Luxeon Led at 3.8 Volts

Zinje

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Hi!

I don't have any experience with leds but was interested in the Luxeon 1 watt. Does it make much difference as far as brightness if you run the led at 3.4v to 3.8v.

If i am using a 9volt battery as a power source...what type of resistor would I need.

I don't know anything about electronics so any suggestions as far was what is optimal.

Thanks!

Zinje
 

BentHeadTX

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From what I have gathered on this forum, the 1 watt LS uses 3.5 to 4.5 watts. Now if you put two of the 1 Watters in parallel.... that would equal 7 to 9 volts with two of them. Considering that a 9 volt battery would not overdrive the LS LEDs with two of them in parallel.... it would seem that using two of the LS 1.25 watt LEDs would work fine with a 9 volt battery.
Then again, what the hell would I know!
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BentHeadTX

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Ooooops... my first sentence should read:
"From what I have gathered on this forum, the 1 watt LS uses 3.5 to 4.5 VOLTS. Now if you put two of them in SERIES....that would equal 7 to 9 volts with two of them.
I am suffering from dain bramage today.............
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Zinje

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Well i'm planning to just use 1 of the 1watt LS with a 9 volt battery for direct current. I'm just going to add on a little switch for experimenting. What type of resistor would I need and can i just make it inline without using a board?

Thanks!

Zinke
 

Zinje

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I was just looking at the luxeon 1 watt datasheet and it says that the max voltage is 3.99 volts. That is why i wanted to push 3.8 volts using a 9 volt battery. I have read in some post that some are running at 4.5 volts. How efficient is this?

If thats the case I will just run direct current with 3 AA batteries. Will this be a problem?

Your throughts?

Zinje
 

StuU

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Take the difference between your supplied voltage and ideal LS voltage (9-3.5 = 5.5 V). Divide this by your target current in amps. The LS typically runs at 350ma or .35amps.

So...5.5 divided by .350 amps equals 15.7 ohms.
Or...5.5 divided by .200 amps equals 27.5 ohms.
Or...5.5 divided by .100 amps equals 55.0 ohms.

Don't feel you need to be exact on the ohms rating. However, with the LS and it's higher wattage rating, you will need to use resistors with higher wattage ratings. You can either buy specialized resistors or just string several Radio Smack standard resistors together to increase the wattage rating, ie three 1/2 watt resistors of 10 ohms run in paralle will give you a 3 ohm effective resistance with a suitable wattage rating.

Also, you will be wasting a *whole lot* of unnecessary current to be running an LS off a 9 volt source and resistoring it down. Better to run a voltage closer to the Fw (forward voltage) of the led...like 4.5V. Or to run 2 LS in series which will use up 7V instead of 3.5V. Also, you can wire the resistor(s) together without setting up a PC board.
 

Zinje

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Awesome!

So I can just mod a 3 AA flashlight to run a 1 watt luxeon with no resistors right?
 

INRETECH

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Normal Alkaline batteries have a built in ESR which will limit inrush current, but using NiCad or other batteries with low ESR could damage the LEDs on turn on unless you use a small Resistor to prevent this

Ohms law can be a wonderfull tool, but its just a start - high current on semiconductors can lead to different results - always have a current meter handy
 

Jonathan

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Originally posted by INRETECH:

Ohms law can be a wonderfull tool, but its just a start - high current on semiconductors can lead to different results - always have a current meter handy
<font size="2" face="Verdana, Arial">Well said! Ohm's law does _not_ apply very well to diodes....well, it really does, but you have to treat R as a variable which changes with V and with temperature.

Zinje's post above points out to one of the confusing aspects of powering LEDs. 'Just what is the proper voltage for them anyway?'

The datasheet will show a range of Vf, which for Luxeons goes from something like 2.8V to 3.99V. This _DOES NOT_ mean that you could power any particular Luxeon at any voltage in that range.

