Big LED circuit question

tetsujin · Post by **tetsujin** » Thu Apr 05, 2007 2:51 pm

Hey,

Normally I feel pretty confident in my ability to make good decisions about LED circuits - I'm generally the play-it-safe type, driving LEDs at lower-than-maximum currents using reasonably stable voltage sources and series resistors - but I'm working on something where that kind of strategy would be slightly difficult.

Basically it's a circuit with around 200 LEDs. I need to keep the wiring relatively simple, so putting a series resistor on each LED really isn't an option. With the way the LEDs are to be arranged, it's not really a layout that I could wire as a matrix, either. (well, I could, but it'd be really tough.) Really, the whole project would be much easier if I could wire most of the LEDs in parallel. So I'm trying to work out the best way to wire the thing up and power it. I thought about using a current-regulated source, but I'm concerned that if I do that, and some of the LEDs get damaged or lose connection, the rest will be overdriven and burn out. (This assumes the LEDs are mostly wired in parallel... I suppose if I operate the LEDs well within spec that danger is greatly reduced, though...)

The LEDs are about 100 each or white and green - the whites normally drop around 3V, the greens around 2V - so I'm thinking of putting all the whites in parallel, all the greens in parallel, and then putting the two circuits in series and using a 5V source or a 6V source with a current-limiting resistor. ('course, at 2.5mA per LED, a current limiting resistor dropping 1V and passing nearly half an amp of current would be only about 2 Ohms...)

Or alternately, there could be three circuits in series - with half the whites being in one circuit and the other half being another - that way each white LED could pull twice as much current as each green LED does, which might work better. And then I could run it from a 9V battery with a 1V current limiting resistor... I'm not really sure what would work best, though.

Anyway, if anybody has any insight, feel free to fire away. Bear in mind:

Fiber optics are not an option here
I am rather skeptical than an LED calculator will be able to help
The power supply must be portable

Thanks in advance for any help you may be able to offer...

Sparky · Post by **Sparky** » Thu Apr 05, 2007 4:34 pm

the dropping resistor will need to a a 1/2 watt resistor dispating 0.5 watts of heat into the model.

I think you're right about using a voltage regulator. If it runs of a regulated 12 volt power supply (like for a Ham Radio or car stero) or batteries than you should be ok.

Even a voltage regulator is going to get warm. Is it possible for you to convert some of the model structures intodworking vents?

tetsujin · Post by **tetsujin** » Thu Apr 05, 2007 5:04 pm

Sparky wrote:the dropping resistor will need to a a 1/2 watt resistor dispating 0.5 watts of heat into the model.

Yeah, I'd realized that earlier but forgotten... I'm a bit concerned about battery life given that the circuit's gonna run around 3 Watts...

I think you're right about using a voltage regulator. If it runs of a regulated 12 volt power supply (like for a Ham Radio or car stero) or batteries than you should be ok.

It's gonna run off batteries - reasonably small ones, hopefully - I just have to figure out how, exactly. :)

Even a voltage regulator is going to get warm. Is it possible for you to convert some of the model structures into working vents?

Well, ventilation shouldn't be too much of a problem...

Madman Lighting · Post by **Madman Lighting** » Sat Apr 07, 2007 2:13 pm

200 LEDs?

Wow. Try looking up some of the commercially available LED driver chips from National Semiconductor or Linear Technologies. They usually have application notes that tell you how to use them. With a couple of those, you could make a constant current regulator power supply designed specifically for driving LEDs.

Makes me wonder if theres a market for such a product...

Balok · Post by **Balok** » Sun Jun 10, 2007 12:37 am

I've got 40 LEDs in my MF engine (see link) and that heats up so you can run it more than 5 minutes at a time in the model. 200 is a lot, but if you don't run it for too long or the LEDs are far enough apart or the model is fan cooled
maybe that would work.

Madman Lighting · Post by **Madman Lighting** » Sun Jun 10, 2007 1:35 pm

Impressive!

Looks like you've got a classic inverter driven flasher circuit going there and lots and lots of LEDs.

Question: Where is the heat actually comming from, the LEDs or the board that drives them? I know 40 LEDs is a lot but even high power LEDs only give off a few miliwatts of heat....

Balok · Post by **Balok** » Sun Jun 10, 2007 2:02 pm

I feel the heat on the backs of the LEDs and on the wiring in back. The LEDs I used are from the Electronics Goldmine part #G13706. They describe them as "Blinding output". To some extent the acrylic may act as a heatsink. The board and the resistors on the board stay pretty cool. With some tubing around the LEDs and wiring and a little fan blowing in one end or through the middle towards both ends, it would keep it cool. You're right its a 557 with both sides running in astable mode. The accent LEDs, which are interspersed, go in and out of phase with each other and the light builds up sometimes and goes lower at others, all pretty quickly.

