CPF'er "in training" needs insight

[poolman966554]

So, as a few of you may have seen my other thread about building a lightbar with 12 xml T6-3B.. i was in a clueless state on what driver i needed to run all these at 2100- 2400ma..

Anyway, as i was doing the math, i figured for the $3 it would cost, i ran down to the local component store and purchased a few resistors. I bought 3 - .68 ohm 10watt resistors..( math says i only need 3w, but space is not a factor here so i figure id overshoot wattage tremendously for the pennies it cost)

my supply voltage was amazingly solid once i got meter on my atv battery while running. the low and high revs equaled a solid 14.42(idle) thru 14.48(revved up)

so, this is my plan.. 14.4V - 12.8Vf / 2.3A = .6956 ohm resistance right? (V-Vf /2.3A)
cree says xml2 is at 3.2v at 2.3a

I would like to "try" and place 4 xmls in series with a single .69ohm resistor, and hope to run at 2300ma per string.

I think it will work flawlessly until another device it powered up, such as headlights, especially "my winch"

Now heres the real reason for this thread..

The voltage drops from 14.4 down to 12.6 under massive load (at battery) when winch is activated.

I imagine the leds will shut off under winch load, due to lack of voltage.. but return under no load.

Is there any component harm going on here?

FWIW, if no harm is done to leds, im ok with this. as winch is only operated for 2 sec at a time.. up and down to drop/raise plow. Thanks!
also, its a late night a coming home from a casual drink. I hope my math is right!

Btw! I bought "cement?" style resistors, vs the silicone that they had. i figured cement was square, so i could heat sink it better..
If anyone feels like they wanna shed some light between the difference, any knowledge is welcome! thanks!!

I can still swap these cement/ ceramic kind if anyone thinks its necessary, but overkilling @10w, not sure if neccessary
p.s. i know a hbflex boost driver will solve my issues, but id need 2 to run this many leds at amperage.. at$40 a piece.. the whole reason im trying this is it only costs me a few bucks.

Let me know what u guys think!!

P.s.s love reading this led forum! if it wasn't for the posters asking question here like this/and having explanations, i still wouldn't have a clue! Im proud to be part of a community that shares the same interests! Thanks again guys!!

 

[Steve K]

You're doing pretty well. Don't forget that the graph of Vf vs forward current is a typical value. I'm not sure if the datasheet really shows what the minimum Vf might be. It does mention the temperature coefficient of Vf, which is -2.1mV per degree C. This means that as it heats up, Vf goes down. As Vf goes down, the current will go up in your circuit. You might want to increase your resistance a little bit until you get some build it up and get some experience with it.

Running the resistors at a lower level of power than their rating is always a good idea. Make sure there is a way for heat to get away from the resistors and out to the air. Keep them (and the LEDs) out of moisture too.

Running the light at a lower voltage won't hurt the LEDs, so don't worry about that. You'll probably still get some light, though, since Vf drops as the current drops, and even at low currents, LEDs produce a decent amount of light.

 

[DIWdiver]

The problem isn't when the winch is running, it's when you turn it off. That's what is called a 'load dump'. The alternator is working really hard to keep the voltage up (and failing, so it's certainly maxed out). When you shut off the winch, the alternator can't back down instantaneously, and you get a surge of voltage that lasts a fraction of a second. I'm working on a fuel pump controller now, and one of the customer specs says it should survive 85V load dump!

I'm going to guess that you won't see anything like that in your ATV. But because you only have a fraction of an ohm of resistance, even a few volts of surge could overdrive the LEDs. The XML can handle pretty substantial overdrive for short periods, and you might have only a 10 or 15 amp alternator, but I'd think seriously about putting a beefy transient suppressor in front of your LEDs.

 

[poolman966554]

Thanks for the info guys!

