On Tue, 11 Sep 2007 09:01:53 -0700, John Larkin
wrote:

On Tue, 11 Sep 2007 16:29:38 +0100, "N Cook"
wrote:

I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

Silicone pads are terrible thermally.

My finger is pretty well calibrated from 50 to 60C. I can stand 50C
forever, 60C for about one second, and interpolate between.

John

My favorite coffee temperature


D from BC