LEDs are diodes, and can ONLY be powered in one way. The have a polarity, a positive and negative (or more correctly, an anode and cathode) end. If you hook them up backwards for anything more than a fraction of a second they will become damaged and quickly burn out. I've not heard of an LED actually melting due to reverse current however.

I suspect that the LED cluster you were using was either rated for a lower voltage, or had no current limiting at all. One thing that CAN cause an LED to melt (though again is more likely to simply burn it out) is excessive current.

Another thing to consider is that although many people think LEDs are devices that always run cool to the touch, this is not true. Relatively dim LEDs used to show that your TV is on certainly run cool, but LEDs that illuminate the road are not. They can dump out several watts of heat just like a halogen, but thisheat must be removed with adequate heat sinking or the LED will become damaged.

The LEDs in my headlights are thermally connected to the aluminum bodies housing the lights, which in turn are clamped to the frame of my bike (though the amount of heat that actually reaches the frame is quite low due to inefficient heatsink design on my part).

jharte, are those "small Union housings" made of plasitc or metal? Just wondering...