a driver circuit, but is hard to find a circuit to deliver 1400mA, especially at such a large drop of voltage from ~13V to about 3V.
Here is what you can do. You said that you wanted to run the run/marker lights at 1/5 the brightness than the turn/brake lights. This will be easy. Suppose we make these tail light brighter than most new stock tail lights by making each tail light cluster of LEDs produce about 175 lumens on the brake/turn mode and 45 lumens on the run/marker mode. This is actually 1/4 difference, but that should work just as well. If you want to stick with the 1:5 ration, we can play with the numbers. 45 lumens will be plenty of brightness considering the old design of the tail light. 175 lumens may actually cause some problems, but if there is losses of light due to the old reflector, then we will be fine. As tail light light brightness increases, the brightness will go from dull, to normal, to impressive, to glaring. We hope to aim for the "impressive" mark. With a car that sits so low, I guess it can use that high of brightness. There are so many factors to consider, but do not let it frustrate you.
Now you can get 45/175 lumens even more efficiently if you use the Red/Orange 3W. It ads a more candy or poppy red color to the light that seems brighter and actually looks cool compared to the average deep reds. Red/Orange 3W LEDs can produce 190 lumens at full power versus the 170 of the red. The light doesn't appear to be that orange (don't let the name scare you), and it may actually be a better match to the original light color of the tail light when you use the light bulb.
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Anyway, with the red/orange, you can get about 175 lumens from the three series attached 3W LEDs powered at 400mA @ 6.75V (2.7W). 45 lumens can be produced if you power each LED at only 125mA @ 6.40V (0.8W).
You can mount all three LED emitters on a piece of round aluminum. This aluminum can be shaped or fitted in a way to fit in the twist-on light base that plugs into the harness.
As for a driver, you have a better choice since the levels of current is lowered. The BuckPuck can still work (though it may be expensive), and even TaskLED's CC1W driver. Email George from TaskLED and he will gladly work things out with you when using one of his circuits like the CC1W or CC5W. He can be very flexible. Some of products have set levels of current that is set by George by either a few resistors, or by programming the chip. He may be able to ccome up with a nice and easy to use custom solution for you. TaskLED's MaxFlex board will not work because it is a boost circuit. It is made to increase the voltage from a few batteries to drive a LED. McGizmo's Downboy may also work, but you will have to check it out.
Again, the cheap LM317 will work. Lets see, for the 3W tail lights at the lower brightness mode, the LM317 will drop about 7.5 volts at 0.125A. Therefore, while the LM317 is regulating the voltage going to the LEDs during the low mode, the LM317 will dissipate about 1W of energy as heat. When powering the LEDs at the higher current for the brighter mode, the LM317 will dissipate 3.25W as heat. Don't forget that the bright tail lights are only on for maybe 10% of the time. Heat may or may not be an issue with this project. You can make the driver circuit external so it can sit in the large, fin-shaped spaced in the Ghia's rear. There you can have a small heatsink to keep the circuit a little cooler. The LM317 is fine running warm.
Finally, if you want to save your money, dont use a fancy driver. A simple 15 ohm 4 watt resistor will power the LEDs at the right current for the high brightness mode. A 68 ohm 1 watt resistor can control the current for the dimmer mode. With a resistor, the switching action between low and high mode is so simple. No relays or transistors are required. Sure these resistors may get warm, but if you set them aside, they will not affect the LEDs at all.
So, what do you think? Do not be at all discouraged by its appearance of complexity. It is really easy to do. With a little time and patience, you will be very happy.
Wow, see how long this post is. You will probably fall asleep reading this! Well, if you made it to this point, then you deserve a treat or something. There is a lot of material to read over and interpret, so I understand if you are frustrated. I hope you understand what I am getting at in this post.
Pretty much all you have to do is lower the current going to your 3W red or red/orange LEDs and your plan is viable. I can help you along the way if you would like.
Please leave any further questions. I will try to answer them the best I can. Most, if not all, of my knowledge of powering LEDs is from other people at this CandlePower Forums. People are extremely helpful here and you can learn so much. Just stick around for a little bit and you will grow to like this place.
EDIT: Ooops, I was just thinking again (that gets me in trouble sometimes). You dont have to go with the Luxeon III Red LED. It is still pretty expensive (around $4.03 each). The new Luxeon K2 cost less and it can handle more heat for longer. It is also more efficient! These only cost about $2.57 each. The K2 was made to be value priced, as well as much more durable. In the specs sheet the K2 isn't as impressive (as in brightness) as the Luxeon III red, but that is okay. We are only using the bottom end of power with the LuxIII anyway. Why not take advantage of a newer and cheaper technology?
-Tony
Dual Intensity driver for turn signals/tail lights?
Linear Mode
