Power Dissipation

hello everybody, have small question regarding power dissipation.

i wondering why multiple people suggest small transistor rated @ 500mw power dissipation 12v 350ma/1a circuits?

here's few exemple :

the transistor commonly refered following :
http://www.onsemi.com/pub_link/collateral/2n2222a-d.pdf
total device dissipation @ ta = 25°c pt 500 mw
total device dissipation @ tc = 25°c pt 1.0 w

http://forum.arduino.cc/index.php?topic=256336.0
350ma * 12, 4.2w


http://electronics.stackexchange.com/questions/235830/npn-circuit-to-switch-12v-1a-dc-load-by-3-3v-1a-dc-using-pir

this 1 use 2 transistor devide amperage assume, still not enough under 500 mw total power disspation. 12v *500ma, 6w.

am not understanding how power dissipation works? did overlooked or assuming heatsink used?

you're confusing power dissipated load power dissipated transistor.

for instance, if supply 12v , load 12Ω, current through load 12v/12Ω = 1a. power dissipation in load 12v x 1a = 12w.
(i'm ignoring voltage drop across transistor because we'll assume it's minimal.)

now if transistor used switching has v_ce(sat) of 0.1v when 'on', transistor's dissipation 0.1v x 1a = 0.1w

(good see you're doing more studying. more better. )

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