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dantwomey 10th November 2002 11:39 PM

Interpreting R-Theta's HS ratings?
Is R-Theta's rating system (see below) directly relateable to the C/W figure so often quoted here?


halojoy 10th November 2002 11:42 PM

it seems that it is the same Rth=C/W

dantwomey 11th November 2002 12:11 AM

So, taking this conversation one step further. Mark Finnis's Aleph 5 construction notes say,

"If we start with the power dissipation, we need to account for 150W/channel of heat. As a rough rule we do not want heat sink temperatures greater then 60C, so we need heatsink with a thermal resistance as follows:

Assume ambient Temp = 20C
For a 60C maximum, we need the temp increase to be <= 40C
Therefore, heatsink Rt = (40/150) = 0.266 C/W (max) per channel
The original Aleph5 sinks ran at about 55C and were temperature protected at 70C. We can afford to go slightly higher than this, with the understanding that the higher the FET temperatures, the shorter the life of the devices. I was fortunate to find a heatsink manufacturer in Oz who is happy to sell direct to the public. The unit I used (diagram) was 300mm x 150mm with 40mm fins and dual flanges for device mounting. The heatsink Rt = 0.25 C/W, which was right on the limit, however I would have the rest of the case for additional heat dissipation."

This means that a three inch section of the following heatsink would be more than adequate. Correct?


jwb 11th November 2002 03:18 AM

Are people running their Aleph's at these temperatures? I designed mine for 70C on the sinks and 100C on the junction. The devices dissipate 24W each and are rated to 175C.

halojoy 11th November 2002 03:32 AM

Don't you add for the termal resistance of the isolation material?
Or do you put the transistors diectly onto the heatzink?

And the surrounding temp is 20 degrees in best case.
I always take the room-temperature as being at least 40 degrees.
The air next to the amplifier on a warm summersday
will be warmer. That is the air the comes into contact with the heatzink.

Worst case is the figure I use, in all my calculations.
That do not say that worst case ever will occur.

wuffwaff 11th November 2002 03:30 PM


it states 0,44K/W for a 3 " piece so if you need 0.25K/W this would only be reached if you use two heatsinks per channel / 4 for an Aleph5. The other possibillity is to use a larger piece (probably 6-7 inches long)


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