Brighter "standard" white LED's

Brighter \"standard\" white LED\'s

Hello,
I'm new to the forum, so I hope my questions are not too ignorant.
I've just become interested in LED flashlights and tried my hand at a "homebrew" light, with some success. I purchased 25 white LEDs from an Ebay seller in Hong Kong. They were rated at 5000-6000 mcd. I purchased a cheap Dorcy 4AA cell flashlight for a few bucks. I bypassed one of the batteries, removed the bulb and installed 4 of the white LEDs.
I had sorted through the bag of LEDs to find 4 whose Vf was within a few millivolts of each other. I put the four LEDs in parallel and used a single series 10 ohm resistor as a current limit. I calculated that the LED current with brand new batteries would be about 30mA (rated maximum), and would decrease from there.
The light works, but just not as well as I'd hoped. I have an ARC AAA that seems almost as bright as the 4 LED light that I built. The light I built does have a nice broad pattern with no "artifacts" from the die, along with a slight bluish tinge which is not at all objectionable (at least not to me).
Is my problem that the LEDs just aren't that good? Is it that I'm not overdriving the LED enough to get it very bright? Where does ARC and Photon and CMG buy their LEDs? The photon III light that I have is incredibly bright (at least at first).

On a related note, where can you get good, very bright white LEDs (standard 5mm) without paying several dollars a piece?

Thanks for any help. Best regards,
Mike M.

Re: Brighter

What you need is Nichia LEDs. Cheaper ones are fine for playing around but when it comes right down to it you'll have to pony up the $$$ and get the Nichias.

Re: Brighter

Nichia is 1.25 each, and you get what you are payin' for

Re: Brighter

Got it. That is what I figured.
So where does one find these Nichia LEDs for sale?
Thanks!
Mike M. /ubbthreads/images/graemlins/help.gif

Re: Brighter

If you go to www.whitelightled.com, you ought to be able to find them. Order the 5mm 20° whites and you'll get the right ones.
I can't think of a less expensive white LED that'll do the job.

Re: Brighter

And welcome, mikemo!

Larry

Re: Brighter

The Arc AAA overdrives its LED considerably. Try cranking up the drive current on yours, like to 50 mA or so. Also, measure the current. Your calculation may be off.

overdriving LEDs

Thank you all. It is becoming clearer. It seems that the problem is from many causes.

Here is what I see. Since I'm using a resistor to regulate the current, the brightness will drop as the batteries age. Brand new batteries give me 30 mA per LED, quickly dropping to 20 or less. I understand that a regulator of some sort would "level" out the light over the life of the batteries. I would like to persue the single resistor design at this point.

Just for fun, I put a set of fairly weak carbon zinc batteries in (the ones that came with the flashlight), then I bypassed the resistor. My current jumped from 60 mA to over 130 mA (32 mA per LED) and the brightness came up.

Then I put a fresh set of alkalines in and measured about 125 mA (32 mA per LED). Just for fun, I bypassed the dropping resistor and BAM!, lots of light. The current jumped to over 400 mA (100 per LED) and the output of the light went sky high. I only left it that way for a few seconds in an attempt not to fry the LEDs. So as I understand it that would be called "overdriving" the LEDs. Also understanding that the heat buildup will fry the devices eventually.

For comparison I measured the current draw from a Photon microlight II with a fresh set of CR2016 batteries. The LED draws well over 100 mA with fresh batteries. I don't know how long that would last, but that at least gives me an idea as to why the microlights are so bright when you change the batteries.

So, here is a design question. If I leave the light as it is designed, 3 AA cells, 4 matched "5000 mcd" white LEDs in parallel, with a single 10 ohm resistor, I get 30 mA per LED with fresh batteries, and a nice low, long lasting light. If I put an additional switch on the light that bypasses the current limit resistor, kind of a "turbo" button, how much risk am I putting the LEDs at? How long can they run at the elevated currents before they go? Will they just "burn out", or will they get weak and change color? What is a normal failure mode?

I'm curious how photon gets away with pushing 100 mA through an LED. I guess it is usually not used for an extended period.

