Heat sink

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Electronics4you

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Why does the resistance between sink and ambient increase, when the power dissipation falls? I makes no sense that a bigger heat sink is required, when the power decreases. What's wrong?
 

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Are you sure the "Q" in that equation is power? I would check that.. I've seen Q as heat transfer or some such...
 
Electronics4you said:
Why does the resistance between sink and ambient increase, when the power dissipation falls? I makes no sense that a bigger heat sink is required, when the power decreases. What's wrong?
Click to expand...

The thermal resistance of a big heat-sink (in °C/W) is low. Your equation is correct.
 
eng1 said:
The thermal resistance of a big heat-sink (in °C/W) is low. Your equation is correct.
Click to expand...

So, for a given amount of power dissipated, as the heat sink gets larger ,C drops?
(C/W) is temp./ watts?
 
The thermal resistance (sink-to-ambient) of a heat sink is its efficiency to release heat to the ambient. As it is a sort of resistance, it must be low enough to protect the junction.
°C/W (or °F/W, if you like) is temperature/watt.
 
Thermal resistance is measured in °C/W, it's the temperature reise above ambient when the it's dissipating 1W. A small heat sink will warm by 10°C when it's dissipating 1W and a large heatsink might only rise by 0.5°C given a power dissiation of 1W, so the thermal resistances are 10°C/W and 0.5°C/W respectively.
 
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