how much power can i put into an led without a heatsink

i have a small model helicopter that has a tiny led that maybe puts out maybe 5 lumens, if i replaced it with an xm-l, it would have no heatsinking, but would it burn itself out?
How much power can i put into an xm-l with no heatsinking (20mA maybe?), or an xm-l with a 16mm star with no additional heatsinking?

Probably 300-350mA for a XML on a 16mm star for extended runs. More if it’s only on for short runs.

Nothing at all, not even a strip of aluminum pie tin? I would think very little. 30mA sounds good to me.

it would replace one of these

Depends on airflow.. If it's going to be moving quite a lot, you can probably put a hundred or two milliamps through it and not overheat it too badly..

Only way to find out for sure is to try it. I would start at about 75-100mA and see how hot the star is after a minute. The LED itself is ok at up to about 125C (the junction will be higher, but its safe up to 150 or so)..

PPtk

One of those in the stock Mag plastic tower will happily run at 1.5A, I've done both XML & XPG like that. Up around 1.8-1.9A things start to get a little melty, but none of the LEDs have died.

Plastic is a terrible conductor of heat when compared to copper or aluminum (well, most plastics anyway). Compared to air, however, it's a fantastic heat-sink material.

Having even a crappy plastic tower is orders of magnitude better than having no heat-sink at all.

PPtk

will it fail by sparking or just stop working?

I assume no airflow, the led is in the same position and similar setup as the red led in the front of this helicopter

If it fails due to heat, it will most likely (quite quickly) get dimmer, and dimmer, and dimmer, and dimmer...ad infinitum.

PPtk

my concern is fire risk, i am happy to possibly sacrifice one of the many unreflowed chips i have little use for (and used without a star for this project)

The LED itself won't spark - it will just create heat. The only real fire risk is if whatever it is attached to gets hot enough to combust.

I think you should be more worried about what this will do to your run time on that tiny battery. I’m assuming its a high c rated bat? If not, careful not to exceed its limits

You should consider the following:

That LED you indicated will have a very low current flowing through it.
You can almost guarantee it is direct driven using a series resistor to limit the current.
The XML will have a higher forward voltage. This means there will be a lower voltage drop across the current limit resistor.
This implies the current flowing through the LED will be even lower.
This means The XML will be painfully dull.

I am assuming it has a forward voltage around 3V, but i don’t know for sure
I am hoping for maybe 20-30 lumens, which would be well under 100mA, and its powered by a small lipo battery, but yes, whats driving it is an important question, a resistor is not an ideal way to control it

If the LED you’re replacing is a red LED, the Vf would be closer to 2v.

its white, i couldn’t find my specific model in a quick search

Yeah, I’m not sure if the circuitry could provide the nessary Vf and the current anyways.

Best to try it out! :slight_smile:

try 5050 SMD LED… there are 3 chips inside it. brighter than 5mm leds.

for XML without heat sink, ~100mA

Solitaire mods have pretty poor heatsinking but 350 mAs is not a problem. An XML on 16 mm copper should have no problem with that current.