Any suggestions welcome. By the way, I'm really not afraid to blow the LEDs. I've got extras and they were only .30 each. I just want to understand what I'm doing.
Thanks again!
Mikemo

Re: overdriving LEDs

I have a cheap-rip off of a Photon that I got at an AutoShow for $3 - it has a White LED and it BLINKS!

I suspect that the current is causing the bond wire to lift off the LED and then when it cools down again to re-connect

A lot of companies overdrive their products, I measured 320ma on the Innova 5xLED product, which means the LEDs are being driven at over 60ma each, far more than they were designed for

The company says that the massive heatsink of the heat allows this to work, but plastic is a VERY poor conductor of heat, the really only heat being drawn out of the die, is those two little wires coming out on the bottom, and they surely can not get rid of more than just 20-30ma

A lot of people put thermocouples or measure packages using a thermal camera (emissivity affects this even more), they are just looking at the PACKAGE, not the actual die inside which could be 10-30c or more hotter

In short, if you want your LED to last, use the reccomended current from the company; as I have stated before "a current meter is your friend" - use it

If you are looking for a very easy way to keep the LED at the same brightness thru the operation of the battery, use a constant current circuit - a LM317 and a resistor

Resistor = 1.25/Current

Re: overdriving LEDs

MIke is fully right about many manufacturers overdriving Nichias 5 mm LED. In a Princeton Tec impact, I measured 80 mA driving a single white LED, and the company states a life of 10.000 hrs for such overdriven LED. I do not believe to this: may be 10K of intermittent usage, not continous.

For your case, calculate the resistor for 200 mA and use all four batteries (6 V), this will give more uniform light output in time. Always take a measurement of the current flowing in the circuit, since sometimes, due to the variances in the LEDs, it is way off the calculated current.
WIth this "mild" overdrive, 50 mA instead of 30 mA, expect a failure between 1K and 10K hrs. operation. Usually, the bonding wire between the connecting lead and the die will break at die junction.

Anthony

Anthony

Re: overdriving LEDs

I agree with TheProphet. 4 cells, and 200 mA sounds good.

If you wanted to pursue the idea of a turbo switch, You could wire it such that it doesn't bypass the resistor, but instead, puts another smaller resistor in parallel with it.
Or, two resistors in series, and the switch bypasses one of them.
This way, you would increase the current, but still have current limiting designed into it.

Re: overdriving LEDs

Thanks for the advice. I'm going to rework the light tonight and add the suggested changes. I've got an LM317 in the junk box so I might just try that out if I can fit it and a heatsink inside the light.

I'm curious about the commercial lights that overdrive the LEDs. They say right on the package that the "bulb" will last 100,000 hours. We are saying that when overdriven they will last significantly less than that. Using the ARC AA as an example, 1000 hours is 200 battery changes. It will probably never see that much use. Also, I wonder what the longevity of the switching supply in the "regulated" lights are.

I guess I could take an LED and put 50 mA through it and see how long it lasts. I'm afraid that when it finally goes out that I won't remember why it was there /ubbthreads/images/graemlins/smile.gif

Thanks!
Mikemo

Re: overdriving LEDs

I had a $1 keychain light from ebay and it uses 2x3v coin cells. I thought "well, I guess I can put 6 volts through that LED even without a resistor." Of course the internal resistance of the button batteries kept the current low enough. I had lots of lights so I connected one of the LED's to 4-AA alkalines.

Wow, that's bright /ubbthreads/images/graemlins/ooo.gif
Ouch, that's hot /ubbthreads/images/graemlins/huh.gif
Hey, it went out /ubbthreads/images/graemlins/frown.gif

Re: overdriving LEDs

Sometimes you'll hear a popping noise when all the magic smoke gets out. And the room might smell funny afterwards.
/ubbthreads/images/graemlins/icon15.gif /ubbthreads/images/graemlins/eek.gif /ubbthreads/images/graemlins/icon15.gif

Re: overdriving LEDs

Georget98;

This is why they warn you to use only alkalines with the Versalux PR-2 LED bulb and MagmaLED. The Versalux in my 2D Coleman Peak 1 Expedition headlamp gets a bit warm, but, no more than that. If it had a metal bulb mount, it would stay cooler. The Versalux in my 2D Mag stays cool, due to the metal bulb mount (better heatsinking).