Heat

The next problem to overcome is the removal of heat from the PN junction. Even the best LEDs only have an electrical efficiency of 20%; this means 80% of the power supplied is converted into heat. The PN junction has a maximum working temperature of around 150 degrees C. If this is exceeded the device will fail. Also as the junction gets hotter the efficiency of the LED drops, hence it is desirable to keep the LED as cool as possible.

In 5mm LEDs the heat is conducted away from the junctions via the leads, this type of package was never intended for high power devices and is limited to around 120mW.

Most power LEDs are surface mount. There is a direct thermal path between the junction and the underside of the package, thus, allowing the heat to be transferred to a PCB. Most power LEDs can be supplied mounted to small aluminium cored PCBs. These are great for amateur use as no special tools are required to mount the LED, the aluminium backed PCB can be screwed or bonded to a heatsink.

The ability of a heatsink to dissipate the unwanted heat is expressed in degrees C/W. For example, with an ambient temperature of 25 degrees, we want to limit the temperature of a 3W LED to 70 degrees. We need a heatsink rated as (70-25)/3 = 15°C/W or better. As a rule of thumb the heat sink surface area (in free air) should be at least the square of the power dissipated in cm^2, i.e. for 3W of dissipation we need a heatsink with a surface area of at least 9cm^2. Note in order to minimise the thermal resistance between the LED and heatsink, thermal compound or resin should be used.

Lamina LED heatsink

For years cavers have used Oldham’s or similar headsets. The Oldham’s headset is very robust and being designed for miners survives very well when used for caving. However, being sealed and made from plastic/bakelite it is very difficult to remove heat from within. It is estimated that the maximum LED power that can be used inside an Oldham’s headset is around 2W.

Oldham’s Headset