using 1WATT emitters?

I was talking to this guy the other day about LED's and I showed him my flashlight (Fenix L2P) and we weer talking about different LED's for EMS work (lights for cars when responding to calls). he works at a large company and is in R&D and told me he would grab me a few emitters.

I didnt think that he was going to but the next day he found me and handed me 40 Lumiled Luxeon Emitter 1W Lambertian's of various colors! needles to say I have a lot to play with now, I just made an order for some misc LEDs aswell.

I've been working with 3500 mcd blue 5mm led's until now. they use like 30mA of current at like 3ish volts. so theres no draw and I can use 1/8 watt ressistors with them. I ran some calculations on these new ones:

Assuming a forward current of 3.7V and a current draw of 350mA we get a wattage of 1.29 WATTS! (WATTS/Voltage = Amps so Volts*Amps = Watts)

Now I ran my LED's specs through an LED calc online (using V=I/R iirc) It tells me i need a 27 Ohm resister capable of 3.3WATTs!?

I could string up 4 25Ohm 5% resistors in parrallel but I dont know if I have that many and with that much load onthe resistors I'll be sucking tons of power and will have to deal with all of that as Heat (I may have to HS the resistor, that seems awfully wasteful)

Now another option (because I only have 12V and 14V PSU's laying around that are capable of an amp or more) would be a small LM317 based circuit to regulate that 12V down to the 3.7V I need. Im not sure if an LM317 can handle an amp (350mA x3 LED's = 1.05A) either way Im going to need to Heatsink that.

I'd like to make a reading lamp with three of the white ones and some night lites with the red ones but Im unsure how to get them the voltage they need as building custom PSU's for them is out of the question for now. Any Idea's for circuit desings that an handle that much power without sucking too much extra current? I suppose I could use a LM317 for every LED and make it three channel, using a pot (or three in this case) would yeild full control without putting much burden on the driver circuits but it seems awfuly wasteful.

while sketching out a few circuits for use with these LED's I had an idea.

could I be going about this the wrong way? I'm no EE student but iirc the diode will only draw the V it needs if Current is limited. so if I made a PSU based around supplying the LED 300mA (as to not stress it) it will then draw the needed V? (Im not sure if thgis will work because the extra Voltage has to go somewhere...)

? I know this plan is flawed but could I be onto something? I'll sleep on it an see what you all come up with

Jerb said:

Now another option (because I only have 12V and 14V PSU's laying around that are capable of an amp or more) would be a small LM317 based circuit to regulate that 12V down to the 3.7V I need. Im not sure if an LM317 can handle an amp (350mA x3 LED's = 1.05A) either way Im going to need to Heatsink that.

I'd like to make a reading lamp with three of the white ones and some night lites with the red ones but Im unsure how to get them the voltage they need as building custom PSU's for them is out of the question for now. Any Idea's for circuit desings that an handle that much power without sucking too much extra current? I suppose I could use a LM317 for every LED and make it three channel, using a pot (or three in this case) would yeild full control without putting much burden on the driver circuits but it seems awfuly wasteful.

Click to expand...

Man, you are lucky! I've run these in series; it works well. Look at the backs to get the bin info. Then, go to the Lumileds website and download the binning data sheet (which is not the regular datasheet). Choose 3 or 4 that get you close to the desired voltage (12 or 14) and wire them in series. Heat sink them. JB Weld works well as it conducts heat but not electricity and the old stars are not electrically isolated from the back of the star. You might need a resistor anyway, but the value should be smaller.

Kevin

Hi Jerb,

I know enough about electronics to know I don't know enough. I do know that these LED's are current devices and you are on thin ice if you are regulating the voltage and not the current. The Vf of the LED can vary from one to the other and it can vary with age as well as temperature. I understand there are a number of constant current devices out now and we have our own members like Dat2Zip and GeorgeS80 who have developed drivers for applications like you are working on. Wayne and I are working on some fixed lighting (12V) fixtures and dimming solutions beyond CPF at present. I am typing this now on a lanai (porch) under 5 overhead Nichia Jupiter LED fixtures. These LED's are being driven at about 1W on full current and can be dimmed down to 0. For some reason, most of the people on CPF can't get past using LED's in flashlights and embrace the exciting world of LED illumination beyond portable. :shrug:

Go to current control though!!! Lumen maintenance is more important in your applications here than it is in a flashlight of intermittent use.

You can bring up your electronic parts and circuit design questions in the electronics forum if you don't get some of the EE guys to answer you here. Goodluck with it and have fun!! You are welcome to post any of your mods or builds here in this forum if you choose! It's time to get some of these knuckleheads to see past their flashlights! :duck:

First off thanks for your kind words, the idea of stringing LED's in a series is brilliant (I cant beleive I didnt think of doing that! heh)

now I've been going through some of my books and I think I may have to employ a Microcontroller here. I'm thinknig that regulating current and voltage are bad idea's and are taking things in the wrong direction. LM317 is wasteful as anything (42% efficiency!?) adn I dont have Pots big enough to dim the LEDs (+ it can be good for them, as you said lumen maintnence is important)

I think I can grab almost any Atmel chip (I do think I have a few laying around somehwhere) and do a simple PWM. that along with 3 strung together I'll need a much smaller ressitor.

Since its going to be a bed lamp and digital I can then do some really cool stuff, digital encoders to dial brightness or maybe a Qprox touch control circuit. I could do a simple two transistor touch circuit, make like five of them and have them activate different brightness levels on the Atmel chip.

thanks again guys. ill post a write-up if anything becoms of this

Hello Jerb!

Considering the efficiency when using one LED and a resistor, you can just as well use 12 V incans.

HOWEVER, you mentioned that you wanted to run three of them, so why not run them in series? If you have a Vf of 3.7 V, that would total 11.1 V and with a 2.7 ohm resistor, you should end up near 350 mA and 92.5 % efficiency.

If you can spend some more money, there are several constant-current regulators on the market.

Good luck and please do ask if anything is unclear!

I think this thread will indeed prosper in the electronics forum, which is also the forum it should rightfully be, considering the title and the lack of connection to McGizmo stuff, not counting Don's post :nana:
Moved.
bernie