I need some constant current driver help

frenzee

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Sep 4, 2006
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I could sure use some help selecting the right driver for this circuit.

I am working on an automotive light that uses 16 Luxeon IIIs arranged in 4 strings of 4 each in series @ 350mA drive current. I only have a very limited amount of PCB space to work with (<1" diameter) and have to fit four drivers into that small space each connected to one of the strings. I am considering OnSemi's NUD4001 and LM317 in a TO-263 packaging. Does anyone know enough about LED drivers to point me in the right direction? Can I drive 8 (or all 16) Luxeons from a 12V supply and get away with fewer drivers? Any help would be appreciated.
 

Gryloc

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Is this forward lighting (white), or rear lighting (red)? Either way, sweet! I am working on a project using 18 K2's per headlight (going kind of slow due to classes). Contact Andrewwynn here on CPF (PM him if you desire). He is pretty helpful! If you cannot get a hold of him, try Georges80 from TaskLED, or Dat2zip (Wayne Yamaguchi from the Sanwich Shoppe). They also have the expertise in LED drivers. Also give Newbie a try. He is very helpful and knowledgeable of almost any aspect of using any sort of high-power LED.

You are in good luck. I used a circuit that andrewwynn originally designed for use in high power incandescent flashlights, but was found to work in high current LED projects like mine. The circuit he designed (that I recommend) is called the Hotdriver. It is pretty much a beefy voltage regulator (but do not let the fact that it is a voltage regulator scare you). It also has a current limiter so you can set the maximum current that you want to send to your LEDs to protect them from thermal runaway. The circuit is only about 1 inch in diameter. In addition, you can dim all the LEDs from an external potentiometer if you wish. I have mine set up so a small High beam/Low beam relay activates either of two trimmer pots that are preset (by me). With this set up, I can set each beam mode to have a certain brightness. You can have one fixed resistance, instead, if you would like.

This one circuit can handle two headlight clusters (18V @ ~12.5V) with ease, so a smaller scale project like yours will work with no problem at all. Of course, to use this circuit, you will have to set up your LEDs into a parallel/series configuration to work (four strings of 4 LEDs in series).

I bet price is a consideration, but have you considered fewer LED's at higher currents? With 12 Luxeon III LEDs, you may get a max of ~550lm @350mA. 12 * $3.45 (per emitter) = $41.40. You might as well go with the 'ol Luxeon I if you only plan to drive them at that current.

Maybe you should try the K2. These are great because of the lower thermal resistance and the higher temperature handling. These are tough and for the same price as the Luxeon III. Color consistency is awesome, too. I got TXOH when I ordered 36 of them around September last year, and every time I ordered more later on, I got the same bin. If you have to, you can up the current safely and get a bit more output. Sure these appear to be "inefficient" at max power, compared to others, but you can save money by going with only 6 of them powered at less than 1A each.

Try the Seoul Semiconductor P4. You can get the same brightness with only 4 of these beasts when powered at around 700mA. These are more efficient (and consume much less power) and will cost you about the same (or perhaps less) as the 12 Luxeon III emitters. 4 LEDs will be so much easier to wire up, also. Trust me on the wiring. 18 can be a pain to solder, as 12 may be. You can also use less optics and a much smaller heatsink. With more room, you can have the luxury of using bigger optics for an even brighter beam. If you desire, push 1A each and you will get 640lm out of just four of these things.

With the P4, you can use an even smaller circuit (I can think of anything at this exact moment) so you have room for you protection circuitry. The car's electrical power supply is so dirty and screwwy, but I can explain more later. I wish I knew your target brightness. If you are trying to get the most brighness with 12 emitters regardless, try 12 P4s. At 350mA, you will achieve ~1080 lumens. You can an absurd amount of brightness at higher currents (more than 2000lm)!

I am not trying to tell you what to do, I just hope that I can help you by making things easier and simpler, all while saving you some money. Might as well as make use of the latest technology. I am not caught up in the moment with the crees and P4s like some on here because others are. I am just sold on the P4 after some testing of my own. This thing is spectacular, and I cannot get over it. There is a place in my heart for the Luxeon I and III and K2, but I feel that it is just smart to utilize new technologies in such an application as yours. My next headlight for another friend will most likely use 7-10 P4's instead of the 18 power-hungry Lux III or K2.

