I found out something interesting today.

LEDs lose resistance as they heat up. That's why it's so tricky to run multiple LEDs in parallel. Given that mass production isn't perfect, every LED varies in resistance from the spec, and the lowest-resistance LED in the array will allow the most current to pass through it, which heats it up -- but instead of that being a self-limiting behavior as with most other electronic parts, it's self-reinforcing instead because the heat lowers the LED's resistance further and even more current is allowed through and even more heat builds up. Usually a resistor is added before each LED in order to limit the effect that heat will have on each LED's resistance.

All the more reason why it's important to inject heatsink goop into the head of every flashlight using an LED array.

And such was the story of little Johny, and his puppy named "Thermal Runaway"

Batteries prefer heat and LEDs prefer cold. The perfect flashlight has a 120F battery tube and a -80F LED mount..

StarHalo said:

Batteries prefer heat and LEDs prefer cold. The perfect flashlight has a 120F battery tube and a -80F LED mount..

Click to expand...

But when not in use, the batteries would be kept at 33F

Marduke said:

But when not in use, the batteries would be kept at 33F

Click to expand...

Correct

I heard from one of my thermal engineers that excessive thermal goop serves more as an insulator than a conductor. Its intent is to merely fill in "small" air gaps between two thermal bodies. If the goop itself is used as a heat sinking material, then its effectiveness is reduced.

I should have asked him why... because it doesn't real make sense to me, but I was busy at that moment.

Because the heat conducting properties of the "goey" stuff is a lot better than air, but a lot worse than most metals.

Compared to dead air space the thermal pasta is an excellent conductor.

Compared to aluminium or copper it is an insulator.

So - you use the thinnest possible amount at the junction between 2 metal blocks to create a joint that has no air gaps.

perfectly said, jankj.

We're not going to start talking about lapping the heatsinks/stars/LEDs or extolling the properties of one thermal compund over another? May I suggest that anyone interested look up overclocking CPUs and there will be lots of information. More than you'll ever care to read.

Quite so. Electronically speaking, it would be easier to build a array in series.

As the LED heats up, the resistance is lowered. The voltage over the LED is decribed by..


..and I (current) is constant in a series circuit. Therefore as resistance (R) drops, the voltage (V) will drop proportionately, making the circuit self-regulating.

Also, a high voltage/low current design is usually preferable to a low voltage/high current one with regards to electric loss and dimensioning of conductors.

T.