Posted to sci.electronics.repair,sci.electronics.design
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Mica versus Silicone pad insulators
"John Larkin" wrote in message
...
On Tue, 11 Sep 2007 16:09:33 GMT, D from BC
wrote:
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
It's interesting that the tongue is the most temperature-sensitive
part of the human body, and the most resistant to temperature damage.
John
That being the case, why is it so easy to burn your tongue on a hot drink or
hot food ?
Arfa
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