. This means that as it heats up, Vf goes down. As Vf goes down, the current will go up in your circuit. You might want to increase your resistance a little bit until you get some build it up and get some experience with it

I bought a 5amp analog meter im gonna install temporarily in series with one of the strings to monitor the amperage during temp swings. The quad is kept in a non heated garage, so starting temps are in my favor here, and cold weather hopefully keeps amperage at bay.. i might have come up with something else if i wanna go mudding in the mid summer @ 90' ..

Is there a potentiometer i can use here(instead of a resistor) inline to control amperage if it creeps up into the danger zone? iwould think a pot with a ammeter inline would help keep an eye on things. Ive never used a pot before, so not sure if they have them with a 5-10watt rating.

I'm going to guess that you won't see anything like that in your ATV. But because you only have a fraction of an ohm of resistance, even a few volts of surge could overdrive the LEDs. The XML can handle pretty substantial overdrive for short periods, and you might have only a 10 or 15 amp alternator, but I'd think seriously about putting a beefy transient suppressor in front of your LEDs.

hmm. ive never heard of such component.. thanks for the tip. Im definitely interested in aquiring one.. any particular one you might want to recommend for my situation?

 

[poolman966554]

Also, ill add im a HVAC tech. I am kinda concerned now about summertime overheating.. In furnaces, there is a device called a "flame roll-out switch" Its purpose is to sense temp,and break a circuit if temp is exceeded its thermal limits. My question is, what temps are in the "danger zone" for the xm-l2?

I have auto-reset switches in all temp ranges from 140 f to 220 f. I think it would be a good idea to mount a safety switch in the middle of heatsink on each string to detect temp accurately.

Thanks!

Edit: http://www.mobilehomerepair.com/media/img/nordyne/626352.jpg
This is the style safety i would like to surface mount to heatsink (although that one is a manual reset)

Edit Secondly, will 18ga wire handle the 8amp Max i should ever see? or should i wire everything with 16ga?

 

[DIWdiver]

Mounting that cutout switch on the heatsink is a great idea, one I've advocated in my driver threads. As to the temperature, that's a bit more difficult to answer. The temperature that's important is the DIE temperature. The XML can handle 150C at the die. At that temperature it will degrade fairly rapidly. At 85C, the LED will last 10,000 hours or more. My rule of thumb for flashlights that will never see 1000 hours of use, is keep it below 125C. That's just my guess as to what's reasonable.

So what does that mean at the heatsink? You're going to put 6.5-7 Watts into each LED, of which around 2/3, maybe 3/4 gets turned into heat. So you're looking at 4-5 watts. For argument, let's say 5. The thermal resistance of the LED package is 2.5C/W, so the die will be 2.5C/W*5W=12.5C hotter than the outside of the case. On an aluminum star, you might see another 1C/W, and in the interface to the heatsink you might see another 0.5-1C/W (guessing), for a total of another 7.5-10 C. That means that the die will be 20+ degrees hotter than the heatsink under the star. You'll certainly have at least a few degrees between the star and the thermal sensor, so I'm thinking that you should shoot for a max of 80-90C at the cutout.

If you know enough about thermodynamics, and the particular materials and techniques you are using, you could make a more precise calculation.

In home wiring, 14ga is rated at 15A. Thus, 18 would be rated at 6A. And that's a very conservative steady state rating. I don't know how automotive ratings compare. Certainly it could handle 8A for short times.

 

[poolman966554]

awesome detailed answer. ill play safe and go with 16ga wire.

i think ill install 170f (76C) rollouts, maybe even two per string to monitor greater surface area. if it gives me trouble, then ill troubleshoot further. thanks again guys!

Oh, what should i do for a transient suppressor? a google search shows many, some from $.30 to $100.. and some look like a simple diode.. Not sure why/ what im looking at here.

 

[DIWdiver]

This should be a good one.http://www.digikey.com/product-detail/en/ICTE12-E3%2F54/ICTE12-E3%2F54GICT-ND/2880323

 

[poolman966554]

This should be a good one.http://www.digikey.com/product-detail/en/ICTE12-E3/54/ICTE12-E3/54GICT-ND/2880323

Will this TVS be adequate for entire circuit? or will each series need one?

Thanks again!