Try this link. Reply to it if you would like to. It is the thread where I conceptualized, designed, and created my project. There are several nice pictures there to look at. I am still trying to finish it (with my very limited time and tools at college), but it is getting there. It is not an impossible idea!

https://www.candlepowerforums.com/threads/117661&page=1&pp=20

Please tell me what you think and keep us updated. I hope I was helpful. It may be helpful to give us as much info about your project as possible (the specific use or your project and your budget, etc). Whatever configuration of LEDs you choose, contact Andrewwynn about his circuit, the Hotdriver. It is well priced and it is a nice driver. I wish you the greatest luck with your project. Whew! I typed enough for tonight... :grin2:


-Tony
 

frenzee

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Sep 4, 2006
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216
Led_Blind, betalight, thank you for your suggesitons, but I can't use a pre-fabricated controller. There just isn't enough room. I need make this fit onto a single or double-sided PCB < 1" in diameter, so I have to build this thing from scratch.

Gryloc, thank you soooo much for you input. I've been following your thread. This a trailight and blinker application, therefore I have to use amber and red Luxeons. Cree and P4 are not an option. The phsical dimentions are tight to say the least: The housing inlet which this will go into is less than 1" in diameter (22mm) with four notches which will allow the four strings of "shaved" Luxeon side-emitters to barely sqweeze through. The driver PCB sits on the bottom of the copper "tower" onto which the LEDs are mounted. I can't drive this any harder than 350mA otherwise everything will get heat soaked. Furthermore, the LEDs run much more efficiently at lower current levels. I opted for Lux III's instead of Lux I's because Lux III's are a LOT more tolerant of over-current and heat abuse and at least in the case of ambers when driven at the same current, Lux III seems significantly brighter although I don't have any photometric data to prove it. Right now I have one blinker made up of four Lamina 2000's direct driver at 1.0 Amps and it's brighter that the stock incandesents, and it's happy as far as heat dissipation. But I want to push the limit if you will, with Luxeons. My design goal is to get the brightest tail light and blinker light possible. Cost is not that big an issue right now, because this is a prototype, but I'm sure it will come into play if I had to make another set for a hobbyist or a friend.

Gryloc, which IC did you use in andrewwynn's driver? Did you use a power MOSFET in the power stage? 18V is good output, but it won't work for me becasue you can drive at most 6 series LED's at that voltage and 6 is not evenly divisible into 16, unless I change the number of LEDs to something like 12 or 18 which is not practical due to the dimentions that I have to work with.
 

Gryloc

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Jan 20, 2006
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Cincinnati, Ohio & North Lewisburg, Ohio
frenzee,

Ahhh, rear with amber/red. Alright. I am still wondering how you will pull this off. You want 12 LEDs per tail light/turn, or 12 total? Will both colors be used? Will one set have to flash and the other stays solid?

I really need to know what the cluster size you plan on using is. What is your target brightness? These are important factors!

For one, if you want to have the brightest flashers around, then you must really do not mind paying for tickets. Legality is a problem if you are pushing even 6 LEDs per taillight powered at 350mA each. That is approximately 280 lumens red, and 230 lumens amber! Stock tail light bulbs are i believe ~400 lumens each, BUT the light that exits is less because more than ~2/3s of the light is filtered out by the red plastic filter. That means that you see about 133 lumens normally out of the rear of the car per taillight.

I am not just criticizing you because you are doing something daring, it is because it is actually dangerous to have too bright of tail lights. I had a Luxeon Star I R/O third brake light and I had to tone down the current because it was too bright to the standard tailights I was using.

I just recently chatted with someone else on here who wanted to use Lux IIIs for their taillights, but they had originally had it set so it would produce over 400 lumens in red! I bet you would be happy running three Red or R/O Lux III's in series running at a very maximum of 200 lumens. You get bright light, good efficiency, and low heat levels. Plus, the driver circuit for this would be a breeze.

The Hotdriver circuit I described does not boost any voltage, so you will never see 18V normally. This is just the very maximum that the circuit could ever tolerate. The voltage can be regulated from anywhere to below 9V to less than a volt below your input voltage (so about ~12.5V max if your car's electrical system produces around 13.5V). You can use any configuration you want but the total Vf of the series attached LEDs at the right current cannot be higher than the max. So, with 4 attached in series, that is about 10V. The circuit can reduce the voltage to that level with no problem. Also, I don't know what IC andrewwynn used. I do not know how his circuit was wired (it is his intellectual property), however, the IC he used does have some circuits made for that chip that you are allowed to mess with. The IC works hand in hand with the mosfet to control the voltage and current.

So, with the way I am reading, you want 12 3W reds or ambers at 350mA. Please do no forget that is about 510 lumens per taillight during brake mode. That is just dangerous. Sure it is cool seeing how far you can go with brightness, but this is absurd. Please consider what I said about the filter and recommended levels.

From what you said about the four Lamina 2000's at 1A. That is approximately 160 lumens each at those currents. With 4, that is 630 lumens in red. Come on. That is already too much! The Lux III's at those currents wont be brighter than the Laminas anyways.


