series/parallel LED circuit voltage regulated.

[huntxtrm]

I am trying to design and build a led circuit to run leds on my boat. I am planning on running a total of 99 bright white leds series/parallel. 3 in each series. in 33 parallel circuits. My question is, My battery is deep cycle 12 volt dc. I want to regulate the voltage to approx 10.5 volts or so, to make the leds last longer. I have read that I can use a LM317 to do this. I have seen various scematics for this. can anyone help me design a circuit that can handle this many leds? Will a lm 317 with R1 @ 240 ohms and R2 at 1780 ohms handle this? Any and all help would be helpful. Plus fun for me to build. Thanks. Leds I plan to use are 30 ma, 3.4v typical, 4.0 v max 120 mw power discipation. 18,000 mcd. by the way is that bright for white leds? I hope? Thanks.

 

[Mitchekj]

I need to write something for the basic use of LEDs. I see these questions all the time, here, there, everywhere. (I work in the LED lighting field, btw.)

An LED is to be driven with a current source, not a voltage source. Yes, they will work with a voltage source, and may even work well. You will run into a number of issues though. I will spare everyone the lecture (see other posts about LEDs with responses by Steve, Resq and myself.)

To be brief: you'd probably want to use those LM317s in a current regulating mode of operation, one on each parallel leg of LEDs. 30mA LEDs are generally not considered high brightness. 350mA, 700mA, some run at 1A. Those are your high power, high brightness LEDs... and of course, thermal managment becomes the key issue with those. The highest efficiency LEDs around only turn ~25-30% of the power into light, the remainder becomes heat. A LOT of heat. Heat kills LEDs.

If you really want to run these with a voltage source regulated w/ an LM317 (or something like it) you're going to need to source one w/ a current rating of ~2A+. And heat sink the bejesus out of it.

 

[55pilot]

I have been a member for only a few days and my first impression is that the forum should be renamed LED Point Oh so many repeat questions about LEDs!

Far too many people think they can use LEDs just like incandescent bulbs. They have no concept of voltage driver versus current drive or even an awareness that there is a difference. Then there is heat sinking. Heat what?

The sad part is that this is not just hobbyists. Even professionals are really messing up. Last summer I was talking to an application engineer for a foreign LED manufacturer. She mentioned that they were considering restricting US sales to only "qualified customers" where the a potential customer will have to be interviewed and deemed qualified before being allowed to use this company's product. She said that the reason was that many OEMs were grossly misusing their medium power LEDs, seeing high failures in the field due to overheating and blaming them for unreliable products. Loosing sales, to them, was a better option than getting a reputation for poor quality.

---55p

 

[huntxtrm]

mitchekj,

Can you give me some help on designing this circuit then. You apparently know what you are doing. What I am trying to do is build led strips around the outside perimeter of my fishing boat. Leds's 2" appart. Aprrox 99 on each side of the boat. I want to use them to cast a halo of light into the water for fishing, landing fish and attracting bait. They do not have to be super bright like a 1w flashlight led, just give off a descent amount of light to the water about 1 foot below my splashrail. I want to use white leds, or best a combination of white and green, switched independantly from my console in the boat. the voltage I have to work with is a 12volt deep cycle battery. the voltage fully charged is around 12.5v and can drop to 11 or so on the downhill side. that is why i was contemplating voltage regulation. that way when the battery is on the down hill the leds wired for 10. 5 would still be just as bright. I just want steady brightness. Can you recommend a basic circuit design for this? Parts list etccc. would be a great help thanks. they don't market anything like this I have found and this is how I want to set up my boat. Thanks. for any help.

 

[huntxtrm]

Oh and it is an aluminum boat, in the water. so heat sink should not be a problem.

 

[huntxtrm]

found a useful tool on the internet. Check it out. http://led.linear1.org/led.wiz

 

[huntxtrm]

I put in the info I had, and it spit me out a schematic. But do I still need to regulate the voltage, since battery voltage is gonna vary, or that of no concern?

 

[Mitchekj]

What are these LEDs, anyhow? Radial through-hole 3-5mm? SMT?

That tool gives you exactly what you already had... 33 parallel strings of 3 in series. Do you have a datasheet for the LEDs, by the way? Maybe a part number? That may help. With a max Vf of 4.0Vdc, (at 30mA?) it may be tough to regulate at 30mA, voltage or current wise. You'd need some level of overhead voltage, regardless of the method. You'd end up driving the LEDs 20-25mA or so, all depends on the LED. I'm not certain how bright that's going to be, other than "not extremely."

Some testing may be in order.

Reading your application, my main concern would be corrosion. These will need to be sealed or otherwise isolated from the cruel, cruel sea. Salt spray will probably reduce this project to a green paste in a matter of months otherwise.

