resistor getting hot with 1 watt led

[sillypig]

hi all why is my resistor getting hot with 1 watt led i'm using a 1 watt resistor and a 12 volt power supply
the led spec is
Forward Voltage: 3.4V~3.6V.
Forward Current: 350mA.

i'm using a 27 ohm 1 watt resistor.
just been on
and it recomends a 3.3 watt why ???

 

[MrAl]

Hi,

The power in the resistor is the source voltage minus the LED voltage, then that squared, then that divided by the resistor value in Ohms.
So (12-3.5)^2/27=2.7 watts so use a 5 watt resistor.

The above comes from:
take 12 the power supply voltage, subtract the LED voltage (3.5 nominal) and that gives you 8.5, then multiply that by itself so we have 8.5 times 8.5 and that gives us 72.25, then divide that by the resistance (27) and we get finally 2.6759 which i rounded to 2.7, which is the actual wattage. But we need to go higher than that in wattage to keep the temperature at a reasonable level so we double that and come up with close to 5 watts. A 10 watt resistor would be even better.

 

[audioguru]

Resistors have a very high temperature when operated at their power rating in free air (not enclosed).
Your resistor is destroying itself because its power is nearly 3 times too high.