Originally Posted by Walker
Actually... I was going to ask what size the fan was.
I'm pretty sure it's a 50mm fan. The ebay ad says the 10 kW dimmer module is 86 x 49 x 54mm. The heat sink must be about 50mm x 75mm with 20mm fins. Let's do the maths.
We'll plug some numbers into a heatsink design calculator.
Power dissipated 41.7W (assume triac has voltage drop of 1.0 V @ 10000 / 240 = 41.7 A)
Max ambient temperature 25 °C
Max junction temperature 125 °C
Thermal Resistance - Junction to Case 0.6 °C/Watt (based on BTA40 triac data sheet)
=> Calculated thermal resistance required of heatsink = 0.8 °C/W
Now let's calculate the thermal resistance of the heatsink under forced convection (i.e. fan blowing on heatsink) using another calculator.
Volumetric Flow Rate 0.0075 m^3/s (50mm PC fan @ 12 V)
Number of Fins 10
Fin Width 0.002 m
Fin Length 0.05 m
Fin Height 0.02 m
Sink Width 0.075 m
Fin Thermal Conductivity 215 W/m·°K (Aluminium @ 125 °C)
=> Calculated thermal resistance of heatsink = 0.73 °C/W
which is just
below the required limit of 0.8 °C/W. So at 10 kW with a 50 mm PC fan blowing on it continuously the heatsink is just barely OK to keep the triac below its rated temperature of 125 °C. So the heatsink is sized adequately to do its job - it's just going to get very very hot doing it.
Going back to the first calculator, if we plug in the numbers for one 5500 W element (need to dissipate 21.9 W), it turns out that the heatsink is adequate to hold the triac at 78 °C. So even with a fan on it the heatsink will still get hot to the touch running one element, and should not contact e.g. the exterior of a PVC enclosure.
I think it's fair to say that 10 kW is right on the limit of what this dimmer is usable for, but 5.5 kW is probably OK with suitable care and attention. Either way you'll need a fan on the heatsink and a mesh guard or something around to keep your fingers from getting burned.