-Tony
 

frenzee

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Sep 4, 2006
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Thanks tony. Actually it's 16 total single-color LED's per lamp. I guess I don't understand what would be dangerous about this circuit and AFAIK there is no legal limitation on tail light brightness, at least here in CA (have you seen how bright the new Range Rover tail lights are?).

This is really a very conservative design. I delibertely kept the power levels low (~10V x .350mA x 4 = 14W) in order to avoid overheating problems. I did build this other unit with 10 Lux III's (5x2) driven at 2 Amps total. Problem is it requires forced air cooling, and even then it's pushing the limits, i.e. 10 minutes constant on time before it gets to 100C which I don't want to exceed. And plus it is somewhat bulky and I also have concerns over the micro-fan's reliability - it it breaks down it will take the LEDs with it. It is pretty bright, brighter thanstock which is a 25W 7443 bulb, however my tail light is not exactly a parabolic dish, so most of the light goes up, down and sideways anyway. It won't blind the rear car, if that is what your concern is, unless you were to stare at it point blank. However, it should get their attention when you brake, which is my goal to being with.
 

Gryloc

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Jan 20, 2006
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Cincinnati, Ohio & North Lewisburg, Ohio
Yeah, I forgot it was 16 instead of 12. I just want you to watch out. I mean 750 lumens in red for each taillight is over 5.5 times greater what it should. I would hate for you to give LEDs a bad name or bring up concerns about maximum brightness of taillights. Dont be one of the few that may ruin it for the rest of us. There are still funny people out there that may flip out over little things ike this. If you should install these, try testing them first in your driveway. Try parking your vehicle and get a second vehicle (if available) and park several yards behind it. Then try the two modes of brightness (another friend would be helpful with this). It wouldn't hurt to try it in the day and at night after your eyes adjust to your headlights so you have a clue what you are dealing with. If it is glaring, or if it is very distracting, I would suggest lowering the current a bit until it is better.

Don't forget that even at 350mA, each LED will deliver 47 lumens each (check out the specs sheet). I tried one of the Luxeon III's powered at 1.4A (190 lumens for the R/O) in a tailight that had a poor reflector inside, and that still seemed more than sufficient. You can drive these at 150mA each and still get 300 lumens at 5W.

Just consider this. You do not think that it is dangerous, but it really can be. You will be throwing out 1500 lumens total with the two taillights. I believe that that is excessive. I applaud your attempt to create a bright light with LEDs (which I love doing, too), but do not be careless. Good luck...


-Tony
 

frenzee

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Sep 4, 2006
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Thanks Gryloc. If I may point out, in my experience at least, Lumiled's spec sheets are over rated. They test the LED's in 25C under ideal condistions. Also I think you may be looking at RedOrage lambertians. Side-emitting reds are only rated at 31 lm/W and ambers at 27.3 lm/W and that's probably from a high-bin emitter which I am not likely to find in small quantities. I am driving these at <1W per emitter at significantly higher than 25C. I'd be lucky if I get 20 lumens from each. Have a look at this smal vid clip I took of the Lamina unit mentioned above. This unit and the 10 LED 2A Luxeon unit are roughly comparable in brightness, although I don't have a video of the second one. The glare you see is a camera artifact and due to the UV emissions.

Well since I didn't get an answer to my original quesiton, I will go ahead with testing the NUD4001. I will post the results when I get done.
 
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SemiMan

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There is absolutely a specification in California on how bright an on-road tail light can be as well as specifications on the beam pattern. Well with 12 lux-1 you are going to hit the mininum, but likely way to bright and a danger to your fellow motorists who will be temporarily blinded.

You could use single boost circuit that could put out say 700mA at 24V.

Watch for surges, automotive can be a nasty environment.

Semiman
 

frenzee

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OK, I did some research and you are correct. Although Calforinia Vehicle Code doesn't mention anything about brightness minimum or maximum, it does call for compliance with the federal law which is 49 C.F.R. 571.108 which in turn refenreces SAE standard J222 and specifically figs. 1A and 1B. It looks lke the min/max for a stop light are 80/300 cd. measured at the horizontal axis. I wish I had a light meter to get exact measurements, but considering that a typical 100W bulb puts out 120 cd, I think and hope that I'll be in a safe range as my light is nowhere near as bright as a 100W bulb.

SemiMan, do you have any specific recommendations? If I split the LEDs into two groups and use 24V I woun't be able to balance the current across the two paths, or at least I don't know how to. If I use a single circuit, I will need 48V and all the boost circuits I've seen that are capable of 48V output, are designed for back lighting with 5mm's and only supply around 20-40mA at most.

Yes, I know how dirty automoble voltage can be - spikes, drops, over-voltage, you name it. If the chip doesn't have built-in over- and reverse voltage protection, hopefully a clamp diode will take care of it.
 
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