how hot is too hot?

[James S]

When slightly overdriving regular (non luxeon) white LED's they do get warm, and sometimes hot to the touch. I'm trying to strike a balance between lots of resistance for a new battery and a little bit to get more out of an almost dead battery.

If the led is hot to the touch (not burning but definitely more than warm) is that too hot and will the phosphors prematurely degrade? I don't have an easy way to take their temperature.

This is not a production model or anything serious, just trying to get a handle on the temperature. If I can hold it then they are OK? Or should I notch up the resister another rung?

Thanks,
James

 

[EMPOWERTORCH]

When I first started making and modding torches I was told to err on the side of caution as Nichia LED's are more sensitive to overload than the old style gallium LED's.
My first BT1 used an underdriven Nichia 8000mcd green LED and the output was pretty good, but it soon started fading on the cheap waltham batteries I was powering it off (roughly 60 or 70 hours.) The first production batch of Bt1's was the same.
On the second batch I chose to slightly overdrive the LED's at about 32 mA (for fresh batteries).
The rated current for the NR86C (Maplin part no) is 30 mA, but I'm sure that both Maplin and Nichia put a large safety margin on that maximum to cover themselves!
With fresh batteries the new BT1's were brighter for longer, even though they were consuming more current. The rated current is reached pretty soon as AAA's have thier own internal resistance which also limits the current. The LED's get mildly warm but do not get overly hot and the metal bezel does a good job in soaking away the excess heat. The difference is seen when the torches are used side by side... the beam I get from the newer torch is picking out far more campsite and woodland, etc. With NSPG's the frequency of the light is also shifted upward so that the output appears more cyan (though not overly so otherwise you're seriously overdriving the LED!)
You can effectively heatsink the LED's by connecting the cathodes to a large piece of metal. In most commercial torches there is a large contact ring which completes the circuit made by the switch strip from the rear of the torch. In permanent mods the leads themselves are soldered directly to this disc.

PR bulb mods are a bit more problematic as the mass of the bulb cap is very small. However, in a metal torch good thermal contract between bulb and torch should provide adequate heatsinking.

You must bear in mind that the LED's total lifetime may be shortened by overdriving. The 100000 hour lifetime for LED's is an estimate (as InGaN LED's haven't yet been around for 100000 hours).

I hope that the info above proves to be useful in your quest!

Matt.