 

[huntxtrm]

5mm LED RL5-W18030
SPECIFICATIONS
Part Number: RL5-W18030 - Super-White LED (GaN/InGaN), Chip Manufacturer: TEKCORE
absolute maximum ratings: (TA=25°C)
PARAMETER SYMBOL RATING UNIT

Power Dissipation PD 120 mW
Continuous Forward Current IF 30 mA
Peak Forward Current (1/10th duty cycle, 0.1ms pulse width) IFM 70 mA
Reverse Voltage VR 5 V
Operating Temperature TA -25~+80 °C
Storage Temperature TSTG -30~+100 °C
Reverse Current (VR=5V) IR 100 µa
Lead Soldering Temperature (3mm from body) 260C (for 3 seconds)

optoelectric characteristics:
PARAMETER SYMBOL MAX TYP UNIT TEST
View Angle of Half Power 2ø1/2 30 Degree
Forward Voltage VF 4.0 3.4 V IF=20mA
CIE 1931 Chromaticity Coordinates ÿP x=.277;y=.277 IF=20mA
Luminous Intensity IV 18000 mcd IF=20mA
Absolute Irradiance 9.08667E+3 uW IF=20mA


Dimensions in millimeters (inches)


Spectral Graph


Here is a link to the sight from where I was going to buy them.

http://www.superbrightleds.com/cgi-bin/store/index.cgi?action=DispPage&Page2Disp=/specs/W18030.htm



I use this boat in fresh water, plus the whole circuit is going to be incapsulated. I don't understand the led brightness. 18,000mcd is not very bright?

 

[Mitchekj]

Well, 18,000 mcd is bright... for a low power LED. To compare, a 100W incandescent bulb is around 130,000 mcd. (There are other things to take into account, such as beam angle, but you get the drift.) Edit: Specifically, if you focus the 100W bulb into a narrow beam, like the LED (30 degress or so?) you're looking at millions of mcd.
A quick check shows that the 18,000 mcd LED at 30 degrees is about equal to 4 to 5 lumens, high power LEDs are well past 100 lumens, and usually more like 120-150 degrees beam angle.

In my experience, LEDs running at 20-30mA are for indication (on/off, etc.) and very small illumination (small channel letters and the like). To get into the realm of illumination (say, a downlight or streetlight) you're going to need to use arrays of high power (350-700mA) LEDs.

I don't know exactly what type of light output you're looking for here, how much light is enough? How many incandescent bulbs would you use to get the right light output, for instance?

 

[huntxtrm]

I generaly use 1, 12volt incandascent rv bulb for the same purpose. it is in a clip on aluminum light fixture. I am basicaly just wanting to widen the area by using leds. I have made a fishing light using 70 green leds, it worked well. they were of about the same design. I was told I didn't wire them correctly, because I didn't control the current correctly. they started dieing out. they ran off of a 9 volt. I want a halo of light around my boat, not extremely bright light. just enough to light up the surface. 1 foot below. Low energy usage, low maintenance. Thanks

 

[Mitchekj]

Ah, ok. If those are enough light, then you're good to go.

You didn't use any current limiting resistors in series w/ the LEDs, which is why they started to die. If you use a constant voltage source, you will need these resistors. Otherwise overcurrent will kill them.

The tricky part: the LED is an active load, it has a dynamic 'resistance' which makes it tricky to select a resistance value for said resistors to run them at the correct current. In the case of these 5mm guys, the manufacturer states 30mA MAX, at 25c ambient (room temp.) All of their specs and testing were done at 20mA. This is common. You would want to run these LEDs at 20mA, constant current. See first attachment from an earlier LED discussion, this will get you a 20mA const. current for each LED, which is technically the way to go. You'd need 33, instead of the 5 shown, and they'd all be white LEDs, but you get the idea.

Now the only problem, voltage overhead. You said the battery drops to ~11V at times? This will be an issue, if the Vf of the LEDs are higher than about 3.25V. The LM317 as shown drops 1.25V across the resistors, so w/ 11V in, you're getting 9.75 max out. If the Vf of your LEDs are 3.4V typically, you're going to be running them at less than 20mA when the battery dips. A possible way around this is to use discretes; two transistors and a couple resistors for each leg of 3 LEDs to supply a constant current, this would lower your overhead to the millivolts, but may add cost, and a little complexity. That's probably the way I'd go, though.

