Tony Harris wrote:
>
> Ahhhhhhh - ok - so I add the junction to case 5degC/Watt to the
> 15.6degC/Watt of the heatsink to get a total of 20.6degC/Watt total...  Am I
> on the right track?
>
> So if I were to throw that heatsing on my 7805, I could move up to 250mA and
> only have a temp of 87.55degC + ambient? and assuming my ambient temp is
> ~21degC, my total temp would be at ~108DegC, which would be less then the
> max of 125DegC....(assuming total volts to 7805 is 22Vdc)....
>
> Do I have my calculations correct?


Hi Tony, your calculations sound ok but your
design philosophy might need revising. ;o)
Any soldered component run over about 50'C
(that's about 25'C over ambient) REALLY reduces
reliability. The PCB will darken, the solder
will be exposed to expansion and heat stresses
and it will soon get dry joints (disaster on
a voltage regulator!)
-Roman

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