Jinx wrote:

>>7805 is still up around 8-ish volts. Probably need a 2W, you'll
>>have work that out from actual current passed.
>>
>>HUH?  You calculated the ENTIRE dissipation is "about 0.5W"
>>    
>>
>
>Hi Tom,
>
>Added a margin for wallwarts being notoriously over label voltage,
>derating for high ambient, and just plain conservativism. A 1/2W
>wire-wound may be OK running at 50C + Tcurrent, but another
>sort (eg 1/2W metal film or carbon) may not for extended periods
>
>If wallwart is actually 12.0V, that's 0.497W for the drop across
>the regulator for starters
>
Hi Jinx,

O.K. and if you dissipate about half the heat in a resistor that is what 
wattage?
(Actually about 4/7 of the heat...)
Appears to me to be a waste of a 2 watt resistor, and probably a real 
safe bet for a half watt
of ANY composition.

In my experience, wall warts are VERY close to their voltage ratings 
when fully loaded.
I suppose if a wall wart had a 35% loss ratio...
(It would get pretty warm too...)
But out of the hundred or so I do have of various voltages, none of them 
vary by more than 20%.

I guess if your power is like Bonaire (island), you could get primary 
variations of more than 20%.

Still, bonding the regulator to the metal case is a reasonable solution.

-- 
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  |  __O    Thomas C. Sefranek   WA1RHP@ARRL.net
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