luxeon star current rising

LedCrazy

Newly Enlightened
Joined
Oct 29, 2003
Messages
3
I purchased a streamlight luxeon 3aa light recently.
I measured the current drain from the supplied
duracell batteries.
It was .51 amps. I removed the disposable alkalines
and ran the light on nickel metal hydride rechargables.
There was a higher current consumption for the first 5
minutes of .55 amp falling off to about .44 amps shortly
after. 30 minutes later the current drain reduced to .4 amp.

I went back to the orignal batteries and noted .52 amps
led load (this is a resistored led only). The following day
a ran rechargables for several hours went back to the orignal duracell
batteries to measure current consuption now at .53 amps
Now I purchased another vom amp meter. I shows .53 amps also (ok).
After another day running rechargables I go back to the
orignal duracells and measure current it seems to increase .1 amp
per 3 hours of (overdriven) runtime using rechargables.
After about a week of running rechargables (3 hours daily)
my orignal duracells show .55 amps of current draw
(confirmed using two different meters)..
I'm now getting nervous about this overdriven
led being damaged and pulling more current .
I decided to check the current draw from other brands
of batteries. Enegizers show .66 amps (damn).
Riteaid cheapes .6 amp. 99 cent store batteries
show .61 amps. My led lite now pulls .56 amps from
orignal (unused new) duracell batteries.

Will the reference current measured using the orignal
batteries keep increasing till the led diode short circuits.
Is this the effects observed by overdriving leds.

This streamlight luxeon star 3aa light is stock unmodified.
My research of luxeon 1 watt led indicates .35 amp at 3.42
volts to be the rating. So any current measured above .35
amp I term as overdriven? What happens when you blow a luxeon
does it go short circuit, open circuit, pull more current as
its being spent (reduced lifetime) by overdriving ?
 

Doug S

Flashlight Enthusiast
Joined
Jun 20, 2002
Messages
2,712
Location
Chickamauga Georgia
I commend your careful observations. I look forward to hearing if others have observed something similar. If they haven't, it doesn't mean that it is uncommon, possibly not many have looked as carefully as you.
I once observed something that is possibly related and discussed it in the following thread. I don't think anyone else said they had seen something similar.

Luxeon Vf change observed
 

evan9162

Flashlight Enthusiast
Joined
Apr 18, 2002
Messages
2,639
Location
Boise, ID
one other slight possibility to consider (I know it's a long shot), but might be that the changing of batteries could possibly be rubbing gunk/oils/etc off the battery contacts, thus slightly reducing the resistance.

It's just a slight possibilty though.
 

INRETECH

Flashlight Enthusiast
Joined
Aug 22, 2002
Messages
1,318
Location
HILLSBORO, OR
A LED being a diode is not a linear load like a resistor, being a semiconductor device - its "resistance" will drop as its temperature increases

This is a problem with large current devices, a slang called "thermal runaway", as the device gets hotter - its resistance gets lower, and it allows more current - which causes the device to heat up more, and repeats - eventually unless the current is limited somehow, the device will destroy itself

The cure for this problem is to properly limit the current to the device, I always suggest the most simplest - using a LM317 in Constant-current mode; although you do lose voltage in the process

Another way is to make sure the device has enough heat-sinking and lower the current, so the process doesn't get out of hand

"A current meter is your friend"
 

Chop

Flashlight Enthusiast
Joined
Jul 22, 2003
Messages
3,635
Location
Louisiana
I am no electrical engineer, but in my experience, as temperature rises, resistance decreases and you get more current over the LED. I think this is right anyway. The key to a resistored light is heat sinking. Get that heat out.

As far as overdriving a luxeon goes, I wouldn't worry about it. With any 1W LED light I get, that is constant current regulated, I overdrive it to at least 700mA. On many, I drive them up to 1A. Guys with bigger lights drive them as high as 1.5A with proper heatsinking.

So the life of the LED may be reduced to 1000 hours rather than 100,000 hours at spec, but even at 1000 hours, I'll probably loose the light before the LED self destructs.

I have a few M@g lights that I have converted to 1W luxeons and I direct drive them off of 3 1.5V cells with no resistance at all. I'm getting ~1A across the LED with no heat problems in these big lights.

Personally, I'd open up the light you have and get rid of the resistor and just worry about getting the heat out. A direct drive on 3 1.5v cells is pretty impressive. If you do ever fry the luxeon, just replace it. They're cheap enough.
 

LightScene

Enlightened
Joined
Sep 12, 2003
Messages
939
[ QUOTE ]
I'd open up the light you have and get rid of the resistor and just worry about getting the heat out. A direct drive on 3 1.5v cells is pretty impressive.

[/ QUOTE ]

What about direct drive with 3 NiMh? What kind of current will that generate?
 

Chop

Flashlight Enthusiast
Joined
Jul 22, 2003
Messages
3,635
Location
Louisiana
LightScene,
I don't use NiMh cells, but it's my understanding that those have less internal resistance than non rechargeables, so you'll probably get more current. Lots of guys here use them without problems. Just put one together and take a reading. If it's too much current, resistor it.

I just want to note here, that I've put some 1W luxeons through some punishment and you'd be surprised at what they'll take if properly heatsinked.

I really can't comment on what your light's current will be. I don't know what batt configuration will be, or what the internal resistance is of the components in your light.

If you've never modded a light before, a good place to start is with a 2D cell M@g light. Get yourself some 1" PVC pipe to make a 3C cell batt holder. It WILL fit in the 2D light if you trim the spring in the tail cap so that it fits flush with the bottom rather than on the rim. Just make sure you get good contact to the spring. You'll need to remove the anodizing from the bottom of the cap. I used lye, but I'm sure that sandpaper would do it too. Do a search here on CPF. Many have done this mod and it's great. It'll be brighter than before and have a much better beam. I have a few that I did and I love them. They were cheap to do too.
 

Steelwolf

Flashlight Enthusiast
Joined
Feb 6, 2001
Messages
1,208
Location
Perth, Western Australia
Z: As temperature goes up, resistance goes up IN A WIRE (or other standard resistive loads). With a semiconductor (like LEDs), as temperature goes up, resistance goes down.

Remember the equations, V=I*R and P=I*V.

Also remember that we are working with a fixed voltage source, ie batteries.

So if resistance increases with temperature, then from the first equation, current must decrease in proportion. And from the second equation, power consumption also decreases. Decreased power consumption means decreased amounts of heat produced. Decreased heat will lower temperature and hence lower the resistance again. No thermal runaway is possible here.

If resistance decreases as temperature increases, then from the first equation, the current has to increase in proportion. From the second equation, we see that the power consumption also increases. This means more heat is produced and the temperature continues to climb, the resistance falls further and the current draw increases, resulting in further increase in power consumption. Thermal runaway has occurred.

Back to topic, it is interesting to see the current consumption increase each time you cycle back from using rechargeables. Before we draw a conclusion, can we take out the variable of the alkaline cells by using a metered power supply? Failing that, can we get some measurements on the current draw using the rechargeables and the starting voltage of both the rechargeables and the alkalines?
 
Top