I did use the javascript LED series resistor calculator, but they don't give much in the way of specs at the EG so I might have got something wrong, but I think they just heat up more than one would expect.

Madman Lighting · Post by **Madman Lighting** » Sun Jun 10, 2007 2:13 pm

Neat. I looked up your part, a surface mount blue LED eh?

I've not fooled with those much yet and since its surplus there's no detailed spec sheet to check on heat output. Sounds like you might be over driving them though. Try cutting your current drive by 10% and see how hot you are then. I bet your LEDs last a lot longer too.

Scott Hasty · Post by **Scott Hasty** » Sun Jun 10, 2007 2:43 pm

Balok wrote:To some extent the acrylic may act as a heatsink.

Actually a good heatsink will be a moderate to good conductor of heat, like aluminum. Acrylic acts as the opposite and probably one of the reasons you are getting so hot, along with the aforementioned over-driving the current to LEDs.

Scottie

Balok · Post by **Balok** » Sun Jun 10, 2007 3:23 pm

Madman Lighting wrote:Neat. I looked up your part, a surface mount blue LED eh?

I've not fooled with those much yet and since its surplus there's no detailed spec sheet to check on heat output. Sounds like you might be over driving them though. Try cutting your current drive by 10% and see how hot you are then. I bet your LEDs last a lot longer too.

Yeah awhile ago when I first starting using these they advertised them as having a 130 Deg viewing angle, which is the main reason I started using them for this ap. I'll do a test to see at what point it starts heating.

So I guess the acrylic is a heat insulator then? Guess it helps to some small degree. Glass could be better. That's currently beyond my ability to shape with the tools I have.

Scott Hasty · Post by **Scott Hasty** » Sun Jun 10, 2007 7:09 pm

Balok wrote:
Yeah awhile ago when I first starting using these they advertised them as having a 130 Deg viewing angle

130, WOW, that would've piqued my interest as well!! Pretty impressive!

Scottie

Balok · Post by **Balok** » Sun Jun 10, 2007 7:46 pm

Lets see at 7 volts, very slight warmth. At 8 some acceptable warmth (won't be damaging the plastic). At 9 the heat builds up after a few minutes. You can easily touch it, it won't burn or anything but it is quite warm after awhile. It was designed for NiMH batteries at 9.6V. Guess 8 volts is what I'll recommend to whoever gets it.

Chacal · Post by **Chacal** » Mon Jun 11, 2007 9:33 pm

Scott Hasty wrote:
Balok wrote:
Yeah awhile ago when I first starting using these they advertised them as having a 130 Deg viewing angle

130, WOW, that would've piqued my interest as well!! Pretty impressive!

Scottie

That's pretty much the normal angle for a LED without the "lens" enclosure. The different angles you get on LEDs are gotten simply by varying the focal distance (moving the crystal and its contacts further down the resin). When you don't do anything, just leaving the resin block flat-topped, you get about the 130Âº. Get a common 5mm LED and cut its enclosure close to the crystal, then polish the end and you'll get a nice (close to) 130Âº viewing angle LED.

Scott Hasty · Post by **Scott Hasty** » Mon Jun 11, 2007 9:56 pm

Chacal wrote:
Scott Hasty wrote:
Balok wrote:
Yeah awhile ago when I first starting using these they advertised them as having a 130 Deg viewing angle

130, WOW, that would've piqued my interest as well!! Pretty impressive!

Scottie
That's pretty much the normal angle for a LED without the "lens" enclosure. The different angles you get on LEDs are gotten simply by varying the focal distance (moving the crystal and its contacts further down the resin). When you don't do anything, just leaving the resin block flat-topped, you get about the 130Âº. Get a common 5mm LED and cut its enclosure close to the crystal, then polish the end and you'll get a nice (close to) 130Âº viewing angle LED.

DAMN YOU!! Now I'll be in the workshop with a Dremel covered with LED resin!!!!!

Scottie

Sparky · Post by **Sparky** » Tue Jun 12, 2007 11:20 am

Belt sander is the quickest way, then polish with 1500 grit automotive sand paper. . .

en'til Zog · Post by **en'til Zog** » Thu Jun 14, 2007 12:48 am

...and finish the flat end with a drop of FUTURE or KLEER acrylic floor 'wax'.

Shiney!

jwrjr · Post by **jwrjr** » Thu Jun 14, 2007 10:32 pm

If you use the Future, Kleer, or most any acrylic clear gloss you don't have to be Quite so careful about the sanding. The clear gloss fills the grooves left by the sanding.