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It all depends on what the current that you are putting into the regulator and what current the regulator is designed to run at. for example you can get a regulator that only supplies 100mA or you can get a regulator that suplies 1A.
Oops. Yeah, I'm kinda tired right now. You can draw 5 watts, if you run it at the default 5 volts.
National semiconductor does say the LM 7805 can supply over 1 amp, but they don't say how much more. Fairchild says their equivalent chip, the MC 7805, can supply up to 1 Amp. Probably 1.5 amps is its highest limit.
You are right Sebi, the 7805 has thermal overload protection. I guess you could use it at 1.5 amps, with a large heat sink. Panji, you could also use the LM 350, which provides 3 Amps of current.
o, i think that you are tired too Sebi. you said that the total disipation is 5W. i think that you meant that the power available from the regulator is 5W for a 1A current and 7.5 for a 1.5A current. i have used many 78xx regualtors at 1.5A, but with good heatsinks. in some cases i preffer to use the LM317 regulator, because they are about the same price and the LM317 only requires 2 more resistors.
and i think that the dissipation for the 78xx regualtors can be as much as 30W but i am not sure.....ill check the datasheets