XR-E -versus- MC-E at 1400mah?

[kramer5150]

I bought the 10-pack of DC boards (1400mah, 4x7135) from DX. My initial plan was to use these to drive Q5 emitters in 6P sized DIY hosts, from 1x18650.

Now I am starting to second guess myself, given the thermal-cooling limits of lights this small.

Would the MC-E be more thermally stable running off 1400mah?.. and therefore emit a more consistent amount of light through the duration of the run? I plan on using these lights for long-ON cycles.

I am not looking for a lumen-burner here (~150 Lumens is my goal). More importantly, I want something cool running that won't thermally nuke itself if left ON for 30-45 minutes straight.

thanks in advance!!
P.S... I tried the search tool but quickly was overwhelmed with all the endless discussions surrounding these two emitters.

 

[Justin Case]

I don't know about the junction, but for a P7 driven at 250ma per core, I get zero perceived heating of the LED case, the heat sink, and the driver (an SOB10000) in a 4C Mag mod. I would expect that in a P60 drop-in using an MC-E at 350ma per core (I'm assuming a 4P MC-E since you plan to use 1x18650), you should not generate much heat either. One glitch appears to be that you will get a very low Vf when driving the MC-E at 350ma per core compared to the Vf at the usual goal of full power drive current of 700ma per core. So my guess is that the AMC7135 will be that much less efficient since the nominal voltage of the 18650 will be that much greater than the MC-E's Vf.

 

[kramer5150]

interesting....
So the Vf of the die varies depending on the drive current?

Would I damage the 7135 circuit if Vin is considerably greater than Vf of the LED?

:thinking:

thanks

 

[Justin Case]

Maybe I was confusing. By drive current, I mean forward current. See Page 4 of the the Cree MC-E datasheet here for the graph of If vs Vf.

At If = 350ma, eyeball estimate for Vf gives about 3.2V. At 250ma, Vf ~ 3.1V. At 700ma, Vf ~ 3.4V.

No damage as long as Vin is within the specs of the AMC7135. At higher Vin (e.g., 6V), the board gets hot because it has to dissipate the excess input as heat. Basically, efficiency is Vf/Vin. Thus, at the start, efficiency could be around 3.2V/4.0V = 80%. Then the efficiency might settle in for a while at 3.2V/3.8V = 84%. It's not bad, and it actually gets better as the battery drains. I wouldn't worry about it.

 

[kramer5150]

ok thanks

 

[Justin Case]

The board might have to dissipate about 0.8W of heat. Most likely, the LED will have to dissipate far more wattage. You are feeding it roughly 1.4A * 3.2V = 4.5W and most of it still goes to heat rather than light.

For comparison, a DX6090 drop-in's driver generates about 4.5W and is probably 67% efficient when running in full regulation. That means it dissipates about 1.5W of heat.