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bazla82

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Hi

Looking for some help with the Cree MK-R led. What im struggling with is CC or CV... I only want to run one of these with a narrow spot lens with an external supply/driver. cree states it has a forward voltage of 11.7 vdc and can at maximum take a current of 1.25a 15w.

Can any one help me please?

regards bazla
 
If you have the option, I would go with constant current instead of constant voltage. Every LED has a different forward voltage, and each has a different voltage-vs-current slope. The datasheet shows you the average but each emitter is slightly different.

If you design a circuit around the idea that 11.7V is forward voltage, and you attach an LED that came off the assembly line with a forward voltage of 9.9V, you will smoke it almost immediately. For the MK-R, a tiny difference of 1.5V can mean an increase of 1.2amps. Same too if your battery voltage spikes unexpectedly.

My suggestion would be look for a constant current driver that operates at the amperage you're looking for. The driver will guarantee the current through the LED and will compensate for the varying Vf that each LED is manufactured with.
 
In addition to the above, the voltage/current behaviour of an individual LED is temperature-dependent (in a way which can result in positive feedback and escalating current draw as the LED heats up), and the behaviour of an individual LED even at fixed temperature can permanently change a little as it gets 'broken in', with the voltage required for a given current dropping a little (ie the current drawn at a given voltage increasing) after a new LED has spent a while running at decent power levels.
 
Thank you very much for your help.

I am just getting into LED's, guess you have to start somewhere.

Now I am most probably wrong...(and doubt I make sense) but if I have a CC driver rated at 1.25a... does that mean the voltage of the driver can be any VDC aslong as its high enough to match the Vf of the LED or higher to compensate?
 
Thanks

So does this mean that I can supply this LED with a 1.25a CC Driver that say is 14vdc and the driver will compensate to the Vf?

And the better the thermal transfer solution.. the harder I can drive it?
 
bazla82 said:
Thank you very much for your help.

I am just getting into LED's, guess you have to start somewhere.

Now I am most probably wrong...(and doubt I make sense) but if I have a CC driver rated at 1.25a... does that mean the voltage of the driver can be any VDC aslong as its high enough to match the Vf of the LED or higher to compensate?
Click to expand...

Correct. If your input voltage is higher than the Vf of the LED, your driver will likely bleed off the excess voltage as waste heat, so the driver board should be heatsinked into the body of the light somehow as well.

bazla82 said:
Thanks

So does this mean that I can supply this LED with a 1.25a CC Driver that say is 14vdc and the driver will compensate to the Vf?

And the better the thermal transfer solution.. the harder I can drive it?
Click to expand...

Correct!

Be a bit wary of driving your LED beyond it's rated maximum. If you notice the light starting to turn blue, it's because the yellow phosphor has begun to burn off. This is irreparable damage, so it's advisable to stay within tolerances. That said, with a proper thermal solution the heat won't build up fast enough to cause that to happen.
 
ok thats great.. cree alsoo state that the maximum junction temperature is 150 oC for the MK-R is this maximun operating temperature? if so can i place a thermal cutoff in the circuit to shut it off if it gets to this temperature?
 
You can't put a heat sensor on the LED die, so wherever your heat sensor is it will be reading less than die temperature, (possibly a lot less) so any cutoff temperature should be correspondingly lower to allow for the die being rather hotter than the sensor.

Also, a die temperature of 150°C isn't a temperature you should be aiming for if you want the LED to last, and the hotter the die the less efficient the LED, so keeping it much cooler is definitely desirable.
 
bazla82 said:
ok thats great.. cree alsoo state that the maximum junction temperature is 150 oC for the MK-R is this maximun operating temperature? if so can i place a thermal cutoff in the circuit to shut it off if it gets to this temperature?
Click to expand...
you can't really mesure junction temp, at the junction. you need to consider termal resistance of your stars, and heatsink. you will have abot 15-20c gradient on a star, another 20 or so star to heatsink. that is assuming you solder the led to the star correctly, (or it was soldered correctly) with no voids\air under the heatpad, and soldering paste, is not some low quality junk, or expired. also assuming your star firmly sits, on heatsink, and both are perfectly flat.
when i build the lights, i try to make sure all of those conditions are met, and i limit heatsink to 75c, (too hot to hold).
also 150 is absolute max. you don't wanna drive your led at that temp for long.

