Cree MC-E Catastrophic failure.

how were you driving this LED?

A pre-built module that supplies 700mA constant current. Ends up being about 12.something volts for that LED module which looks within spec.

And I've checked, it works as advertised.

John Tserkezis schrieb:

Hello,

the heatsink worked well when you rode the bike, enough airflow at the heatsink. But when you did your first test in a still room, there was no forced airflow at the heatsink and the airflow caused by the temperature differences, the thermal convection, was not enough.

The heatsink had some fins, I suppose. But what direction had these fins, horizontal fins would be perfect when riding the bike, but in the still room vertical fins would be better.

If you do another test with a new LED in a still room, you should use a ventilator to simulate the forced airflow when riding the bike.

Bye

The original heatsink I had calculated to be enough (assuming the air movement you're likely to get outdoors due to a breeze, but otherwise standing still) and I had verified that while riding, it was no problem at all.

It appears I've underestimated the worst case conditions for this though, so this time, I'm overestimating the heatsinking requirements. :-)

I'm using significantly larger heatsinks with vertical fins (double sided to increase surface area). What I'll also do is drill a small hole in the case near where the led star base is, enough to position a temperature probe. So I'll get a closer look at that it's running at.

The resulting heatsink is going to be absolutely ridiculously larger than whatever I've seen suggested (or calculated) but hopefully the visual appeal will make up for that.

John Tserkezis schrieb:

Hello,

you may try a heatpipe between the LED and the heatsink, the thermal conductivity is much better than that of aluminium, but a heatsink of copper is also worth a try, not as good as a heatpipe, but better than aluminium.

Bye

I don't have copper like that lying around, and I don' t fancy buying something like that either.

I had thought of a heatpipe, but there are additional "problems" to overcome if I do that.

The 1cm thick block of aluminium the led is bolted to is part of the structural arrangement of the entire light, so making it thinner will prevent me from mounting anything securely.

The heatsink can be bolted directly to this 1cm block so I don't really need a heatpipe.

The most important bit, if I *did* use a heatpipe, I'd have additional structural problems because I would have to mount it much more securely than otherwise. (the heatpipe would move/vibrate and fracture).

The general rule when it comes to bicycles (same as go-karts) is that however tight you make things, they're not tight enough. Crossing your fingers helps. Once they're bolted on, you need to test it by absolutely manhandling it.

If it comes loose in your hand, it WILL come loose on the road. If it does not come loose in your hand, it MIGHT not come loose on the road. This is where the crossing your fingers comes in.

You may have smooth roads where you are but here, in Sydney (Australia) we do not. :-(

and the clear reflector has come off the die).

perusing, so I'm asking here. The reliability spec

than I have here, so I'm at a a loss.

block about 1cm thick, within a PVC pipe section, and

heatsink compound applied. The only surface I did

mounted on - that was left "factory standard".

about 15-20 minutes, the temperature probe

The heatsink was quite warm to the touch at that

I have been using a 5W LED array (Bridgelux) as a reading lamp with only a 6 x 5 x 2 cm CPU heatsink from an old computer - without any thermal problems. It runs warm to hot - but not too hot to comfortably maintain touch though.

temperature, or anywhere near the actual die star for that

the heatsink temperature barely at "warm to the

friends. It ran for several seconds before I moved the

measured and then dismantled the assembly (it was O/C)

edges), and found the lens came off quite easily with my

It seems that the die lost thermal contact with the heatsink. You obviously had a screw loose. It only takes a minor degree in the torque attaching the LED die to the heatsink to make a big difference in thermal conductivity.