If you try to regulate the voltage (instead of the current) with the LM317 as you had first suggested, then you're still facing the same issue... 1.25V dropout. But, let's say this IS the way you want to go, you can try to regulate the 12V to 10.5Vdc, using an LM317 (rated for 1.5A or higher, btw, and heat sink it) as you suggested earlier. Then you will need to add a resistor in series with each LED leg. Using the typical Vf from their datasheet, you'd be looking at ~10.2Vdc for each leg (3.4V each LED.) You need to drop the extra 0.3Vdc across a resistor:

(10.5 - 10.2) / 0.020 = 15 Ohms

See second attachment. Edit: And I don't condone running it this way, Vf is the issue. Each LED will be different, some LEDs will be running less then 20mA, some more, etc. etc. Making any sense? Sorry it's a little long winded and cluttered. hehe.

 

[huntxtrm]

those are very similiar to what i came up with. Check out this schematic. Will it work? or is it back to the drawing board? I am a newbie at this, this is pretty much put together from stuff i have seen on the internet. Please tell me of any ideas you have or changes you would make. C1 is .1 uF, C2 is 1uF, R1 is 240 ohms 1/4 watt, R2 is 1780 ohms 1/4 watt. I think output is gonna be 10.52v. do the resistors on the lm317 make it current limiting? Do the caps stabalize the voltage? Should I still use some sort of resistor in each series string of leds. Or should i shoot for running single led's not in series, all wired paralles with independant resistors. What is best. Don't be scared to hurt my feelings, I'm a big boy. Thanks.

 

[Mitchekj]

Yeah, what you posted is almost exactly what I posted (2nd attachment) except you're missing the resistors in each of the LED strings. Those are indeed necessary, to limit the forward current through the LEDs if you go the regulated constant voltage route. The value would all depend on the Vf bins of the LEDs, but 15 Ohms would work if the LEDs are indeed all right around 3.4V that the datasheet said was typical. (My guess, out of the 200 total LEDs you'd be getting, some will be 3.2 some will be 3.8, a few will be 4.0, and everything in between... as they warm up, the Vf drops, current rises accordingly, etc. etc. Driving the LEDs with a constant current source would solve that whole issue, btw.)

The caps on the LM317 are to improve transient response, filtering, etc. Nice to have, especially if it's some distance away from the battery. Keep in mind, when your battery dips to 11V, the LM317 will NOT be able to regulate at 10.52Vdc anymore, and will most likely drop to ~9.75V. What that means as far as LED brightness: they will be dimmer, I can't say for sure if it would be noticeable or not, but forward current definitely will drop below 20mA. A change of +/- 10% in brightness is not typically noticeable, but at 20mA, 10% equates to a very small current change.

Your questions: The resistors on the LM317 are voltage regulating. It maintains a 1.25V reference across R1, at 240 Ohms this gives you about 5mA through it (and in turn R2) R2 drops about 9.27V (1780 Ohms * 5.2mA,) together with R1's drop you get the 10.52V value.

The first schematic I posted works similarly, but since R1 is in series w/ the load, you get a constant current output (1.25V / 62 Ohms = 20mA)

 

[huntxtrm]

So which of the scematics is gonna be best for my app. or should i change up one of them to make it work better? My battery will not dip all the way to 11 volts probably down to about 11.5, I hope?
And what resistor values should I go with?

 

[huntxtrm]

circuit board and switch will be on the dashboard, 6 feet or so away from the first portside led. So I need the caps?

 

[huntxtrm]

Oh and one more thing Led string is gonna be right at 16 feet long start to finish.

 

[huntxtrm]

I can draw you a simple drawing of my boat and led placements, and where the switch and circuit board is gonna go. if you want?

 

[Mitchekj]

So which of the scematics is gonna be best for my app. or should i change up one of them to make it work better? My battery will not dip all the way to 11 volts probably down to about 11.5, I hope?
And what resistor values should I go with?

Well, personally, I'd use the first schematic that I posted. Sure, you're going to need 66 of the LM317s in total, but that should be about $20 in total, (get the LM317LZ, $21 for 100 of them.) Small compared to the $200 you'll be spending on LEDs, eh? Why run the risk of them dying an early death? The resisors should come out to about $0.05 - $0.10 a piece, so that's no big deal. (I'd go for 62 Ohm, 1% tolerance, 1/4W, metal film, axial.) Digikey.com has all this stuff in great quantities.

You bring up another issue... length. To further exacerbate the voltage overhead issue, the length of wire (16 ft. is a decent length) will drop voltage due to its inherent resistance. The only thing I can recommend is to use the heaviest gauge wire possible... 16-12 AWG? That may be overkill, especially at low current, but... 18-20 gauge? Maybe someone else can chime in, but I definitely wouldn't try to use wire-wrap wire or the like (30 gauge, etc.)

 

[huntxtrm]

so on the out on the lm317 use 62 ohm? what about the adjust loop on it? do I need caps for the long run of the circuit? And by the way thanks, you have been a big help sofar.