btw yea, that driver looks like it would work, if Vin is in correct range,
 
ok so if i was to drive this led at 1.2a with constant current for a prolonged amount of time then it would fail on me due to overheating?

or if soldering is correct and heatsink is attached with good thermal compound to star and flat... i shouldnt have a problem at that power for prolonged time as heat would be dissapated efficiantly.. not that waste energy is efficient.
 
if you drive it at 1,2A, and heatsink is sufficient, than you should have no problems. even higher current might be ok, as long as you move heat away faster, or at least as fast as it is produced.

are you building a handheld light, or light fixture?
 
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alpg88 said:
if you drive it at 1,2A, and heatsink is sufficient, than you should have no problems. even higher current might be ok, as long as you move heat away faster, or at least as fast as it is produced.

are you building a handheld light, or light fixture?
Click to expand...

i am trying to build my first high power / high lumen spot light
 
you mean handheld, right??

in that case mk-r is a poor choice, spotlight needs a narrow beam (spot), you just can't focus that multi die large led, there is no optics for it, to make 5-10 degree beam. (2*-5* halfangle, as it usualy written in specs)
i build pbly dosen of spotlights from scratch, after many tries, i figured the best set up is multiled set up with either xp-g or even better xp-e, (or xpg2-xpe2).
mkr is good for wide area lights,, not focused narrow beam.

this one turned out to be a nice thrower, 4xpg 38mm 4* full angle lens. this light has the best looking beam i build so far. but new lights are in the making, with even more power\throw,.
this one also has 75c switchsitting on the heatsink, once it heats up to that, it just shuts off, until it cools, but even in hot room it takes more than an hour nonestop to heat up that much. in real world outside, it never gets that hot. other lights i'm building will have output cut in half, instead of simple shut off.
 
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No its not going to be handheld but infact part of a unit that will be mounted roughly 3 meters from the ground. presenting a 4 inch diameter bright spot. i was going to try this ledil lens with it... http://www.ledil.com/node/2/p/7721 which is stated 15 deg. This is my first attempt with led so im fully prepared for epic failure. I also intend to pass the light through a second optic to get a uniform round spot at target.

Any suggestions to help me acheive this would be mega.
 
that 15* is half angle, you don't need second optic, he beam will be uniform and round without it. second lens will only create more losses,.
i used such lenses, i like them. you will need to either cut thread in your heatsink\radiator, or use nuts and bolts, i personally like cutting hreads beter, you can easy replace led if anything, on the spot, with nuts on the other side, it is a bit more complicated.
this is same lenses with xml led, the screws go thru holder\led and firmly press the led against the heatsink. this is the best way to mount those.
 
ok great thanks for that advice... I see your heatsink is rather large.. the one I have obtained is only 23mm diameter and 30mm deep and now seeing yours I think mine is too small here look do you think I need to reconsider this?
 
yours is too small, for 15watt led you need heatsink about as big a mine, i run 2 xml on that heatsink, i only feed them with 2,1A and after an hour working indoors, it get to 55-60c. the hottest you want your heatsink to be is about 75c.
yours is no good, may be for 1 watt it would be ok, but not more, also, the pins are way to close for passive colling, that heatsink is made for forced cooling, (fan). you want your fins to be at leat 10-12mm apart.


if you use it indoors, just get a computer cpu heatsink with fan, that would be enough for15w, even for 50w
 
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ok so bearing all this in mind i looked up this heatsink with fan.. i am trying to keep the design as small as posible... if you know of a way to do this without fan that would be great knowledge shared. :)
 
small size with no fan, only if you'll be using it outdoors at -30c. otherwise, you can't go around laws of nature.

passive cooling needs surfice area to dissapate heat, the more watts the more square inches\cm you need.\

rough estimate formula that i follow, most of the time, is 1 watt needs about 50square cm of area.
 
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