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Originally Posted by Joe Ross
As far as heatsinks, it depends on how much current you plan to draw and the package of the regulator. Looking at the national datasheet, at 25 degrees C, the TO-220 can dissipate about 2 watts, although for safety, lets use 1 watt. The equation for power dissipation can be approximated by Pdis~=(Vin-Vout)*Iin. Since Vin is 12v and vout is 5v, you can solve for Iin of 1/7 of an amp. Since that's Iin, your output current (Iout) could actually be 350ma. I have a feeling that's more then enough for your application.
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Joe,
Voltage regulators are not transformers,
unfortunately. The regulation is obtained by means of power dissipation, as you pointed out. I agree I_{in} (is this LaTeX?
) is 1/7 of an amp, but Iout must be essentially the same, minus some small leakage to ground. To make 1/7 of an amp at the input become 350 mA at the output you considered that the input power is equal to the output power, as in an ideal transformer (Vin*Iin = Vout*Iout), but that contradicts your first statement about power dissipation and thus, Iout is not 350 mA.
1/7 of an amp is approximately 142 mA, which is still a good number for sparksandtabs's use (digital logic circuits).