[Amps] making sinks black

[Steve Thompson]

>>Most heat sinks now days have relatively tall closely 
>>spaced fins. For practical porposes the heat sink, from a 
>>radiation stand point, could be looked at as a block of 
>>material the length, width and height that would not 
>>include the fins.

If the fins are too close together, then the convection airflow between 
them can get choked with turbulence and, again, the heat loss ends up 
little better than a solid block. I tried an experiment with a heatsink 
that had fins 5mm (3/8") apart, mounted it vertically with some 
resistors for heating. It ran slightly cooler after I milled off every 
alternate fin.

If you are calculating expected temperatures, look carefully at the 
manufacturer's data for thermal resistance - often it's specified at 
high temperatures above ambient, way over what you want your circuit to 
run at. The thermal resistance will be higher at lower temperature rise. 
A fan blowing an air stream that you can just detect on the back of your 
hand will roughly halve the thermal resistance.

Steve