The heatsink and derating conversation is what I try to avoid when I post "use 6 ohm/50 watt resistors" for LED loads.
Example of Power Resistor Derating
Aluminum encased resistors allowed for reduced size vs 'free air' power resistors. They are expected to be bolted down to a surface which acts as a heatsink. There is a minimum heat sink area which must be present to achieve its power rating.
Also, at ambient temperature rises, there is a wattage derating which is applied.
Using a Dale/Vishay power resistor datasheet gives light to the issue of heat sink area and ambient temperature.
- Vishay_Pwr_Resistor_3.JPG (152.01 KiB) Viewed 579 times