Each Luxeon will require a particular Vf to supply normal operating current. The Vf of each Luxeon will fall _somewhere_ in the datasheet range. Some Luxeons only need 3.2V to reach full power, others need 3.6V. If you connect a 3.2V Luxeon to a 3.8V supply, then you will toast it.

Finally, as Inretech noted, people are not driving the Luxeons with 4.5V. They are using 3 alkaline cells with a nominal voltage of 1.5V each, for a total _nominal_ voltage of 4.5V. But when you put alkaline cells under load, their voltage drops and the actual voltage delivered to the Luxeon is considerably lower, in the 3.5 to 4V range (depending upon the cells and the Luxeon).

With these same 'direct drive' flashlights, using 3 NiMH cells, with a nominal total voltage of 3.75V, you actually get more current flowing through the LED, which quickly toasts.

As StuU noted, the voltage dropped in the resistor is wasted in heat production. However the other side of this coin is that the more voltage dropped in heat production, the less sensitive the system is to changes in voltage.

Good luck!

Jon
 

INRETECH

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Its important to have the series resistor, it provides a soft-turn on for the LED and allows you to control the amount of current

With a low ESR device like a Nicad, the inrush current to a semiconductor could be in the 100s of amps in the first few microseconds without having a limiting device

Having done design of dimmers that run 100A per circuit, I can assure you inrush current is a problem
 

Icebreak

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INRETECH

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Do not think in voltage mode, think current mode - since as others have noted, each color runs at a slightly different voltage

Get yourself a nice 50 ohm power reostat or a bunch of low value resistors like 10-20 ohms

Start with a high resistance, and slowly lower the resistance until it gets close to 350ma then replace/solder it correctly
 

Joshua

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OK, go to radio shack and buy a battery holder. The have ones that hold 4 AAs in series. You will have to make a dummy battery in the holder to close the circut.
Yes 3 AAs will direct drive a 1 watt white LS/O. This is what I use. I run 4 white LS/Os on 3 AAs each as a power source.
Remember, NORMAL BATTERIES.
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ElektroLumens

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Originally posted by Zinje:
Hi!

I don't have any experience with leds but was interested in the Luxeon 1 watt. Does it make much difference as far as brightness if you run the led at 3.4v to 3.8v.

If i am using a 9volt battery as a power source...what type of resistor would I need.

I don't know anything about electronics so any suggestions as far was what is optimal.

Thanks!

Zinje
<font size="2" face="Verdana, Arial">You should pay more attention to the amount of current you run through the emitter. And also, how well you sink it is very important. You can fry the emitter at only 3.2 volts and 350mA, if you do not sink the heat properly. I have run them up to 1,500mA and 4.5 volts, with no apparent problems, using superb heat sinking.

Unless you use fantastic heat sink, you cannot run the LS much higer than 350mA. Heat sinking is equally or more important than the amount of current you run through them.

Use a .1 ohm resistor to check the current. Measure the voltage across the resistor, and multiply it by 10, to get the current. Your DVM will not give you an accurage measurement of current, as it has too much resistance to measure accurately.

Wayne J.
www.elektrolumens.com
 

ElektroLumens

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Originally posted by INRETECH:
If you run the Luxeon LED on a 9v battery, it will not last very long at all
<font size="2" face="Verdana, Arial">That's for sure. Like 'BLINKO', it's zoiched!
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I did it once by accident, thinking it was a 5 watt.
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Wayne J.
 

ElektroLumens

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Zinje,

If you want to run a LS from a 9V, get a 5 watt emitter. If you sink it well, it works good. The internal resistance of the 9V works to keep the current to tolerable levels. That's what I did when I made the 'Pocket Zorch'. It's a piece of aluminum, with a 9V holder attached. The 9V will run for about 20 minutes using the 5W. Otherwise, use two 1 watts in series. That should work fine too.

It doesn't make much sense to use a 9V battery, and resist it down for a 1 watt emitter.

Wayne J.
 

INRETECH

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Actually, I meant even at the correct current settings, the LED will not run long due to the small amount of power in a 9v battery

What is the Amp/Hr of a typical 9v battery, about 450mah ?
 
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